Osseous Tissue and Bone Structure

6.1I. Intro to the skeletal system

A. includes the bones, cartilages, ligamentsB. Functions: five primary

1. support- provide framework for muscle attachment 2. Stores minerals & lipids

-bones store calcium and phosphate ions -store lipids in yellow marrow

3. produce blood cells- red marrow produces red and white blood cells4. protection

-protect soft tissues and organs- ribs protect heart and lungs

5. leverage-can change direction and amount of

force produced by muscles- allow for precise motion

6.2II. Bone classifications

A. Shapes-typical adult skeleton has 206 bones- divided into 6 shape based categories

1. long bones- long and slender- found in arm, forearm, thigh, leg,

fingers & toes -femur is the largest &heaviest bone in the body 2. flat bones- thin parallel surfaces

- form the roof of the skull, sternum, ribs-protect underlying soft tissues - attachment site for muscles

3. sutural bones- (Wormian bones)- small, flat, irregular shaped- in skull- jigsaw borders

4. irregular bones-complex shapes-short, flat, ridged surfaces- spinal vertebrae, some skull bones

5. short bones-small & boxy-wrist & ankle bones

6. sesamoid bones (shaped like sesame seed)- generally small, flat-develop inside tendons -located near joints-kneecap

B. Bone markings PAGE 187 -surface features

-grouped as projections, depressions, openings- can provide info like age, sex, size

1. elevations/projections-form where tendons and ligaments

attach -where bones join (joints)

2. depressions/grooves-form sites where blood vessels or nerves

lie along surface or go through bone

C. Bone structure1. parts of long bone

a. diaphysis- shaft, made of compact boneb. metaphyses- area between shaft and end

(epiphyses)c. epiphyses- expanded area at end of shaftd. medullary cavity- space in shaft for red or

yellow bone marrow2. bone tissue

a. compact tissue -relatively solid, dense -forms protective layer around medullary

cavityb. spongy bone (cancellous) -open network, porous -covered by thin covering of compact bone

(cortex)

6.3III. Bone Matrix and Cell types

Osseus tissue is a supporting connective tissue

Why? –specialized cells-matrix of extracellular protein

fibers & ground substance A. bone matrix

1. bone weight 2/3 from calcium phosphate 1/3 from collagen fibers 2% of mass contributed by cells

2. Calcium phosphate interacts w/ calcium hydroxide-forms hydroxyapatite crystals

3. Calcium crystals are hard but brittle-can tolerate compression-can’t tolerate twisting or sudden impact

4. Collagen fibers are very strong -provide framework for crystals to form on-are flexible, can bend-don’t resist compression

5. fibers &crystal combination allows bone to be strong, somewhat flexible, resistant to shattering

B. Cells in bone1. osteocytes-mature bone cells

- make up most of cell population-each occupies a lacuna, a pocket in layers

of matrix(lamellae)- can’t divide- rely on canaliculi (passageways through

lamellae) to connect lacunae w/ one another & nutrient source

- Functions: *create chemicals that dissolve matrix

& rebuild the matrix, stimulating deposition of crystals*can convert to other cell types in case

of bone injury (released from lacunae)

2. osteoblasts- produce new bone (process of osteogenesis)- make & release proteins for matrix- organic component of matrix is called osteoid- elevate calcium phosphate levels to trigger the deposit of salts- turn into osteocytes once surrounded by matrix3. osteoprogenitor cells- stem cells-divide to produce cells that become osteoblasts- maintain osteoblast populations- important in repairing fractures- located on outer surface of bone (periosteum) & inner layer lining medullary cavity (endosteum)

4. osteoclasts- remove recycle bone matrix- giant cells- 50+ nuclei-not related to other bone cells, but

come from stem cells that produce macrophages

-secrete enzymes that dissolve matrix and release stored minerals (osteolysis)

-helps to regulate calcium & phosphate concentrations in body fluids

5. balance bwtn osteoblasts & osteoclasts is important in maintaining bone health---too much osteoclast activity will weaken bones

6.4 IV. Structure of compact & spongy bone

A. Compact bone1. osteon- basic unit of mature compact bone a. osteocytes are arranged around the

central canalb. central canal (Haversian canal)-

parallel canal carries blood to/from osteonc. perforating canals- perpendicular

canals, supply blood to -osteons deeper in the bone -medullary cavity tissues

2. thickest where stress comes from limited direction

- shaft of long bone

B. Spongy bone1. lamellae are not arranged in osteons2. matrix forms struts and plates (trabeculae)- open network

a. no capillaries – nutrients reach cells by diffusion along canaliculi on surface of trabeculae3. located where

- bones aren’t heavily stressed -stressed from many directions

4. lighter than compact bone-reduces weight of skeleton-easier for muscles to move bones

5. framework supports and protects cells of bone marrow

a. spongy bone contains red bone marrow-responsible for blood cell formation- in epiphysis of long bones (femur)- interior of large bones (sternum)

b. spongy bone contains yellow bone marrow

- adipose tissue -energy reserve

C. Periosteum1. covers superficial layer of compact bone (except at joint)

2. functions:- isolate bone from surrounding tissues- provide route for circulatory & nervous

supply- participates in bone growth & repair

3. near joints, is continuous w/ connective tissues that lock bones together

- fibers of periosteum interweave w/ collagen fibers of tendon, which become cemented into lamellae as bone grows

- provides strong attachment, strong pull on tendon or ligament usually breaks bone rather than fibers of tendon or ligament

D. Endosteum- layer that lines medullary cavity

1. covers trabeculae of spongy bones, lines surfaces of central canals

2. made of layer of osteoprogenitor cells,

- not a complete layer- some matrix is exposed- osteoclasts & osteoblasts remove

& deposit matrix at exposed sites

6.5 Bone formation & enlargementV. Skeletal Growth

A. Determines1. body size & proportiona. begins 6 weeks after fertilizationb. skeleton is cartilaginous at this pointc. growths until age 25 +/-B. Ossification- process of replacing tissues w/ bone1. calcification- depositing of salts, occurs during ossification2. Types of ossification:a. endochondral- bone replaces cartilage

b. intramembranous ossifications- bone develops directly from tissue (mesenchyme or connective tissue)

C. Endochondral ossfication (cartilage)1. most bones come from hyaline cartilages that are smaller models of the bones they will be2. cartilage humerus or femur in embryo continues to grow through

a. interstitial growth- expansion of cartilage

b. appositional growth- production of new cartilage at outer surface

3. cartilage is gradually replaced by bone at the metaphysis, increasing bone length4. Former line of epiphyseal cartilage is often seen in x-rays of adults (epiphyseal line)

D. Intramembranous Ossification1. osteoblasts differentiate w/in a mesenchymal or fibrous connective tissue2. occurs in deeper layers of the dermis

a. results in dermal bones- flat bones of skull, lower jaw, & collarbone

E. Blood & nerve supply to bone1. bones need blood supply for growth & maintenance

a. osseous tissue is highly vascular2. three major sets of vessels develop:

a. nutrient artery & vein- supply the diaphysisb. metaphyseal vessels-supply inner surface of epiphyseal cartilagec. periosteal vessels- provide blood to

superficial osteons of shaft3. sensory nerves penetrate cortex w/ nutrient artery

a. innervate: endosteum, medullary cavity, epiphysesb. injury to bones is very painful due to

sensory innervation

6.6VI. Bone formation & resorption balance

A. remodeling- bone matrix is recycled & renewed1. replaces matrix

a. bone is unchangedor

b. bone changes shape, internal structure, or mineral content

2. older deposits removed & released into circulation

-new circulating materials are absorbed &deposited

3. balance btwn osteocytes, osteoblasts, & osteoclasts

4. bone turnover rate is high ( 1/5 of skeleton recycled & replaced annually in young adults)

a. spongy bone replaced several times/year, compact bone is mostly unchanged

b. metals can be incorporated into bone matrix (similar to calcium)- later release of metal ions; potentially dangerous

c. excess removal= weak, brittle bones

6.7VII. Effects of exercise, hormones, & nutrition on bone

developmentA. Exercise

1. remodeling of bone allows bone to adapt to new stress

a. minerals in stressed bone generate tiny electric fieldsb. osteoblasts are attracted to area and

begin to produce bone-stressed bone grows

2. shapes of bone reflect forces applied to thema. surface markings (bumps, ridges) indicate tendon attachment

b. bumps become larger to withstand increased forces (increase in muscle power)c. stressed bone- thicker & stronger vs weak and brittle bones of couch potato

3. skeleton can change rapidly- bone loss in casted leg

a. can lose up to 1/3 of mass in a few weeks!

b. can rebuild quickly w/ normal weight bearing activity

B. Hormone and Nutritional effects: *Normal growth/maintenance depend on several

factors1. constant dietary source of calcium & phosphate2. hormone calcitriol is needed for Ca absorption (synthesized from vitamin D)3. vitamin C- needed to stimulate osteoblast differentiation (lack of can result in scury=low bone mass & strength)4. Growth hormone & thyroxine stimulate bone growth5. differences in sex hormones account for variations in body size & proportion of men & women- trigger epiphyseal closure

6.8IIX. Calcium’s Role in Bone function

A. skeleton as calcium reserve1. calcium -most abundant mineral in

body (99% in skeleton)2. calcium ions -important to activities of

*muscle cells (esp. cardiac muscle cells)

* neuron cells3. too much/little calcium results in

unresponsive/over excitable neurons & muscle cells

B. Calcium Homeostasis- regulates Ca concentration1. maintained by hormones2. when Ca ion levels become low:

*parathyroid hormone (PTH) is released into blood stream

a. has 3 effects:i). Stimulates osteoclast activity-

release Caii). Increases rate of intestinal

absorption iii.) decreases rate of excretion at

kidneys* calcium ions enter body fluids, levels

return to normal

3. when levels are above normal* calcitonin is releaseda. 2 functions

i). Stops osteoclast activityii). Increases rate of excretion at

kidneys* calcium ions leave fluids as osteoblasts

produce bone matrix & calcium is excreted at kidneys

6.9IX. Fractures

* Crack/break in bone *bone is impacted suddenly, subjected to

extreme load, or struck from unusual directionA. Repair process

1. step 1: blood vessels are broken, bleeding occurs

a. clot (fracture hematoma) formsb. disruption of blood flow kills

osteocytes, resulting in dead bone around break

2. step 2: cells of endosteum & periosteum undergo rapid cell division

a. daughter cells migrate to fracture zoneb. external callus- collar of cartilage & bone forms around bone at fracturec. internal callus- forms w/in medullary cavityd. at edges of callus, cells differentiate into osteoblasts to begin building a bridge btwn bone fragments on either side of break

3. Step 3: osteoblast replace cartilage of external callus w/ spongy bonea. external & internal calluses form a brace a fracture- cast removed at this stage

4. Step 4: osteoclasts & osteoblasts continue to remodel

area (4 months-1 year)- eventually replace callus w/ living bone

6.10

X. Effects of AgingA. Osteopenia

1. inadequate ossification- everyone becomes slightly osteopenic w/ age

2. reduction in bone mass begins btwn age 30-40

3.osteoblast activity declines, osteoclast activity continues at same level

4. women lose 8% skeletal mass/ decade, men 3%

a. more mass lost at epiphyses, vertebrae, & jaws (results= fragile limbs, tooth loss, height loss)

5. osteoporosis- reduction in bone mass is enough to impact bone function

a. Bones are very porous and prone fractures

b. affects 29% women & 18% of men over 45

c. can develop as a secondary effect of many cancers (bone marrow and breast cancers release an osteoclast activating factor)