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HISTOLOGY REVIEW - 4 BASIC TISSUES AND THEIR SUBTYPES

I. EPITHELIUM • Covering and Lining

o Simple Squamous - blood vessel lining o Simple Cuboidal - kidney cortex, sweat gland body o Simple Columnar - renal papilla o Stratified Squamous Non-Keratinized - esophagus, vagina o Stratified Squamous Keratinized - skin o Stratified Cuboidal - sweat gland duct (lining classification) o Stratified Columnar - ducts of pancreas and parotid o Pseudostratified Columnar - trachea (cilia) o Transitional/Urothelium - urinary bladder (chemical protection, stretching abilities)

• Glandular o Serous - secretes water protein (salivary amylase by salivary gland) - cells stain dark with H&E; round

nucleus and secretory granules. Serous cells can secrete their product into the lumen of the mucous tubule by intercellular canaliculi. Serous demilune is a crescent shape group of serous cells which cap the end of tubules of mucous cells.

o Mucous - secretes mucus (salivary gland) - cells stain light with H&E; dark flat nucleus located at the basal end of the cytoplasm.

o Goblet - secretes mucus (unicellular gland) - a mucous cell in the intestinal lining o Sebaceous - secretes lipid (oil glands of skin) - cells close together with little intercellular substance b/t

them; organized as a bag full of cells (no lumen). The cells are the secretory product. The sebaceous glands form a short duct which usually empties into a hair follicle. They have 3 stages:

• Undifferentiated Stem Cells - arise from the basal layer near the basement membrane and form new sebaceous cells; flatter and cytoplasm stains darker.

• Synthesizing Stage - larger, lighter, euchromatic chromatic nucleus (synthesizing lipids) • Secreting Stage - smaller, darker, heterochromatic nucleus (done synthesizing and ready to be

secreted) 3 Types of Secretion

o Apocrine Secretion - epithelium surrounds a lumen where product is secreted; product contains small part of cell’s apical cytoplasm and plasma membrane (e.g. sweat or mammary gland).

o Eccrine/Merocrine Secretion - product is stored in secretory granule and released into a lumen with NO loss of cell membrane or cytoplasm; used by mucous and serous cells.

o Holocrine Secretion - whole cell is secreted (sebaceous gland)

II. NERVE • Peripheral Nerve Tissue - Consists of peripheral nerves and ganglia (cell bodies of neurons) • Central Nervous Tissue - Consists of nerve fibers as tracts and collections of neuron cell bodies called nuclei

III. MUSCLE

Muscle Type Cell/Nucleus Contractile

Apparatus

Attachment of Myofibrils to Sarcolemma

Gap Junctions Ca2+ Supply

Ca2+ Sensitivity Response

Inherent Ability to Contract

Syncytium Hypertrophy vs. Hyperplasia

Contraction Signal

Skeletal Muscle(st

riated

Long, cylinder, euchromatic,

nucleolus visible, multiple nuclei

under sarcolemma**

Actin filaments

insert on Z disk of

sarcomere; Nebulin and

Titin

Actin filaments attach to

dystrophin, dystroglycan complex, and

laminin-2

None

Triad at A-I Band

(1 T-Tubule, 2 SR Terminal

Cisternae)

Troponin in Actin

Filaments No Morpho-

logical

Hypertrophy, Regenerates via

satellite cells

NMJ Synapse

Smooth Muscle

Fusiform/ Single nucleus at

center, lighter, larger

Actin inserts into dense

bodies Fascia Adherens Nexus Extracellular via

Caveolae

Ca2+/Calmoudlin & Myosin Light Chain

Kinase

Yes Functional

Hypertrophy, Hyperplasia via

same cell division

Spontaneous Autonomic Innervation

Cardiac Muscle striated

Branched/ single central rectangle

nucleus

Striated, Sarcomere, No

Nebulin

Actin filaments attach to fascia

adherens in vertical place of

intercalated disks, desmosomes

Intercalated Disks

Diad at Z-Disk (SR &

Extracellular Fluid, 1 large T-

Tubule)

Troponin in Actin

Filaments Yes Functional

Hypertrophy, Can not

regenerate

Spontaneous, Contracts w/o nerve stimuli;

AP originate in pacemaker cells; Ca2+

induced Ca2+ activation

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IV. CONNECTIVE TISSUE - All connective tissue consists of cells, fibers, and ground substance. - Cells (fibroblasts) synthesize and secrete the fibers and ground substance - Fibers and ground substance form the extracellular matrix of connective tissue - Ground substance is composed of GAGs dissolved in tissue fluid.

• Connective Tissue Proper o Embryonic CT

Mesenchymal cells (main cell type), resemble fibroblasts but have angular shaped nuclei (instead of round). Highly cellular.

Contains abundant ECM rich in proteoglycans and hydrophilic substances. Present in umbilical cord (Wharton’s jelly) and pulp of developing tooth

o Loose (Areolar) CT More cells than collagen fibers, more ground substance Cells include fibroblasts, plasma cells, lymphocytes (T and B), mast cells, macrophages,

neutrophils, and eosinophils. Elastic fibers are thin, straight and branching. Collagen bundles are thick and wavy. In intestinal villus, showing clock face pattern of heterochromatin in plasma cell nuclei. In mammary gland, surrounds glandular epithelial components. In dermis of skin (above dense irregular CT), has fewer cells and more fibers. Fibroblasts

a) Make all 3 types of fibers (reticular, elastic, and collagen) b) Do not store products. Continuously release ground substance, collagen fibers, elastic

fibers, or reticular fibers as they are synthesized. c) Round nucleus

Plasma Cells a) Oval shaped cell, eccentric nucleus b) Have more cytoplasm than lymphocytes c) 9 microns d) Clock face pattern of heterochromatin e) Often found in lamina propria (loose CT)

Lymphocyte (T and B) a) Less cytoplasm than plasma cells b) 5 microns

Mast Cells a) Arranged along and outside of small blood vessels b) Store products in secretion granules which causes them to stain darkly

o Dense CT Contains more collagen fibers than cells (except in elastic ligaments) 1. Regular CT

Parallel oriented bundles of collagen fibers which are made by tendon cells (modified fibroblasts) and wrapped by less dense collagenous tissue.

Pathways created b/t bundles contain blood vessels surrounded by loose CT called peritendineum internum.

Separated by linear rows of fibroblasts Found in tendons, ligaments

2. Irregular CT Thick, wavy, irregular, and intertwined bundles of collagen fibers Non-uniform array Fibroblast cells are sparse Mast cells, macrophages, and blood vessels Found in dermis of skin (deep to loose CT), aponeuroses, submucosa of GI tract Reticular fibers and elastic fibers dominate

• 3 Types of Fibers (All made by fibroblasts)

o Reticular 2 microns (lymphocyte is 5 microns) Must use PAS (periodic shift reaction) and silver to visualize

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Adult-type CT reticular fibers predominate, forming stroma of organs of lymphoid-immune system, hematopoietic bone marrow, and liver.

Reticular fibers (Type III collagen) dominate and are produced by reticular cells (fibroblasts) Thin, branching structures Form a meshwork in which lymphoid cells are embedded and allow passage of cells and fluid. Found in lymphatic tissues (spleen, liver) and bone marrow Argyrophilic (bind silver salts and appear very dark)

o Elastic Elastin contains 2 unusual a.a.: Desmosine and Isodesmosine (give elastic fibers rubberishyness) Stain well with Fe Hemaoxylin Stain (Verhoeff’s Stain) 10 microns Adult type where elastic fibers dominate Formed by smooth muscle cells Form discontinuous lamellae or membrane in muscular walls of large blood vessels just deep to

endothelial layer of cells Wavy pink bands

o Collagen Family of 26 proteins Collagens are principle structural elements of all connective tissues. Characterized by the presence of a repeated sequence of Glycine-X-Y, where X and Y are

commonly proline or hydroxproline. All collagens are trimers in which at least some and often most of the protein chains are involved

in forming a triple helical structure (triple helix). Groups of collagens include fibrillar and basement membrane collagens. Is a protein and is present in loose or dense CT proper Large collagen fibers are ubiquitously present in CT proper as Type I collagen Collagen is (+) and binds pink Eosin stain (-), 30 microns Types I-III are banded

a) Type I - large collagen fibers, bone, tendon, dentin, and skin. Used in repair cartilage. b) Type II - hyaline and elastic cartilage. Used in repair cartilage. c) Type III - reticular fibers, reticular lamina of basement membranes d) Type IV - basement membrane (basal lamina) e) Type V - amnion, chorion, muscle and tendon sheaths

• Specialized Connective Tissue

o Adipose CT 1. Unilocular Adipocytes(Common or Yellow)

Single large lipid droplet Vascularized

2. Multilocular Adipocytes (Brown) Many small lipid droplets and mitochondria Present in newborn babies and hibernating animals Highly vascularized, non-shivering thermogenesis

o Mucous CT 1. Carbohydrate 2. Binds to PAS

o Hematopoietic CT 1. Lymphatic 2. Myeloid

• Specialized Supporting Connective Tissue o Cartilage

1. Hyaline Cartilage - Type II collagen 2. Elastic Cartilage - Type II collagen 3. Fibrocartilage - Types I collagen

o Bone 1. Cancellous Bone 2. Compact Bone 3. Woven Bone

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I. EPITHELIUM Parenchyma - cellular component of an organ that performs its function Stroma - connective tissue component that forms a framework to provide physical support for the parenchyma cells Intercellular junctions are cell surface structures involved in cell-cell interactions. Functions include: � Seal spaces between cells � Regulate cell polarity � Participate in cell-cell communication Six types of intercellular junctions: � Tight junctions (zonular occludens) junctional complex � Adherens junctions (zonula adherens) junctional complex

Function in cell-cell adhesion Extend around the entire circumference of cells accompanied by a ring of actin filaments Work together with desmosomes Important in cohesiveness of epithelial tissue Initial cell contact is made by filopodia. These interdigitate and adherens junctions are formed to constitute an adhesion

zipper. The adhesion is then expanded 2 Types of Adhesion molecules involved: E-Cadherin (which is bound by β, α - Catenins which is linked to cytoskeleton)

and Ig Family Protein Nectin (which is linked to actin by Afidin). � Desmosome (macula adherens) junctional complex

Maintain tissue integrity by providing intercellular adhesion acting as a link between the cytoskeletons of adjacent cells Connection is via intermediate filaments Contain two types of cadherin molecules, desmocollin and desmoglein Pemphigus Vulgaris acquired autoantibodies to desmogliens. Blisters on forehead, painless.

� Gap junctions (nexus) General distribution on intercellular membranes Function in cell-cell communication In excitable tissues such as nerves and cardiac muscle they provide routes through which electrical impulses are propagated. In non excitable tissues such as epithelia they pass metabolites and signaling molecules between cells. Six connexin protein molecules form a hexamer called a connexin or hemichannel. Connexons from two adjacent cells

connect to establish intercellular communication. Can be regulated by intracellular signals to adopt open or closed configuration to regulate intercellular communication.

� Hemidesmosome Anchor dermis to epidermis (literally) Cell-matrix adhesion in certain epithelial, especially the epidermis, is mediated by hemidesmosomes. Responsible for strong binding between basal surface of epithelial cell and underlying basement membrane. Provide link to intermediate filament cytoskeleton. Genetic mutations give rise to various form of Epidermolysis Bullosa (EB) which is a group of blistering diseases. Bullous Pemphigoid is autoimmune blistering disorder caused by auto antibodies to BP-180 a hemidesmosome

� Focal contact/ focal adhesion Junctions perform key roles of fastening epithelial cells together and to basal lamina

Tight Junctions - Zonula occludens - define cell polarity and control passage of substances b/t adjacent cells (paracellular pathway); beltlike

distribution with no intercellular space; associated with actin microfilaments Anchoring Junctions - Zonula adherens - beltlike distribution just below tight junctions; wide intercellular space; ass. w/ actin microfilaments and

act as an anchor for the terminal web of Microvilli. - Macula adherens (Desmosomes) - spot distribution; wide intercellular space; provide strength and rigidity ass. with

intermediate filaments - Hemidesmosomes - fastens epithelial cells to the basal lamina; associated with keratin intermediate filaments

(tonofilaments)

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Note: Zonula adherens in epithelial cells is the equivalent to the fascia adherens in the cardiac myocyte. Zonula adherens are associated with thin microfilaments in construction of the terminal web (the cytoplasmic region at the base of microvilli in intestinal epithelial cells). Skeletal Myocyte - The actin microfilaments are attached from Z disk to Z disk and at the end of the myofibril, they are attached to the sarcolemma via desmin, dystrophin, dystroglycan complex, and laminin-2. Cardiac Myocyte - the actin microfilaments attached from Z disk to Z disk and at the end of the myofibril, they are inserted into the fascia adherens and the terminal end of the myofibril. Point: Zonula adherens and fascia adherens both serve at anchors for a terminal end and involve the insertion of thin microfilaments.

Communicating Junctions - Gap junctions - functionally connect 2 adjacent cells via a channel formed by connexins.

Connections at basal lamina between cell and basement membrane: - Basal lamina = lamina lucida + lamina densa - Anchoring fibril (Type VII collagen) attach reticular fibers (Type III collagen) to basal lamina.

Epithelial cells show infolds of plasma membrane to increase surface area. Tissue is coherent association of cells differentiated in same direction and having same function.

- Epithelial tissue is close association of cells with very narrow intercellular space containing little intercellular substance and no blood vessels. (Covering)

- Connective tissue (Connecting) - Muscle tissue (Contracting) - Nerve tissue (Conducting)

Endoderm, mesoderm and ectoderm all contribute to epithelial cells. Two major subtypes of epithelium

- Covering and lining epithelium 1) Occurs in layers (strata) 2) Covers body surfaces and lines body tubes/cavities.

- Glandular epithelium 1) Lining of ducts and secretory cells 2) Two types: endocrine and exocrine

Recognize differences between following classification of epithelium:

- Simple squamous cells form a selective lubricated barrier that allows movement & diffusion. - Simple cuboidal cells form a tightly sealed barrier of cells fat enough to have organelles to secrete into and/or transport from

lumen content. - Simple columnar cells are taller, more robust, and provides great variety of cell types. - Pseudostratified columnar ciliated cells have cilia to rid airway of particles and provides reflexes and immune defense. - Stratified squamous form a very protective barrier, needing glandular lubrication. Keratinized versions are highly protective

against abrasion, dehydration and microorganisms (skin). - Stratified cuboidal and stratified columnar cells line large ducts to seal off, but still modify the secretion. - Transitional/Urothelium cells have specialized chemical protection, with an ability to be stretched.

Glandular epithelium makes glands.

- Can be unicellular (goblet cell) or multicellular (submandibular gland) - Can be classified according to:

1) if they have ducts or not 2) if ducts branch 3) morphology of secretory units

The ducts of simple glands do not branch:

- Simple tubular gland - Simple coiled tubular gland - Simple tubular branched gland - Simple acinar or alveolar gland

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Compound glands - Epithelial component performs its function and is known as parenchyma.

1) Mucous acinar cells have nucleus flattened against basal cell membrane with pale cytoplasm and secrete products that are rich in glycoproteins and water.

2) Serous acinar cells have rounded nucleus with cytoplasm containing granules that stain and secrete products that are enriched with proteins and water.

- Connective tissue component gives framework and support, known as stroma. Classification according to mechanism of secretion:

- Merocrine (exocytosis) involves secretory vesicles approaching apical domain of epithelial cell and fusing with plasma membrane. (Salivary glands: mucous and serous cells)

- Apocrine involves pinching off some of apical cytoplasm with contained secretions. (Mammary gland) - Holocrine involves the cell producing and accumulating secretory product in cytoplasm and then disintegrating to release

material. (Sebaceous gland) Dysplasia is growth of epithelial cells into abnormal patterns. Metaplasia is the conversion of epithelial cells into a normal, but different tissue. Neoplasia is the abnormally fast growth of abnormal cells such as carcinoma. II. NERVE Peripheral Nerve Thickness of myelin sheath of nerve fiber is directly proportional to diameter of nerve fiber.

- Contains lots of lipids. - Insulates the axon. - Portion of Schwann cell that contains cytoplasm and nucleus after it has completed myelin sheath wrapping of portion of

axon is termed neurolemma. - In PNS, myelin sheath by one Schwann cell covers 1 mm.

Axonolemma is plasma membrane of axon, which is a process extending from neuron cell body.

- Node of Ranvier is site where myelin sheath terminates from two adjacent segments of nerve fiber. Histological organization of peripheral nerve:

- Endoneurium is connective tissue wrapping around an individual nerve fiber. - Perineurium is wrapping around a group or bundle of nerve fibers (fascicles).

Flattened cells making up this layer have tight junctions between them to create a different environment within nerve fascicle.

- Epineurium is wrapping between perineurium wrapped fascicles and binding all fascicles. 1) Blood vessels are present in epineurium.

Collection of cell bodies in PNS are called ganglia.

- Sensory neurons are pseudounipolar. - While passing through ganglion, axon process takes right angles to enter a cell body and emerge on opposite side. - Lipofuscin pigment is a collection of undigested organelles and are indicative of aging neurons. - Autonomic ganglia are nuclei displaced in an eccentric position within cell body.

Collection of cell bodies in CNS are called nucleus. Axonal injury causes several changes in perikaryon (cell body):

- Chromatolysis (dissolution of Nissl substances with decrease in cytoplasmic basophilia) occurs within nerve cell body. - Volume of perikaryon increases - Migration of nucleus to peripheral position in perikaryon - Wallerian degeneration occurs where proximal segment of axon degenerates close to wound for a short distance, but growth

starts as soon as debris is removed by macrophages. - Macrophages produce interleukin-1, stimulating Schwann cells to secrete substances to promote nerve growth

(regeneration). - Axon and myelin sheath degenerate completely at nerve stub distal to injury and remnants are removed by macrophages. - Schwann cells proliferate, serving as guides to sprouting axons. - After regressive changes, proximal segment of axon grows and branches, forming several filaments that progress in direction

of columns of Schwann cells. 1) Only fibers penetrating these columns will continue to grow and reach effector organ.

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III. MUSCLE Smooth muscle cells have fusiform tapered nucleus in center of fusiform tapered shaped cells. Skeletal muscle has multiple nuclei in each cell just under cell membrane and is shaped like a cylinder, with cytoplasm having bands that run across the cell at right angles to longitudinal axis of the cell.

- Are striated. - Plasma membrane is called sarcolemma. - Endomysium envelops each single muscle cell. - Perimysium surrounds each fascicle. - Epimysium surrounds entire muscle formed by groups of fascicles.

Cardiac muscle is composed of branched cells that have centered nuclei that are box shaped. Common features of striated skeletal muscle:

- Nucleus located just inside sarcolemma. - Dark thin lines running across muscle are Z lines. - Narrow light band that Z line bisects is I band. - Broader darker stained band is A band, composed of myosin filaments. - Myofibril is the contractile unit of skeletal muscle.

1) Sarcoplasmic reticulum (SER) and transverse invaginations of sarcolemma (transverse tubules) are important structures in activation and control of contraction that run in between the myofibrils.

2) Composed of thin filaments (actin) and thick filaments (myosin). Smallest repeating segment of myofibril is the sarcomere.

- Bound on either end by Z disk. - Actin (thin) filaments are anchored into Z disk. - Myosin (thick) filaments are suspended between Z disks and bound together at M-line. - H band is the gap between free ends of actin filaments.

1) During contraction, actin slides along myosin filaments, pulling Z disks together from opposite ends of sarcomere. a) Shortening of sarcomere and narrowing/disappearance of H band.

Myofiber maintains ordered manner of myofibrils via cytoskeletal network of intermediate filaments.

- α -Actinin - anchors barbed end of actin to Z disk - Desmin - are intermediate filaments that keeps myofibrils in order to each other and to the sarcolemma; binds to actin

binding protein ( α-actin of Z disks. - Plectin - links desmin filaments to each other and into network between myofibrils. - Dystrophin - reinforces/stabilizes the sarcolemma during the stress of muscle contraction by maintaining a link between the

cytoskeleton and the ECM. Dystrophin is bound to dystroglycan complex. - Dystroglycan Complex - integral membrane protein bound to Laminin-2 - Laminin-2 - anchored into the ECM

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During muscle contraction, the complex arrangement of Desmin, Dystrophin, Dystroglycan Complex, and Laminin-2 to the sarcolemma (and ECM) keeps the muscle cell from tearing from the sarcolemma. A defect in any of these would result in: Disorder of myofibrils → rupture of the sarcolemma → extracellular Ca2+ rushing into the cell → degeneration of skeletal muscle → Duchenne’s muscular dystrophy Muscular dystrophies involve a defect in gene for dystrophin.

- Symptoms are muscle weakness, atrophy, and myofiber degeneration. - Caused by weakening or loss of link between cytoskeleton and extracellular matrix, resulting in disarray of myosin and actin

filaments, leading to myofiber degeneration. Within myofibril and each sarcomere segment, structural framework made up on non-contractile filamentous proteins (nebulin) that keep actin filaments in line and regulate their length.

- Titin extends from Z disk to M line, anchored at both sites and keeps myosin filaments lined with one another. Ca+2 activates troponin/tropomyosin complex Process of myosin (thick filament) moving along actin (thin) filament:

- Myosin head bound to ATP, unable to bind to actin filament. - Hydrolyzing ATP to ADP + Pi allows myosin head to bind to actin filament. - Release of Pi allows myosin head to undergo conformational change (powerstroke). - Release of ADP and binding of ATP allows for detachment of myosin head from actin and prepares for new cycle.

During muscle contraction, length of thick and thin filaments does not change.

- Constant length of A band and the distance between Z band and adjacent edge of H zone. - Length of sarcomere decreases because thick and thin filaments slide past each other.

1) Shown by reduction in length of H zone and I band. Transverse tubules carry an action potential inside the muscle cell.

- At A and I band junction level, there is close relationship between two sacs of sarcoplasmic reticulum and transverse tubule (triad).

- Action potential is conveyed to sarcoplasmic reticulum, causing release of calcium. 1) Instantaneous transfer of action potential to release calcium, inducing a smooth coordinated contraction.

Smooth Muscle and Cardiac Muscle have inherent ability to contract, skeletal muscle does not. Collaterals magnify effect of impulse. e.g. - postural muscles Hyperplasia The abnormal multiplication or increase in the number of normal cells in normal arrangement in a tissue.

Hypertrophy The enlargement or overgrowth of an organ or part due to an increase in size of its constituent cells, but not an increase in the number of its cells. Abnormal enlargement of a body part or organ, or, may be considered a normal enlargement as in the increase in size of skeletal muscle due the increase in diameter of each skeletal muscle fiber following strength training exercise.

Morphological basis of regeneration of skeletal muscle fibers.

- Satellite (stem) cells are located at perimeter of adult muscle cells and contained within the same basal lamina. - Actively begin cell division when nearby muscle cells are injured, producing numerous myoblasts. - Myoblasts fuse with one another to form myotubes which results in a giant cell called a myofiber. - A morphological syncytium results.

Extrafusal vs. Intrafusal

- Extrafusal Fibers - do the work/contract - Inrafusal Fibers - monitor the level of contraction; covered in CT capsule and wrapped with nerve endings which travel

back to CNS. When they become impinged upon by the extrafusal fibers (in the contractile state), they send signals back to the CNS that gives positional information to the brain (kinesthesiology). Note: These are muscle, not nerve, despite their physiological function to provide contractile info. to the CNS.

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Smooth muscle cells are tapered (fusiform) with centrally-located nuclei. - No striations but actin and myosin participate in contraction.

a) Actin is bound to cytoplasmic and sarcolemma dense bodies by α-actinin. b) Between actin extending from each density is special myosin molecule that complexes with actin drawing them

close together. (Dense bodies are equivalent of Z disks in skeletal muscle). - Smooth muscle behaves like a single cell due to adhering junctions and gap junctions.

1) Signal to contract is conducted from cell to cell via gap junctions. 2) Force of contraction is integrated across large population of smooth muscle cells via adherin junctions 3) Behaves like a single cell and shares its ionic pool of Ca2+, so called functional syncytium.

- Muscle contraction initiated by release of calcium within cell, eventually leading to phosphorylation of myosin light chain. 1) Ca+2 brought in by caveolae (instead of SR in skeletal muscle), which are permanent depressions of the plasma

membrane involved in pinocytosis. a) Lipid raft is precursor to caveola. b) Protein caveolin binds to cholesterol, causing invagination of membrane. c) Detachment of pinocytotic vesicle from plasma membrane initiates vesicular trafficking to release Ca2+. d) Ca2+ is released into the cell, binds with Calmodulin, which binds with myosin light-chain kinase (MLCK)

which causes the phosphorylation of myosin light chain (instead of Troponin in skeletal muscle cells). e) Inactive myosin is converted to active myosin which binds to F-actin.

Smooth muscle vs. Skeletal muscle

- Both types hypertrophy. - Smooth muscle cells express hyperplasia (via same cell division as seen in uterus). - Smooth regenerate via satellite cells. - Smooth exhibit functional syncytium whereas skeletal express morphological. - Skeletal innervated by NMJ (synapses) whereas smooth is spontaneous (autonomic nervous system). - Skeletal have transverse tubules via triads whereas smooth do not (can only transport Ca+2 through caveolae).

Muscle Type Cell/Nucleus Contractile

Apparatus

Attachment of Myofibrils to Sarcolemma

Gap Jxns. Ca2+ Supply

Ca2+ Sensitivity Response

Inherent Ability

to Contract

Syncytium Hypertrophy

vs. Hyperplasia

Contraction Signal

Skeletal Muscle

(striated)

Long, cylinder, euchromatic,

nucleolus visible, multiple

nuclei under sarcolemma**

Actin filaments

insert on Z disk of

sarcomere; Nebulin and

Titin

Actin filaments attach to

dystrophin, dystroglycan complex, and

laminin-2

None

Triad at A-I Band

(1 T-Tubule, 2 SR Terminal

Cisternae)

Troponin in Actin

Filaments No Morphological

Hypertrophy, Regenerates via satellite

cells

NMJ Synapse

Smooth Muscle

Fusiform/ Single nucleus

at center, lighter, larger

Actin inserts into dense

bodies Fascia Adherens Nexus Extracellular via

Caveolae

Ca2+/Calmoudlin & Myosin

Light Chain Kinase

Yes Functional

Hypertrophy, Hyperplasia via same cell

division

Spontaneous Autonomic Innervation

Cardiac Muscle

(striated)

Branched/ single central

rectangle nucleus

Striated, Sarcomere, No Nebulin

Actin filaments attach to fascia

adherens in vertical place of

intercalated disks,

desmosomes

Inter-calated Disks

Diad at Z-Disk (SR &

Extracellular Fluid, 1 large T-

Tubule)

Troponin in Actin

Filaments Yes Functional

Hypertrophy, Can not

regenerate

Spontaneous, Contracts w/o nerve

stimuli; AP originate in pacemaker cells; Ca2+

induced Ca2+

activation

• Morphological Syncytium - cells are connected and the actions of one physically affects adjacent cells. • Functional Syncytium - cells maintain individual integrity but function as one unit due to free flow of ions b/t cells.

Connected electrophysiologically. • Extracellular Ca2+ induces Ca2+ activation in cardiac muscle. • Spontaneous contraction of smooth and cardiac muscle can be modified. • **Each nucleus in striated skeletal muscle is responsible for maintaining the protein molecules in a defined region of

adjacent cytoplasm.

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VASCULAR HISTOLOGY General structure of blood vessels (tunics or layers):

- Tunica intima lines lumen of vessel. 1) Endothelial layer continuous with endocardium. 2) Subendothelial layer (loose connective tissue, some smooth muscle) 3) External layer of elastic fibers, Internal elastic lamina (fenestrated=perforated)

- Tunica media comprised of smooth muscle cells and fenestrated sheets of elastic tissue 1) Smooth muscle layers concentrically arranged 2) Collagen & elastic fibers 3) Pericytes (capillaries & post-capillary venules)

- Tunica adventitia is the outer layer that contains loose connective tissue, collagen fibrils and nerves. 1) Adventitia of large vessels (arteries & veins) contains small vessels (vasa vasorum) that penetrate outer portion of

tunica media to supply oxygen and nutrients. Three main types of capillaries:

- Continuous (somatic) 1) Pinocytotic vesicles (skeletal muscle) 2) Blood brain barrier

- Fenestrated (visceral) 1) Diaphragms 2) No diaphragms

- Sinusoidal 1) Greatly enlarged 2) Found in liver & hematopoietic systems (bone marrow & spleen)

Capillaries (microvasculature)

- Single layer of highly permeable endothelial cells surrounded by basal lamina. - 7-9 mm in diameter (~ size of red blood cell) but thin enough for gas diffusion. - Will always have tunica intima. - Tunica media is reduced considerably, often absent.

1) Pericytes compose tunica media, contain myosin & tropomyosin; respond to injury. a) Serves in place of smooth muscle. b) Undifferentiated cells that resemble modified smooth muscle cells and are distributed at random intervals in

close contact with basal lamina. - Zonula occludens & desmosomes - Tunica adventitia is not well defined and often blends into surrounding connective tissue. - Continuous capillary

1) Endothelial cells have a complete (continuous) cytoplasm. 2) Found in brain, lung, musculature, thymus & bone. 3) Transport across endothelial cell involves caveolae is bidirectional (transcytosis).

- Fenestrated capillaries: 1) Endothelial cell has many fenestrae (pores) with or without diaphragm. 2) Basal lamina is continuous. 3) Found in tissues with substantial fluid transport like intestinal villi, choroids plexus, ciliary process of eye & kidney.

- Discontinuous or Sinusoidal capillaries 1) Gaps in discontinuous capillaries are much larger than fenestrated capillaries. 2) Found where intimate relation is needed between blood and parenchyma. 3) Basal lamina is discontinuous. 4) Liver & spleen are two main organs containing this.

Synthetic and metabolic activities:

- Prostacyclin (prostaglandin I2 [PGI2]) prevents blood coagulation. 1) Also important vasodilator.

- Plasminogen activator (procoagulant) is used against blood clots. - Interleukin I are pro-inflammatory cytokines. - Growth factors

1) Blood cell colony stimulating factors 2) Fibroblast growth factor 3) Platelet derived growth factor (PDGF)

- Angiotensin I to II conversion (lung)

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- Inactivates: 1) Norepinephrine 2) Thrombin 3) Prostaglandins 4) Bradykinin 5) Serotonin

- Antithrombogenic (PGI2) 1) Thrombomodulin

Collagen and Angiogenesis Source Effector Effect Collagen XVIII Endostatin Inhibits angiogenesis Collagen XV Restin (vascular basement membrane) Inhibits angiogenesis

Collagen IV NC 1 a2 Canstatin NC 1 a1 Arrestin NC 1 a3 Tumstatin

Inhibits angiogenesis

Antithrombin Cleaved form Inhibits angiogenesis Plasminogen Angiostatin Inhibits angiogenesis Arteries are characterized by size

- Arterioles are resistance vessels 1) 0.5 mm in diameter 2) Lacks internal or external elastic lamina (except largest) 3) Weibel-Palade granules in endothelial cells

a) von Willebrand’s factor (factor VIII) involved in blood coagulation. b) Lack of causes one form of hemophilia

4) Have from one to five layers of smooth muscle in tunica media. 5) Regulate distribution of blood to different capillary beds by vasoconstriction and vasodilation in localized regions

(determining systemic blood pressure). - Muscular arteries (medium vs. large); medium-sized arteries are distributing arteries.

1) Possess internal elastic lamina 2) Many layers of smooth muscle in media (more than 5 layers) 3) May have external lamina, adipose cells, lymphatics, vasa vasorum, and nerves depending on size.

- Large elastic arteries are conducting vessels. 1) Conducting arteries (aorta) are elastic arteries. 2) Distributing arteries (to most major organs) 3) Have two major characteristics:

a) Receive blood from heart under high pressure. b) Keep bloods circulating continuously while heart is pumping intermittently.

4) Pathology: Aneurysm caused by weakening of aortic wall. c) Fatty streaks (“foam cells”) are macrophages & smooth muscle filled with lipid deposits basis of

atherosclerosis that weaken aortic wall. d) Infarcts result in necrosis of surrounding tissue

6) In normal aging, one will deposit additional elastic membranes. a) Increased elastic tissue in tunica media resists deformation, leading to arteriosclerosis. b) Decreased stretch causes arteries to harden due to too much elastic c) Puts a workload on left ventricle; can lead to LVH (Left Ventricle Hypertrophy)

7) Elastic arteries are predominant vessels in which atheromas (atherosclerotic plaques) form. a) LDLs attach to endothelium and displaced to subendothelial space. b) Macrophages invade, take up lipid materials and become “foam” cells. c) Fibroblasts & smooth muscle cells invade space as well. d) Macrophages produce interleukin-1, tumor necrosis factor & growth factors to stimulate growth of smooth

muscle. e) Continued growth causes underlying media to atrophy. f) When atheroma ulcerates, will initiate thrombogenic event causing occlusion of vessel, infarction and/or sudden

cardiac death.

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Veins are characterized by size - Venules have well developed adventitia, thin media. - Small or medium veins have thin subendothelial layer. - Large veins have well-developed intima and thin media.

1) Have own vessels called vasa vasorum. a) Found in adventitia & outer part of media. b) Innervated by vasomotor nerves (unmyelinated sympathetic) to release norepinephrin for vasoconstriction. c) Has baroreceptors and chemoreceptors. d) Carotid sinus e) Arch of aorta

- Valves are two semilunar folds found in small or medium sized veins that prevent reflux. - Tunica adventitia is where longitudinally oriented smooth muscle bundles are arranged.

1) Hallmark of large elastic veins. 2) Help contract muscle and promote venous flow. Sitting too long allows blood to pool and can cause DVT (deep vein

thrombosis). Post Capillary Venules - First place where cells and fluid leave vasculature - Inflammation

1) Macrophages secrete interleukin-1, which induces endothelial cells to express E-selectin. 2) Binds to neutrophil receptor and allows cell to extravasate/ diapedese (mechanism for migration of lymphocyte cells

into tissues). - Rheology (flow characteristics of fluids) helps extravasate leukocytes.

1) Cells stay in middle of flow away from walls where serum undergoes shear forces against endothelium. 2) When leaves capillary bed and enters postcapillary venule, flow slows down aiding ability of leukocytes to come into

contact with wall of vessel and bind to E-selectin. Arteriovenous anastomoses control blood flow.

- Precapillary sphincter is found at meta-arteriole/capillary junction. 1) Can completely stop blood flow. 2) Blood bypasses microcirculatory bed, activates arteriovenous anastomoses, then shunted blood back into venous

drainage rather than in capillary beds. 3) Goal is for thermoregulation and to keep vital organs supplied.

- Glomera is a thick layer of concentrically arranged smooth muscle in arterioles. 1) Found in ears and fingernail beds.

Lymphatic capillaries are blind-ended structures which come together to dump contents into either thoracic duct or right lymphatic duct.

- Require anchoring fibrils which attach incomplete basal lamina to surrounding connective tissue, maintaining patency. - Collect lymph along with proteins like albumin and (in GI tract) chylomicrons, which will be processed by the liver. - Have numerous valves, contain only lymph, proteins, chylomicrons & white blood cells.

Capillary Type General Information Tissues Found In Endothelial Cell Characteristics

Continuous (Somatic)

Pinocytotic vesicles (skeletal muscle)

Blood-brain barrier

Brain, lung, musculature, thymus, and bone

Bidirectional transport via caveolae (transcytosis)

Continuous endothelial cell connected with tight junctions; continuous basal

lamina

Fenestrated (Visceral)

Diaphragms or no diaphragms

Intestinal villi, choroid plexus, ciliary process of eye & kidney

(tissues w/ substantial fluid transport)

Fenestrated endothelial cell with or w/o diaphragm; continuous basal lamina

Siunsoidal (Discontinuous)

Greatly enlarged (30-40µ) Liver and hematopoietic systems (bone marrow and

spleen)

Liver and spleen Large gaps in endothelial cell and fenestrated basal lamina

Capillary Microvasculature Rolled single endothelial cells Diameter is 7-9µ (~RBC) Zonula occludens & desmosomes Pericytes - Comprise tunica media - Contain myosin and Tropomyosin - Respond to injury (pericytes will proliferate to establish capillary beds)

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Tunica Intima Tunica Media Tunica Adventitia Endothelial layer

Subendothelial layer (loose CT, some smooth muscle) Internal elastic lamina (fenestrated; not always

present) Function: Decrease blood flow

Smooth muscle (concentric layers) Collagen & elastic fibers (external elastic lamina)

Pericytes (capillaries & post-capillary venules) Function: cause change in vessel diameter

Loose CT (mostly collagen Type III, elastin, & nerve) Dense Irregular CT (Webmic)

Function: Restricts how much a blood vessel can increase in diameter.

Function Definition Tunica Intima Tunica Media Tunica Adventitia

Large Elastic Arteries

2 Types: Conducting (aorta) Distributing (major

branches of aorta to supply major organs)

Receive blood from the heart under high pressure;

keep blood circulating; contain “foam cells”

Thicker (cushion blood)

No internal elastic lamina

Thickest layer Large amounts of elastic

membranes (~75) that contain fenestrae w/

collagen fibers, fibroblasts, & bundles of smooth muscle which set limit of expansion

by elastic fibers; site of aneurysm

Lots of collagen fibers; blood vessels, nerves,

lymphatics

Medium/Large Muscular Arteries

(named arteries)

Distributing (Ex. Coronary, brachial, and

renal lobar arteries) 2-10 mm

All 3 layers present; fenestrated internal

elastic lamina deep to tunica intima

6+ smooth muscle layers; decreased elastic fibers. Collagen, reticular fibers

present.

External elastic membrane at jxn. of t. media and t. adventitia,

adipose, lymphatics, nerves. ↑collagen, ↓ elastic

towards periphery Small Muscular

Arteries No external elastic membrane

6+ smooth muscle layers; decreased elastic fibers

Provides support, collagen fibers

Arterioles

Resistance (Have the greatest effect to BP)

0.5mm (15-100 microns) 1-5 layers of smooth muscle

in tunica media

All 3 layers present, Weibel-Palade granules

in endothelial cells; lacks internal elastic

lamina (except largest)

1-5 layers of smooth muscle Small amount of collagen fibers around perimeter

Capillaries

Exchange - The close distance of capillaries to

RBC ↓ the distant gas must fuse. Most numerous of all

blood vessels.

1 endothelial cell with basal lamina

(Range: 3-10 microns) (Average: 5-6 microns)

(RBC: 7.5 microns)

Reduced or absent, Pericytes instead of smooth muscle

Poorly defined, blends w/ surrounding CT

Large Elastic Veins

Ex. Vena Cava Longitudinal smooth

muscle responsible for preventing collapse of vena cava and assists in venous

return to heart.

Thick tunic intima Thin tunica media (smooth muscle and CT)

Longitudinally oriented smooth muscle bundles;

vasa vasorum

Small/Medium Veins Capacitance 2 semilunar folds

1-10 mm

Thin subendothelial layer and small amount

of CT

Circular smooth muscle cells separated by collagen Connective Tissue

Post Capillary Venules

Extravasation (diapedesis) of leukocytes & serum 30 microns

Thin t. media (No smooth muscle cells in

any of its walls). Well developed

Lymphatic Capillaries

Collect tissue fluid (lymph), proteins

(albumin), chylomicrons.; have only WBC

Blind ended structures; require anchoring fibers b/c attaching it to basal lamina and CT; numerous valves

Incomplete basal lamina

• Large Elastic Arteries - designed to store energy of systole and release during diastole to maintain forward blood flow

while heart is relaxing to fill b/t contractions. Smooth muscle cells produce the collagen fibers and also make the elastin and organize it into either elastic fibers or elastic membranes.

• Veins have a larger diameter. Arteries have a thicker wall. • To tell the difference b/t a post-capillary venule and lymphatic capillary: Post-capillary venule will usually have an RBC. • Smooth muscle cells in the tunica media are physically connected via adhering junctions called fascia adherens and

chemically connected by gap junctions. • Muscular arteries are the same except for size. Differ from large elastic arteries. • Vasa vasorum - small blood vessels that supply nutrients to outer tunica media and tunica adventitia. Especially

important for large elastic arteries, large arteries, and large veins. • The average RBC is 7.5 microns is greater than the average capillary (5-6 microns). This relationship, along with the

ability of the RBC to fold, allows for the RBC to be dragged slowly though the capillary to facilitate the exchange of CO2 with O2.

• The larger diameter of the post-capillary venules results in the slowing of blood flow and gives WBCs the opportunity to marginate the vessel wall and if stimulated by active endothelium, results in WBCs leaving the vessel and migrating into the tissues (diapedesis).

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HEART HISTOLOGY & THE CARDIAC MYOCYTE Three main layers of the heart:

- Epicardium has veins, nerve, ganglia and adipose tissue and covered by visceral mesothelium. - Myocardium is a functional syncytium of striated cardiac muscle fibers forming three major types of cardiac muscle: atrial,

ventricular, specialized excitatory and conductive. - Endocardium has veins, nerves and conducting cells (Purkinje fibers); also where fibroblasts come from. - Fibrous skeleton has three main components (septum membranaceum, annuli fibrosi, and trigona fibrosa).

Cardiac skeleton

- Is derived embryologically from endocardial cushion tissue via delamination of endocardial cells in AV canal (epithelial → mesenchymal transformation)

- Very dense CT - Provides physical support for valves. - Serves as anchoring point for all cardiac muscle fascicles. - Separates upper part of heart from lower part except for AV bundle which is how the top and bottom are able to

communicate. - Small muscle strands can span cardiac skeleton, short-circuiting the physiology of conduction (Wolff, Parkinson & White

[WPW] syndrome). Must be corrected by cardiac ablation. Myocardial Fascicles Have a helical configuration for deriving mechanical advantage during contraction (systole). To propagate electronic wave initiated at Sinoatrial Node, two components are needed:

- Specialized junctions between myocardial cells (intercalated discs). - Specialized cardiac tissue (Purkinje fibers) assembled into bundle of His and AV node.

Wall thickness is greatest in ventricles. - Cardiac muscle will hypertrophy in response to increased work load. - Common cause is hypertension, leading to left ventricular hypertrophy (LVH) which is measured by an ECHO.

Myocardium, fibrillar organization in cardiac myocyte and endomysium surrounding each muscle fiber all show high microvasculature.

- Necessary to meet high metabolic needs. - First branch of aorta is Coronary Arteries which supply blood to the heart.

Heart produces small secreted peptides to help regulate blood volume and pressure: - Atriopeptin, atrial natriuretic polypeptide (ANP), cardiodilatin, and cardionatrin. - Aid in fluid maintenance, electrolyte balance and decreasing blood pressure through actions on kidney.

1) ANP granules are distributed throughout myocyte. 2) Increase in body fluid causes atrium to stretch which causes a release of the granules into the cardiac interstitium thus

into the capillaries of the endomysium. 3) ANP travels to tell kidneys to excrete fluid. Body fluid decreases and causes a decrease in pressure in atrium thus

stopping the release of ANP. Lipofuscin Granules

- Represent materials that could not be handled by lysosomal (-ase enzyme) system. - Cardiac myocyte removes the water, packs material tightly and “sweeps it under the rug.” - Considered “aging granules” that can give indication of cell’s age (they build up over time).

Intercalated Disc

- Is compound cell junction comprised of 3 junctions which are distributed where cells meet end to end. - Due to branched nature of cardiac myocyte, can interact with several other cells and propagate wave of contraction more

efficiently. - Maintains cellular integrity but does have gap junctions to function as coordinated unit. Creates a functional syncytium. Vertical Plane – Tensional force is greatest Fascia adherens work like zonula adherens + terminal web but doesn’t encircle cell, but rather presents an area or fascia. Mimics a z-disc Anchoring point for last sarcomere of myofibril. Macula adherens (desmosomes) are anchoring points helping hold myocytes together during forceful contraction cycle (systole). Horizontal Plane – Electronic coupling of cardiocytes Gap junctions allow free flow of ions between myofibers, helping to propagate SA nodal impulse across atrium towards AV node.

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Myocardial Infarction - Myocytes die and are replaced by scar tissue created by proliferating fibroblasts of the endomysium. - Uncouples cardiac myocytes and interferes with normal SA nodal propagation. - Resulting tissue damage leads to release of lactic dehydrogenase (LDH-1) and creatine kinase MB from dead cardiocytes

which can be detected several days after the MI. - Creatine Kinase is composed of 2 dimers, M and B. - CK-MM isozyme predominates in skeletal muscle and heart. - CK-BB isozyme is present in brain, lung, and other tissues. - CK-MB is characteristic of myocardium and becomes elevated 3-4 hours after a MI. - Cardiocyte-Specific Troponin I (cTn1) - is specific to heart and an increase in

Cardiac myocytes have high level of consumption for glucose and need large amount of ATP.

- Alternation of row of myofibrils with row of mitochondria is common pattern. Cardiac myocytes differ from striated skeletal myofibrils:

- T-tubule complex is at Z line (disc) as opposed to A-I band in striated. - T tubule itself is larger in diameter in cardiocyte. - Instead of triad, only one sarcoplasmic reticulum is joined to a T-tubule (diad).

Other features of cardiac myocytes: - Ca+2 induced calcium activation. - No neuromuscular junction. - No nebulin surrounding thin filaments.

Cardiac Conduction System:

- SA Node in right atrium initiates a wave of depolarization, sweeping across atria by gap junctions in intercalated discs. - Wave ends when meets components of cardiac skeleton. - AV Node located above cardiac skeleton depolarize so that it can propagate to the ventricles via the Bundle of His. - First branches of the Bundle of His go directly to papillary muscles connected to valves of heart. - This occurs so tension is generated on valve leaflet just ahead of the contraction of ventricle, avoiding prolapse of valve into

upper chamber.

Purkinje Fibers - Are found in AV node, bundle of His and distributing fibers in ventricles. - PFs differentiate from myocytes that are recruited by adjacent arteries (e.g. coronary artery). - Thus, PFs are specialized cardiac myocytes, NOT neuronal tissue. - Much larger in diameter than normal cardiac myocytes, allowing for rapid conduction. - They function physiologically as nerves but they are embryologically derived from cardiac myocytes. They have myofibrils

and intercalated discs.

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CONNECTIVE TISSUE Ingredients of connective tissue are cells, fibers and ground substance.

- Fibers and ground substance produced by fibroblast cells. Components of connective tissue proper:

- Ground substance and fibers compose the matrix. - Fixed cells are stable ones always found in connective tissue.

1) Adipose tissue has many adipocytes and is a specialized connective tissue. Embryonic connective tissue (mesenchyme) has lots of ground substance.

- Contains abundant ECM rich in proteoglycans and hydrophilic substances. - Present in umbilical cord (wharton’s jelly) and in pulp of developing tooth.

Loose (areolar) adult connective tissue has the following embedded in connective tissue: - Elastic fibers are thing, straight and branching. - More cells than collagen fibers. - Collagen bundles are thick and wavy. - In intestinal villus, showing clock face pattern of heterchromatin in plasma cell nuclei. - In mammary gland, surrounds glandular epithelial components. - In dermis of skin, has fewer cells and more fibers.

Dense adult connective tissue has two forms: regular and irregular - Contains more collagen fibers than cells. - Dense irregular is found in dermis of skin, submucosa of digestive tube, and other sites.

1) Contain thick, wavy and irregularly arranged collagen fibers. 2) Fibroblasts are sparse, separating collagen bundles. 3) Reticular fibers and elastic fibers predominate.

- Dense regular is found in tendons and ligaments. 1) Regularly arranged collagen bundles separated by linear rows of fibrocytes.

Reticular tissue is tissue where reticular fibers predominate, forming stroma of organs of lymphoid-immune system, hematopoietic bone marrow, and liver.

- Characteristic of lymphatic tissues. - Reticular fibers (type III collagen – argyrophilic, meaning bind silver salts for staining) are synthesized by fibroblasts

(reticular cells) and are thin and branching structures. - Form meshwork where lymphoid cells embed and allow passage of cells and fluid.

Elastic tissue is connective tissue where elastic fibers predominate. - Characteristic of walls of large blood vessels and ligaments. - Synthesized by smooth muscle cells. - Form discontinuous lamellae or membranes in concentric arrangement around lumen, appearing as wavy pink bands, and

gives this tissue elasticity. Special types of connective tissues:

- Adipose tissue has thin rim of cytoplasm with nucleus pushed over to edge. 1) Unilocular adipocyte (white fat) have single large fat inclusion resulting from coalescence of multiple lipid droplets,

and pushes nucleus to eccentric position. a) Functions to store energy within the body.

2) Multilocular adipocyte (brown fat) are mitochondria-rich cells surrounded by abundant blood vessels. a) Dissipate energy instead of storing it. b) Generates heat by uncoupling production of ATP in the ETC from movement of H+ across inner mitochondrial

membrane down gradient. c) Achieved through uncoupling protein-1.

3) Mesenchymal cells give rise to preadipocytes that differentiate into brown adipose tissue and white adipose tissue. a) Insulin-like growth factor-1 [IGF-1] bound by insulin stimulates both adipogeneric pathways. b) Primary fat formation in fetus is brown fat.

- Cartilage and bone have specialized cells and ground substance. 1) Cartilage has noncalcified ECM, whereas ECM of bone is calcified.

- Hematopoietic tissue is found in bone marrow of certain bones Extra-Cellular Matrix (ECM): Proteoglycan Aggregate = Hylauronan (backbone) + core proteins + linker proteins + glycosaminoglycans Proteoglycan = Core protein + GAG

- Rich in hydroxyl, carboxyl and sulfate groups. 1) Become hydrated.

- Glycosaminoglycan molecules vary between tissues: 1) In connective tissue of skin is Dermatan Sulfate.

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2) In cartilage is Chondroitin 6-sulfate and Keratan Sulfate. 3) In bone is Chondroitin 4-sulfate.

- Bacteria that secrete hyaluronidase can break down hyaluronic acid backbone. 1) Streptococcus bacteria secrete so can spread infection. 2) Staphylococcus bacteria does not secrete so form localized areas of infection (boils).

Fibronectin assures adhesion of fibroblasts to collagen support. - Is a glycoprotein and has binding sites for cells and collagen.

Laminin is glycoprotein with binding sites for cells and collagen. - Located only in basal lamina.

Fixed cells of the connective tissue: - Fibroblasts synthesize and continuously secrete mature proteoglycans and glycoproteins and the precursor molecules of

various types of collagens and elastin. - Mast cell - Reticular cell - Pericyte - Adipocyte - Undifferentiated mesenchymal cells

Synthesis of collagen by fibroblasts: Cell types that make collagen: fibroblast, osteoblast, chondroblast, odontoblast

- Step 1: RER - Synthesis of preprocollagen and released into RER. Signal peptide is cleaved and forms triple helix of polypeptide chains as procollagen. Hydroxylation of lysine and proline occurs within RER and requires ascorbic acid (vitamin C) as cofactor. Glycosylation and disulfide bond formation within RER.

- Step 2: Packaging and secretion of procollagen from Golgi apparatus. - Step 3: Enzymatic removal of most of nonhelical domain of procollagen to form tropocollagen within extracellular

environment (procollagen peptidase). - Step 4: Self-aggregation in staggered array of tropocollagen to form collagen fibril (lysyl oxidase). - Step 5: Side-by-side cross-linking of collagen fibrils forms collagen fibers (mediated by FACIT collagen and

proteoglycans). The cross-links formed are responsible for the tensile strength of collagen fibers. Summary of Collagen Synthesis

Step 1: (RER) Triple helix procollagen made in cisterna of RER. Step 2: (Golgi) Packaging and secretion of procollagen from Golgi apparatus. Step 3: (Extracellular Space) Modification by procollagen peptidase done extracellularly to form tropocollagen. Step 4: Formation of collagen fibril. Banding of fibril due to staggered arrangement of tropocollagen. Step 5: Formation of collagen fiber (mediated by FACIT Collagen and Proteoglycans)

1) Type I, II and III collagen are all banded. a) Type I found in one, tendon, dentin and skin, providing tensile strength. (α1, α1, α2) b) Type II found in cartilage. c) Type III composes reticular fibers.

2) Staggered arrangement of tropocollagen molecules cause banding of collagen. Pathology of collagen:

- Ehlers-Danlos syndrome is caused by either defect in procollagen peptidase molecule (removes non-helical ends of procollagen molecule) to prevent conversion of procollagen to tropocollagen or gene mutated influenced defect in lysyl hydroxylase mediating the hydroxylation of lysine (cross-linking of tropocollagen to form collagen microfibrils). 1) Symptoms are hyperelasticity of skin and joint dislocation.

Synthesis of elastic fibers Cell types that make elastic fibers: fibroblasts (skin, tendons), chondroblast, chondrocyte (elastic cartilage), and smooth muscle cells (large blood vessels).

- Step 1: (RER) Synthesis of 3 elastic components of an elastic fiber: Proelastin (containing desmosine, isodesmosine), microfibril-associated glycoprotein (MGAP), Fibrillin 1, 2.

- Step 2: (Golgi) Packaging and secretion of proelastin. Proelastin is converted to tropoelastin. - Step 3: (Extracellular Space) Assembly of MGAP, Fibrillin 1 and 2, and tropoelastin (forms immature elastic fibers). Fibrillin 1: force bearing structural support Fibrillin 2: regulates assembly - Step 4: Immature aggregate to form mature elastic fibers - Note: Elastic cross-linking is minimal. Allows for stretchability. - Deficiency in Fibrillin 1 causes Marfan syndrome where patients are tall with long arms, legs, fingers and toes

(arachnodactyly). Mitral valve prolapse, dilation of root of aorta, and aortic dissection are common.

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Mobile cells of connective tissue: - Plasma cell

1) Antibody production; derived from differentiation of B lymphocytes (B-cells). 2) Have eccentric located nucleus, clockface pattern of heterochromatin, and basophilic cytoplasm. 3) Secrete products by constitutive method (proteins travel directly to membrane). 4) Each plasma cell makes an antibody to the specific antigen that was presented to it (monoclonal antibody). Each plasma

cell makes a different antibody based on the “chunk” of the bacteria that was presented to it so an arsenal of antibodies can be made (polyclonal antibodies).

a) Antigen taken up by macrophage and stored in phagocytic vesicle b) Phagocytic vesicle fuses with lysosome to become a phagosome. Lysosomal enzymes break down antigen into

small peptides which bind to MHC molecules inserted in the phagosome membrane. c) Phagosome fuses with the plasma membrane and the peptide-MHC is exposed to and binds to T cells. d) T cells secrete cytokines and interleukins which bind to adjacent B cells and induce B cells to differentiate into

plasma cells. e) Plasma cells produce and secrete specific Igs which bind to the free antigen in the extracellular space to

neutralize the damaging effects. - Lymphocytes for antibody production. - Neutrophils become increased in bacterial infection with tissue. - Eosinophils increase in parasitic infestations and after antigen-antibody events. - Macrophage cleans up small and large debris in tissues (e.g. spleen); increase in levels during chronic infections.

1) Derived from monocytes circulating in blood; escape through post-capillary venules. 2) Indented or irregular nucleus 3) Phagocytic vacuoles in cytoplasm 4) Lysosomes 5) Basophilic cytoplasm due to large amount of RER 6) Present antigens to immune cells (such as lymphocytes) in acquired immunity. (Antigen Presenting Cells)

a) Take up antigen stored within phagocytic vesicle. b) Lysosome fuses and antigen is broken down into small peptide fragments, which bind to major

histocompatibility complex (MHC). c) Antigen is presented on surface of membrane to T-lymphocyte.

Mast Cells and Anaphylactic Shock is the most severe type of anaphylaxis, occurs when an allergic response triggers a quick degranulation of mast cells (mediated by IgE) causing a release of large quantities of immunological mediators (histamines, prostaglandins, leukotrienes) leading to systemic vasodilation (associated with a sudden drop in blood pressure) and edema of bronchial mucosa (resulting in bronchoconstriction and difficulty breathing). Anaphylactic shock can lead to death in a matter of minutes if left untreated. - Mast cells arise from basophils in blood stream.

1) Have numerous granules in cytoplasm with synthesized products that act as vasoactive mediators: f) Allergen (antigen) binds to 2 adjacent IgE receptors on mast cell. g) Cytosolic Ca2+ is mobilized h) Granule mediators (histamine, tryptase), lipid mediators (leukotrienes, prostaglandins), and cytokines are

released. 2) Histamines dilate blood vessels so that blood flow increases and neutrophils can escape more easily from leaky

vessels to go into tissues to kill bacteria. 3) Proteoglycans contribute to packaging and storage of histamine and proteases. 4) Leukotrienes (not released in granules, but are released as metabolites of arachidonic acid) and cytokines help

recruit neutrophils and macrophages during infection. 5) Immunoglobulin E (IgE) is bound to mast cell membrane after initial allergic response, causing massive binding of

antigen to IgE molecules anchored and release of histamine to abnormally dilate large veins and resulting in anaphylactic shock.

6) Tryptase is unique marker of mast cells. 7) Connective tissue mast cells differ has more cytoplasmic granules than mucosal mast cells.

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CARTILAGE Three major types of cartilages all which are made by chondrocytes which derive from the mesenchyme:

1. Hyaline Cartilage (“glassy”) Avascular - Important b/c it limits the size of cartilage b/c diffusion becomes rate limiting. Perichondrium EXCEPT at the articular surface of joints - Can NOT repair!!! Perichondrium has outer

fibrous layer, an inner chondrogenic layer, and blood vessels. Type II collagen and proteoglycan aggregates make up the territorial and interterritorial matrix which

surround the chondrocytes. The extracellular matrix of hyaline cartilage has a dual role:

1. Shock absorber 2. Synovial lining of capsule of joint provides lubricated surface for movable joints.

Where you can find it: o Temporary skeleton of embryo o Articular cartilage in joints o Respiratory tract (nose, trachea, larynx, and bronchi) o Costal cartilages

2. Elastic Cartilage Avascular Perichondrium Type II collagen, proteoglycan aggregates, and elastic fibers (orcein stain) make up the territorial and

interterritorial matrix which surround the chondrocytes. Elastic fibers dominate Where you can find it:

o Pinna of ear o Eustachian tube o Epiglottis

3. Fibrocartilage Types I collagen No perichondrium - Can NOT repair!!!! Consists of chondrocytes and fibroblasts surrounded by type I collagen and less rigid ECM. Chondrocytes are aligned in rows along stress lines separated by type I collagen fibers. Considered intermediate between hyaline cartilage and dense fibrous tissue. Where you can find it:

o Intervertebral disk o Meniscus of knee o Aponeuroses o Pubic symphsis o Achilles tendon insertion and TMJ

Two bones articulating with each other form a joint.

1) Articular cartilage is at surfaces where bone actually articulate. 1) Underneath this is layer of compact bone. 2) Underneath that is spongy bone (core of bone) where hollow areas provide space for hematopoiesis. 2) If a bone has an Epiphyseal plate, you know it developed by endochondral ossification.

Chrondroblast.

1) Youngest cartilage cell 2) Synthesizes Type II collagen but not completely surrounded by it (not trapped). 3) Contain lipids, glycogen, well-developed RER, and Golgi.

Chondrocyte

1) Completely surrounded by cartilage in lacuna (trapped). 2) Type II collagen and other organic molecules form lacunae. 3) Territorial matrix is matrix surrounding chondrocytes which consists of randomly arranged Type II collagen

surrounded by GAGs. 4) RER well developed - where collagen synthesis occurs 5) Golgi well developed - where GAGs are assembled and sulfated; cisternae fill with secretory matrix 6) Anaerobic Glycolysis 7) Chondrocytes have significant nutritional requirements at all ages but cartilage is avsacular. So, its cells receive all

nutrients by diffusion through ECM.

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Interstitial growth 1) Process of making chondrocytes from pre-existing chondrocytes. Must occur inside a cartilage mass. 2) After cartilage cell divides, they share common lacunae, and form isogenous group. 3) Lacunar Rim - inner layer of territorial matrix (loose collagen fibrils). 4) Territorial matrix - at the periphery of the chondrocytes (mostly GAGs and Type II collagen) 5) Interterritorial matrix - lies outside territorial matrix (fine collagen fibrils). It takes up bulk of cartilage tissue and is

what continues to grow throughout life. Growth in size of cartilage is due to interterritorial matrix and it is slow. 6) As they secrete extracellular materials, move further apart. 7) Provides basis for lengthening of lone bones during endochondral ossification.

Appositional growth

8) Process of making chondroblasts from undifferentiated cells in the chondrogenic layer of the periochondrial edge. 9) Secrete type II collagen. 10) Adds new layers of cells and extracellular matrix to surface of cartilage. 11) Only hyaline and elastic cartilage have perichondrium. 12) Hyaline at articular surface of joints lacks a perichondrium. 13) Two layers include fibrous and chondrogenic, with the inner chondrogenic being the layer to produce chondroblasts. 14) Chondrocytes formed by appositional growth can later contribute to interstitial growth.

Mutation in gene expressing transcription factor Sox9 causes Campomelci Dysplasia

1) Sox 9 controls formation of Type II collagen and proteoglycan aggrecan (Effects Hyaline and Elastic Cartilage) 2) Causes bowing and angulation of long bones 3) Hypoplasia of pelvic and scapular bones 4) Abnormalities in vertebral column

Cartilage injuries result in formation of repair cartilage from perichondrium. Repair Cartilage:

1) Contains undifferentiated cells with potential to differentiate into chondrocytes that synthesize components of cartilage matrix.

2) Has matrix composition intermediate between hyaline and fibrous cartilage (contains both Type I and II collagen). 3) Note: The fact that hyaline articular cartilage and fibrocartilage both lack a perichondrium is the reason why when

you injure joints or knee, the do not heel well b/c the repair cartilage can only come from perichondrium which these structures, coincidentally, do not have.

Specialized extracellular matrix of hyaline cartilage has a dual role:

4) Acts like shock absorber. 5) Provides lubricated surface for movable joints. 6) Produced by synovial lining of capsule of joint. 7) Analysis of synovial fluid is valuable in diagnosis of joint disease.

Ground substance of cartilage and CT proper is proteoglycan aggregate. It is the “water of hydration.” CT proper is where sweat comes from. The dominant GAGs are in the following tissues:

1) CT of skin: Dermatan Sulfate 2) Cartilage: Chondroitin 6-Sulfate and Keratin Sulfate 3) Bone: Chondroitin 4-Sulfate 4) Sulfated molecules in cartilage and bones are what help give consistency to theses tissues (cartilage is like hard gel). 5) Bone is similar but eventually becomes mineralized with Ca2+ hydroxyapatite. 6) Bacteria that secrete hyaluronidase can break down the hyaluronic backbone in CT of skin.

o Streptococcus bacteria - secretes hyaluronidase and thus can spread infection. o Staphylococcus bacteria - does not secrete hyaluronidase thus can not spread infection but forms boils.

How chondrocytes survive:

8) In cartilage, chondroblasts and chondrocytes are sustained by diffusion of nutrients and metabolites through aqueous phase of ECM.

9) In bone, deposits of calcium salts in matrix prevents diffusion, thus must be transported from blood vessels to osteocytes through canaliculi.

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BONE Two types of bones based on gross appearance:

1) Compact bone appears as a solid mass. 2) Spongy or cancellous bone (spongiosa) consists of a network of bony spicules or trabeculae delimiting spaces

occupied by bone marrow. Two types of fine structure which makes up bone:

1) Primary (Woven) - All bone tissue begins as primary and is formed via either: a) Intramembraneous Ossification - formation of bone spicules with adjacent mesenchyme b) Endochondral Ossification

4) Secondary (Lamellar) - Nearly all bone is replace with secondary. General architecture of the long bone:

1) Epiphysis at end of long bone. 2) Metaphysis between epiphysis and diaphysis. 3) During growth, metaphysic and epiphysis are separated by the epiphyseal (growth) plate, which is responsible for

lengthening of long bone. a. In adults, defined as epiphyseal line. b. Endochondral ossification utilizes interstitial growth of cartilage to produce lengthening. c. Nutrient canal is “hole” in outer compact bone, and provides canal or passageway for blood vessels and lymphatic

vessels to reach core of bone. d. Epiphyseal plate and adjacent spongy bone represent the growth zone, responsible for increasing length of the

growing bone. 4) Diaphysis (shaft) bordered on either side by metaphysis. 5) Boundary between metaphysic and diaphysis is defined as level where spongy bone ends and marrow cavity begins.

a) Spongy bone provides room for hematopoietic tissue to produce blood cells in marrow, yet provides light but strong bone for support.

b) Does not form in haversian system. 6) Periosteum covers outer surface of bone, with exception of articular surfaces and tendon and ligament insertions, is

highly vascularized. Formed by two layers:

a. Outer layer contains abundant collagen fibers, blood vessels that penetrate Volkmann’s canals, and thick anchoring collagen fibers (Sharpey’s fibers), that penetrate outer circumferential lamellae deep in the bone.

b. Inner layer contains osteoprogenitor cells (osteogenic layer). 7) Endosteum consists of cells that belong to stroma of marrow or are derived from osteoblasts, main source of lining cells

of cortical haversian canals. 8) Compact bone located at periphery of long bone, except for articular surface and at tendon ligament insertions. 9) Concentric array of lamellar bone with osteocytes concentrically arranged between lamellae.

a) Bone has small canaliculi, within which are filipodial processes connecting to nearby osteocytes by gap junctions, providing nutrient flow from Haversian canal to osteocytes farthest away from canal.

10) Haversian canals are located within lamellar bone found within compact or mature bones and contain blood vessels, nerves, lymphatics, and endosteum (with osteoblasts and osteoprogenitor cells). a. Are connected with one another by transverse or oblique canals known as Volkmann’s canals, containing blood

vessels from marrow and some from periosteum. Two types of bones based on microscopic organization of ECM:

1) Lamellar bone, typical of mature or compact bone. a) Consists of lamellae, largely composed of bone matrix (mineralized substance deposited in layers or lamellae),

and osteocytes, each occupying a cavity or lacuna with radiating and branching canaliculi that penetrate lamellae of adjacent lacunae.

b) Displays four distinct patterns: o Osteons or haversian systems are formed by concentrically arranged lamellae around longitudinal

vascular channel. o Interstitial lamellae observed between osteons and separated from them by thin layer known as

cement line. o Outer circumferential lamellae, visualized at external surface of compact bone under periosteum. o Inner circumferential lamellae seen on internal surface subjacent to endosteum.

2) Woven bone, observed in developing bone.

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Osteoprogenitor Cells 1) Give rise to osteoblasts Osteoblasts

1) Osteoblasts are linearly arranged to form an osteoblast monolayer but are polarized (synthesis side facing bone they are laying down).

2) They synthesize the organic matrix of bone called the osteoid (Type 1 collagen and proteoglycans) which is deposited in the form of bands or lamellae. Eventually, osteoblasts become trapped in the osteoid to become osteocytes.

3) Osteoblasts also control the mineralization of the matrix. 4) Remain in communication via gap junctions b/t filopodia processes within canaliculi to transport nutrients (max 100

um). 5) Have abundant RER for protein synthesis. 6) Has alkaline phosphatase on cell surface (ectoenzyme) that hydrolyzes monophosphate esters at high pH, but

disappears when cell ceases protein synthesis and becomes embedded in mineralized bone to become osteocyte. 7) Major protein products are:

a) Type I collagen - tough fibers Noncollagenous Proteins b) RANK Ligand - Ligand for receptor activation of nuclear factor kappa B (RANK) that is present in osteoclast

precursor cells. c) Osteocalcin required for bone mineralization. d) Osteopontin to mediate formation of sealing zone. e) Bone sailoprotein to mediate binding of osteoblasts to extracellular matrix through integrins.

8) Can tell from chondrocytes because chondrocytes does not usually line up next to each other like epithelial cells. 9) Osteomyelitis is inflammation of bone caused by Staphylococcus aureus bacteria attaching to bone sialoprotein.

Osteocytes 1) Eventually osteoblasts are trapped within osteoid (in lacunae) and become osteocytes (most mature or terminally

differentiated cells) when matrix is calcified. 2) Responsible for maintenance and turnover of bone matrix, thus cytoplasm is full of RER. 3) Growth and remodeling of bone is dynamic process. 4) Older Haversian systems may be partially or completely eaten away by osteoclasts.

Osteoclast is highly polarized cell associated with shallow concavity. 1) Has lots of SA 2) Actin filaments used to form a sealing zone and confined space. 3) In Howship’s lacunae, osteoclasts use acids to dissolve mineral salts leaving behind the osteoid. 4) Active surface facing lacuna has ruffled border. 5) Actin filaments at ruffled border where plasma membrane is applied close to the bone forms sealing zone, together with

integrin and osteopontin. 6) Formed at clear zone (lacks cell protrusions) to help prevent acidic conditions from leaking to other regions. 7) Multinucleated cells and contain abundant mitochondria. 8) Bicarbonate-chloride exchanger ensures maintenance of cytoplasmic pH neutrality. 9) Chloride channel prevents excessive rise of intracellular pH. 10) Carbonic anhydrase II generates protons (H+) from CO2 and H2O. 11) H+ released into Howship’s lacuna by ATP-dependent pump to create acidic environment for solubilizing mineralized

bone. 12) Lysosomal and nonlysosomal enzymes are released into Howship’s lacuna to degrade collagen and noncollagen proteins.

Osteoblasts regulate differentiation of osteoclasts:

13) Monocyte, derived from bone marrow, reaches area of bone formation and remodeling. 1) Receptor for macrophage colony-stimulating factor (M-CSF) is expressed. 14) Monocyte becomes macrophage when M-CSF ligand (synthesized from osteoblast) binds to receptor and induces

expression of RANK. 15) RANKL (ligand) expressed on surface of osteoblast binds with RANK on macrophage and establishes required

osteoblast-osteoclast precursor contact. 16) Mononucleated macrophage becomes multinucleated immature osteoclast (which cannot reabsorb bone). 17) Osteoblast secretes osteoprotegerin, glycoprotein which binds RANKL with greater affinity with does RANK receptor

thus preventing the completion of differentiation of osteoclast precursors into functional osteoclasts. 18) Parathyroid hormone blocks synthesis of osteoprotegerin. 19) Prevents completing differentiation of osteoclast precursors into functional osteoclasts, thus regulating population of

functional osteoclasts. 20) Resting (nonfunctional) osteoclast uncouples from osteoblast. 21) Maturation of osteoclasts is completed when sealing zone and ruffled border appear.

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Osteoporosis is the loss of bone mass leading to bone fragility and susceptibility to fractures: 1) Major factor is deficiency of estrogen leads to increased number/activity of osteoclasts. 2) Bone reabsorption exceeds amount of new bone that is formed. 3) Two major changes in vertebrae are:

a. Wedged upper vertebrae b. Crushed lower vertebrae

4) Three major changes in architecture of spongy bone lead to weak bones: thinner, fewer spicules (trabeculae) and bigger holes in framework.

5) Prevention is estrogen replacement therapy. 6) Side effects include breast cancer and endometrial cancer. 7) Alternative therapy includes alendronate (fosamax), which inhibits osteoclast-mediated bone resorption.

BONE DEVELOPMENT AND REMODELING Two processes of bone formation (osteogenesis) observed in embryo are: 1) Intramembranous Ossification

- Bone tissue is laid down directly in primitive connective tissue or mesenchyme. Requires a vascularized area. - Bone is added in appositional manner to thicken up spicules or trabeculae. - Surrounding tissue is mesenchyme. Contains Type I collagen. - Membrane bones (flat bones of skull, etc.) develop this way. Steps:

a) Mesenchymal cells aggregate without a cartilage intermediate in a process controlled by patterning signals. b) Mesenchymal cells differentiate into osteoblasts and form into a bone blastema. c) Osteoblasts line the surface of the blastema and osteocytes are within the core interconnected by cell processes

forming a functional syncytium. d) Osteoblasts deposit bone matrix (osteoid). e) Blood vessels transport serum Ca2+ which is used in mineralization process to form trabeculae (spicules).

which become the primary ossification center. f) Trabeculae are randomly oriented woven bone forming an irregular network called primary spongiosa. g) Note: Although primary bone tissue formation begins as interstitial process, it soon becomes appositional. h) Osteocytes become trapped within the calcified osteoid. At the surface of the osteoid, osteoblasts continue the

appositional growth of the matrix, mainly by adding Type I collagen and noncollagenous proteins. i) The mesenchymal cells located near the periosteal surface condense to form periosteum. j) The continued deposition of bone on trabeculae forms compact bone. k) In other areas, the thickening does not occur and the CT in the intratrabecular space differentiates into

hematopoietic tissue. l) If trabeculae get too big, osteoblasts will form Haversian Canals from the outside in which allow for the

entrance of osteoprogenitor cells which can develop into osteoblasts.

2) Endochondral Ossification - Bone tissue replaces preexisting hyaline cartilage, template for future bones. - Bones of extremities, vertebral column and pelvis derive from this.

a. Hyaline cartilage with Type II collagen is the template of a long bone. b. Primary ossification center is located in the diaphysis c. Osteoprogenitor cells of the perichondrium form the periosteal collar (bone collar) by way of

intramembraneous ossification. d. The formation of the periosteal collar induces the formation of blood vessels forming the periosteal bud, which

enter through the periosteal collar and branch in opposite directions. e. Secondary ossification centers at the epiphyses. f. Epiphyseal cartilaginous growth plate b/t the metaphysic and the epiphysis will eventually be replace by

bone, forming the epiphyseal line (thick bone). g. Indian hedgehog (Ihh) stimulates chondrocyte proliferation in the growth plate and delays chondrocyte

hypertrophy. h. All the epiphyseal cartilage is replace by bone, except for the articular surface. i. No further growth in length of bone is possible once it disappears at puberty.

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Zones of Endochondral Ossification - Erosion of hyaline cartilage template occurs at the primary ossification center.

- Four major zones can be distinguished, from end of cartilage and to zone of erosion: 1) Reserve Zone

a) Is composed of primitive hyaline cartilage and is responsible for growth in length of bone as erosion and bone deposition process advances mediated by the osteoclasts.

b) Chondrocytes are “running” as osteoclast-mediated erosion “chases”. 2) Proliferative Zone

a) Flattened chondrocytes in columns or stacks parallel to the growth axis. b) Characterized by active proliferation of chondrocytes aligning as cellular columns. c) Dilated cisterna of RER contain matrix proteins. d) Chondrocytes are separated by territorial matrix. e) Chondron - is functional unit of growth. Is a cluster of cells with its territorial matrix.

3) Hypertrophic Zone a) Chondrocytes enlarge (hypertrophy) caused by fluid influx into the cell which causes the septa to appear

thinner. b) Longitudinal and transverse septa becomes calcified which prevents nutrients from getting to the

chondrocytes and causes them to die (apoptosis). 4) Vascular Invasion Zone

a) Blood vessels penetrate transverse septa forming vascular spaces with blood (lacunae) and carrying migrating osteoprogenitor cells and cells that can develop into osteoclasts.

b) Osteoprogenitor cells give rise to osteoblasts that initiate deposition of hydroxyapatite and type I collagen fibers on the calcified cartilage septa at the vascular invasion zone forming osteoid. Osteoclasts also help by removing residual chondrocytes and matrix.

c) Results in formation of bone spicules and, later, trabeculae; thus cancellous bone (woven bone) appears in mid-section of template.

After endochondral ossification, general organization of long bone is remodeled by combined reabsorption mediated by osteoclasts and deposition of new bone.

Growth in Width of Diaphysis

1) As bone grows in length, new layers of bone added to outer portions of diaphysis by appositional growth. 2) Erosion of inner wall results in enlargement of marrow cavity. 3) New bone in form of haversian systems added beneath periosteum (containing blood vessels) by osteogenic layer. 4) Ridges and grooves are lined by osteoblasts that proliferate and deposit osteoid.

a. Ridges grow toward one another and enclose periosteal vessel within a tunnel. b. Adjacent periosteal capillaries within tunnels are connected by transverse blood vessels (Volkmann’s canals, not

surrounded by concentric lamellae). 5) Osteoblasts lining tunnel deposit new lamellae and convert into haversian system, a central blood vessel surrounded by

lamellae. 6) Appositional growth adds lamellae under periosteum in cortical region of diaphysis, becoming outer circumferential

lamellae. a. Osteoclasts erode bone at outer circumferential lamellae-osteon boundary, allowing interstitial lamellae to fill space

between osteon and what remains of outer circumferential lamellae. 7) Osteoblasts lining inner surface develop inner circumferential lamellae in similar fashion, but blood vessels enclosed

in tunnels are branches of nutrient artery formed originally from periosteal bud. Osteopetrosis “stone-like bone characterized by abnormal osteoclast function.

1) Bone is abnormally brittle and breaks like soft stone. 2) Marrow canal is not developed and most bone is woven due to absent remodeling. 3) Can be caused by gene mutation in:

a. Osteoclast proton pump - 50-60% of cases b. Osteoclast chloride channel (ClCN7) – 15% of cases

4) Bone marrow transplantation is treatment but very risky. Rickets (in children) and osteomalacia (in adults) are characterized by defect in mineralization of bone matrix (osteoid), most often caused by lack of vitamin D3.

1) Produces skeletal deformities. Scurvy is caused by vitamin C deficiency, causing improper collagen synthesis and weak bones.

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Fibrodysplasia ossificans progressive (FOP) is inherited disorder of connective tissue. 1) Main features are skeletal malformations and ossification of soft tissues. 2) Ectopic ossifications observed as lumps in muscles of neck and back. 3) Lymphocytes in sites of injury synthesize excess bone morphogenetic protein-1, contributing to development of

skeleton in normal embryo. Joint capsule consists of dense connective tissue with vessels, lined by synovial membrane.

4) Continuous with periosteum and is attached to edges of articular cartilage. a. Articular cartilage is hyaline cartilage not lined by synovial membrane.

5) Synovial membrane is layer of vascular connective tissue covered by synovial cells. a) No basal lamina. b) Capillaries are fenestrated.

6) Synovial fluid is capillary ultrafiltrate that contains mucin produced by synovial cells. Rheumatoid arthritis is chronic inflammatory disease of joints.

7) Activated CD4+ T cells stimulate production of proinflammatory cytokines such as tumor necrosis factor- (TNF- ), interleukin-2 (IL-2) and interleukin-6 (IL-6) to induce proliferation of synovial cells.

8) Synovial cells release collagenase, extracellular matrix metalloproteases, prostaglandins and nitric oxide to destroy articular cartilage and subjacent bone.

1) Synovial membrane becomes thickened by proliferation of synovial lining cells. 2) T and B cells and plasma cells infiltrate connective tissue of synovial membrane. 9) Neutralization of proinflammatory effectors in treatment include: 1) Blockade of cytokine receptor by antagonist or an antibody to cytokine receptor. 2) Soluble cytokine receptor (etanercept) or antibody targeting the cytokine (infliximab). 3) Anti-inflammatory cytokines prevent expression of proinflammatory effectors.