Histology of the Connective Tissue

By

Safaa El Bialy (MD, PhD)

Objectives

• List the structural and functional characteristics of connective tissue and distinguish it from other basic tissue types.

• Describe the functions carried out by different types of connective tissue.

• Describe the three fundamental components found in all connective tissues.

• Describe the biochemical composition and the sites of synthesis of the extracellular matrix components.

• Define the structure and function of the connective tissue cell.

Embryonic Origin (FYI)

All connective tissue cell types are derived from embryonic mesenchyme (mesoderm)

Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a loose aggregate of collagen fibrils and unspecialized cells

Connective Tissue

• Is one of the 4 basic tissues of the body. • CT forms a framework, connecting, supporting and packing tissues of

the body.• It also plays a dynamic role in the development, growth, defense and

homeostasis of other tissue types.

• Structurally it is made up of :1. Cells2. Extracellular matrix * Fibers * Ground substance: Viscous material of glycoproteins and proteoglycans

Types of cells in connective tissue

1. Residents:•Fibroblasts •Reticular cells (type of fibroblast that synthesizes type III collagen)•Mesenchymal cells •Fat cells 2. Visitants:•Macrophages•Mast cells•Plasma cells•Leukocytes•Melanocytes

Fibroblasts

• Fibroblasts • Most common cells in CT• Spindle shaped with elongated

nuclei• Synthesize most components of

connective tissue (Extra-cellular matrix, proteins etc)

• Involved in wound healing• Adipocytes• Also called “Fat Cells”• Nucleus pressed against the cell

membrane “Signet ring”• Functions:

1. energy reserves,

2. insulation,

3. protection and support

Macrophages• Macrophages (Histiocytes)• Almost as numerous as fibroblasts• Most abundant in richly vascularized

areas• Derived from monocytes in bone marrow

and mature to macrophages in CT• Important agent of defense.• secrete :

1. enzymes

2. two proteins of complement system,

3. an important antiviral agent, interferon• Langerhan’s cells in epidermis, kupffer

cells in liver, microglial cells in CNS and osteoclasts in bone all derive from monocytes and are macrophages like cells).

Mast cells• Mast means well fed. Their

cytoplasm is full of coarse granules

• Spherical, central nucleus• Metachromatic granules• Secrete:• Histamine: increase vascular

permeability and smooth muscle contraction

• Heparin: anticoagulant• Tend to occur in small groups

around blood vessels (particularly in dermis, digestive and respiratory tracts)

PLASMA CELLS

• Resemble to lymphocytes• Basophilic cytoplasm due to rough

ER• Eccentric nucleus• Cartwheel appearance• Cytoplasm contains a clear, rounded

area (site of centrosphere and Golgi apparatus

• Frequently found in serous membranes and lymphoid tissue and plentiful in sites of chronic inflammation

• Principle function is the production of antibodies which are synthesized in RER

• Rare in most connective tissues

Leucocytes

•Wandering cells

•leukocytes that migrate from the blood vessels by diapedesis

•This process increases greatly during inflammation

•Neutrophils

•Eosinophils

•Basophils

•Lymphocytes

•Monocytes

•macrophages

Extracellular Matrix• Extracellular matrix (Fibers & Ground substance) is synthesized and

secreted mainly by the fibroblasts • Fibers

Prime function is a support and a strengthening role in1. Fibrous capsule of organs2. Penetrating trabeculae of organs3. Dermis of skin4. Ligaments & tendons5. Cartilage & bone

• Ground substance 1. Acts as a molecular sieve & stops the spread of noxious substances 2. Cellular nutrition & waste removal 3. Plays a vital role in aging. Its amount diminishes with age and wrinkles start appearing.

Fibrous Components

• Connective tissue fibers are long, slender protein polymers that are present in variable proportions in different types of connective tissue.

• In many cases the predominant fiber type is responsible for conferring specific properties on the tissue.

• We have three types of fibers:• Collagen Fibers• Reticular Fibers • Elastic Fibers

Collagen• Collagen is the most abundant

protein in human body forming 30% of its dry weight

• Collagen Producing Cells:• 1. Fibroblast: More than one type

of collagen• 2. Chondroblast: Type II collagen• 3. Osteoblast:Type I• 4. Reticular cell:Type III• 5. Smooth muscle:Type I & III

Synthesis of Collagen (FYI)

Mother cell

Procollagen (Triple-helical units)

Procollagen peptidase

Collagen fibril

Collagen fiber

Extracellular

Intracellular

Reticular Fibers

• These fibers look very similar to collagen but are thinner (0.5-2 um).

• More highly glycosylated.• Form delicate silver-staining

network instead of thick bundles.• Composed mainly of type III

collagen and glycoprotein.• Reticular lamina of basement

membrane (Slide # 6 in epithelial tissues) and around adipocytes, smooth muscle (slide # 29 in muslce tissue) and nerve fibers.

• Reticular fibers constitute a network around the parenchymal cells of various organs (liver, endocrine glands)

Lymph NodeReticular cells and fibers

Elastic Fibers (Connective tissue)

Elastic fibers consist of an amorphous protein called elastin and numerous protein microfibrils embedded in it.

Diameter range 0.1-10um.

Add resilience to connective tissue

Collagen fibers

Elastic fibers

Elastic Fibers

• Elastic fibers are collected in thick, wavy, parallel bundles & separated by loose collagenous tissue with fibroblasts.• Ground substance is sparse.• Elastic connective tissue provides flexible support.•ligamentum flava of the vertebral •column•suspensory ligament of the penis•elastic lamina of blood vessels.

Elastic laminae

Wall of Aorta

Medical application (FYI)

• Marfan syndrome:• Lack of fibrillin which is necessary for the deposition of elastic fibers

• Lack of resistance in tissues rich in elastic fibers

• Aortic aneurysm is common

Ground SubstanceGlycosoaminoglycans • GAGs are polysacharides that

contain aminosugars.• Hyaluronic acid is the largest GAG.Proteoglycans:• They are made up of a core protein to

which glycosoaminoglycans (GAGs) are attached.

Glycoproteins: • Fibronectin-mediates the attachment

of cells to the extracellular matrix.• Laminin-a component of basal

laminae that mediates the attachment of epithelial cells.

Tissue fluids:• plasma proteins of low molecular

weight• Salts:

Proteoglycans Glycoproteins

Connective Tissue Types

1. Embryonic connective tissue

2. Adult connective tissue:

A. Connective Tissue Proper:

1. Loose connective tissue

2. Dense connective tissue

a) Dense regular connective tissue

b) Dense irregular connective tissue

B. Reticular connective tissue

C. Elastic connective tissue

D. Mucous connective tissue

Connective tissue

Embryonic Adult

Mesenchyme Mucous Loose (areolar)

DenseSpecialized

Irregular RegularReticular Adipose Elastic

Mesenchyme

• Mesenchyme is embryonic connective tissue.

• Its stellate and fusiform cells (mesenchymal cells) are derived from mesoderm.

• loosely associated cells and are surrounded by a large extracellular matrix

• They give rise to all the connective tissue of the body.

• These are multipotential cells and persist in adults to give rise to new generations of connective tissue cells especially during wound healing, bone repair and tissue fibrosis

Mucous Connective Tissue

• Resembles embryonic mesenchyme and is rarely found in adults

• Found mainly in the umbilical cord and nucleus pulposus of the intervertebral discs

• This tissue has very few cells (fibroblasts) & fibers.

• Lots of ground substance (mainly hyaluronic acid)

• This tissue yields readily to pressure and returns to its original shape, so it is useful for protecting underlying structures from excess pressure.

Loose Connective Tissue

• Very common type• Support structures under some

pressure and low friction• Fibroblasts and macrophages

are predominant• Collagen, elastic and reticular

fibers are present• Supports epithelial tissues,

small blood vessels and lymphatics

• Fills spaces between muscle and nerve fibers

• Papillary layer of dermis and hypodermis

Dense Connective Tissue

• Same components as loose CT• Fewer cells with predominance of

collagen fibers over ground substance

• Less flexible and more resistant• Collagen fibers arranged in bundles

without a definite orientation → dense irregular CT (dermis, wall of GIT, organ capsules)

• Often found with loose CT• Collagen fibers arranged according

to a definite pattern → Dense regular CT (tendons and ligaments)

Loose connective tissue

Dense connective tissue

Connective Tissue

Fibroblasts

Extracellular matrix

Mammary Glands

Epithelial tissue

Lesser amount of collagen fibers

Larger amount of collagen fibers

Special Types of Connective Tissues

• Adipose tissue• Blood & lymph• Cartilage• Bone

Adipose tissue

• Specialized type of CT• Adipocytes or fat cells

predominate• 15-20% of male body weight

and 20-25 % of female body weight

• Largest repository of energy• Key regulator of body’s

energy metabolism

Adipose tissue

Cartilage

Perichondrium

Chondroblasts

Chondrocytes

Lacuna

Cartilage matrix

Isogenous group

of chondrocytes