Chapter 7 - Bone Structure & Function

Anatomy & Physiology (McGraw Hill)

regions of a long bone

• diaphysis - shaft

  • provides leverage and support for long bone

  • compact bone on the outside, spongy bone on the inside

• medullary cavity - hollow cylindrical space within diaphysis

  • contains red (children) or yellow (adult) bone marrow

• epiphysis - expanded knobby region

  • thin layer of compact bone outside, extensive spongy bone inside

• articular cartilage - hyaline cartilage that covers joint surface of epiphysis

  • reduce friction, absorbs shock

• metaphysis - region of long bone where diaphysis widens into epiphysis

  • transfers force

  • epiphyseal plate (children) - thin layer of hyaline cartilage where bone length originates

  • epiphyseal line (adults) - thin, defined area of compact bone (remnants of epiphyseal plate)

periosteum

tough sheath that covers outer surface of bone (except areas covers by hyaline cartilage)

• outer layer - fibrous, dense irregular connective tissue

  • protects bone

  • anchors blood vessels and nerves to bone surface

  • attachment site for ligaments and tendons

• inner layer - osteoprogenitor cells and osteoblasts

  • bone width - both cells produce circumferential layers of bone matrix

• perforating fibers - anchor periosteum to bone

endosteum

thin layer of connective tissue that lines internal surface of bone within medullary cavity

• osteoprogenitor

• osteoblasts

• reticular fibers

diploe

region of spongy bone between two layers of compact bone

• flat bones

• no medullary cavity

bone marrow

soft connective tissue of bone

• structure

  • cells - developing red blood cells and/or adipocytes

  • reticular connective tissue

• location - spongy bone in most bones, medullary cavity in long bones

• red bone marrow - primarily developing red blood cells

  • widely distributed in children bones

  • selected portions of axial skeleton in adults

• yellow bone marrow - primarily adipocytes

  • developing red blood cells decrease in adulthood, adipocytes increase

  • may convert back to rede bone marrow if red blood cells are needed

osteoprogenitor cells

bone stem cells derived from mesenchyme

• function - divide to produce mature osteoblasts

• location - periosteum and endosteum

osteoblasts

matured osteoprogenitor cells

• structure - cuboidal, abundant rough ERs and Golgi apparati

• function - synthesize and secrete osteoid (bone matrix)

  • calcification - osteoid hardens due to calcium salt deposition

  • mature into osteocytes

osteocytes

matured bone cells derived osteoblasts

• structure - contain cytoplasmic projections to maintain connection with neighboring osteocytes

• function

  • maintain bone matrix

  • detect mechanical stress on bone

• location - entrapped in calcified osteiod

  • lose ability to form new bone

osteoclasts

phagocytic cells derived from fused bone marrow cells

• structure

  • large, multinuclear

  • ruffled border on bone surface increases surface area exposure to bone

• function - bone resorption

• location - resorption lacuna (pit on bone surface)

Osteitis deformans

disruption in balance between osteoclast and osteoblast function

• excessive osteoclast activity - excessive bone resoprtion

  • oseoclasts are 5x larger than normal

  • contain 20+ nuclei (normal is 3-5 nuclei)

• excessive osteoblast activity - excessive bone deposition

  • new bone is poorly formed and unstable

  • susceptible to deformation and fractures

extracellular matrix of bone connective tissue

• osteoid (organic)

  • collagen proteins

  • semisolid ground substance - suspend and support collagen

    • contains proteoglycans, glycoproteins

• crystal deposits (inorganic)

  • hydroxyapatite Ca10(PO4)6(OH)2

    • calcium phosphate crystals (CA3(PO4)2) + calcium hydroxide

  • other salts (i.e. calcium carbonate) and ions (i.e sodium, magnesium)

  • deposit around long axis of collagen fibers

calcification

bone formation; osteiod mineralizes/hardens

• concentration of calcium and phosphate ions increase and precipitate out of solution

  • forms hydroxyapatite crystals that deposit around collagen fibers

• vitamin D - enhances calcium absorption in GI tract

• vitamin C - involved in collagen formation

bone resorption

bone matrix is destroyed by substances released by osteoclasts

• proteolytic enzymes - chemically digest organic components (collagen, proteoglycans, glycoproteins)

• hydrochloric acid - dissolves inorganic mineral (crystals)

  • osteolysis - release of calcium and phosphate from bone matrix

• occurs during bone remodeling, low blood calcium levels

osteons

cylindrical structures in compact bone, basic structural unit

• central canal - cylindrical channel in center of osteon

  • houses nerves and blood vessels

• concentric lamellae - rings of bone connective tissue surrounding central canal

  • collagen fibers oriented in alternating patterns of direction, increases bone strength

• osteocytes - mature bone cells in lacunae between lamellae

• canaliculi - small interconnecting channels that connect adjacent lacunae

  • allow for communication between osteocytes

• perforating canals - run perpendicular to central canals

  • connect multiple central canals within different osteons

  • contain nerves and blood vessels

• circumferential lamellae - rings of bone that surround osteoblasts groups and extend entire circumference of bone

  • external - underneath periosteum

  • internal - over endosteum

• interstitial lamellae - compact bone between osteons

  • leftover parts of osteons that have been partially resorbed

  • incomplete, no central canal

spongy bone anatomy

contais no osteons

• trabeculae - open latticework of rods and plates of bone

  • space filled with bone marrow

• parallel lamellae

• lacunae - microscopic spaces between parallel lamellae

  • house osteocytes

• canaliculi - microscopic channels that connects lacunae

cartilage growth

begins prior to birth during embryologic development

• interstitial growth - grows in length

  • declines rapidly as cartilage matures

• appositional growth - grows in width

intramembranous ossification

formation of bone tissue on thin layer of mesenchyme; flat bones, facial bones

1. ossification centers form in thickened regions of mesenchyme (8 weeks development)

   • mesenchymal cells differentiate into osteoprogenitor cells

   • osteoprogenitor cells become osteoblasts, begin to secrete osteoid

2. osteoid undergoes calcification

   • osteoblasts become entrapped, stop producing osteion and mature into osteocytes

3. woven bone (immature bone connective tissue) formed

   • mesenchyme surrounding woven bone thickens to form periosteum

   • mesenchymal cells produce additional osteoblasts

4. lamellar bone (mature bone) replaces woven bone

   • trabecular spaces filled with compact bone

endochondral ossification (gestation to childhood)

formation of bone in cartilage; most bones of skeleton

1. chondroblasts secrete cartilage matrix, forms hyaline cartilage (8-12 weeks development)

   • chondroblasts trapped within lacunae mature into chondrocytes

   • perichondrium surrounds cartilage

2. cartilage calcifies, periosteal bone collar forms

   • chondrocytes enlarge and resorb surrounding cartilage matrix, producing large holes in the matrix

   • cartilage matrix calcifies, chondrocytes die

   • blood vessels grow toward cartilage and penetrate perichondrium

   • stem cells in perichondrium develop into osteoblasts

   • periochiondrium becomes periosteum

   • osteoblasts under periosteum secrete osteiod, which hardens to form periosteal bone collar

3. primary ossification centers form in diaphysis (12 weeks development)

   • growth of capillaries and osteoblasts extends from periosteum into core of cartilage shaft

   • primary ossification center - osteoblasts secrete osteiod in center of cartilage shaft

   • extends in both directions toward epiphyses

   • healthy bone connective tissue replaces calcified, degenerating cartilage in shaft

4. secondary ossification centers form in epiphysises

endochondral ossification (childhood to adulthood)

1. bone replaces almost all cartilage, except for articular cartilage and epiphyseal plates

2. bones continue to grow in length until epiphyseal plates ossify into epiphyseal lines (adulthood)

interstitial bone growth

dependent on growth of cartilage in zones of epiphyseal plate

• zone of resting cartilage

  • farthest away from medullary cavity of diaphysis, nearest to epiphysis

  • small chondrocytes in cartilage matrix (resembles hyaline cartilage)

  • secures epiphysis to epiphyseal plate

• zone of proliferating cartilage

  • chondrocytes undergo rapid mitosis

  • align within lacunae into longitudinal columns parallel to diaphysis

• zone of hypertrophic cartilage

  • chondrocytes stop dividing, enlarge greatly in size (hypertrophy)

  • walls of lacunae become thin as chondrocytes resorb matrix

• zone of calcified cartilage

  • 2-3 layers of chondrocytes

  • minerals deposited in matrix between columns of lacunae

  • calcification limits nutrients, causes chondrocyte destruction

  • chondrocyte death gives matrix opaque appearance

• zone of ossification

  • walls between lacunae break down, forming longitudinal channels

  • channels invaded by capillaries and osteoprogenitor cells from medullary cavity

  • osteoprogenitor cells develop into osteoblasts, which deposit new bone matrix on remaining calcified cartilage

appositional bone growth

growth in bone width within periosteum

• osteoblasts in inner cellular layer of periosteum produce/deposit bone matrix in external circumferential lamellae (tree rings)

• as bone becomes wider, osteoclasts in medullary cavity resorb bone matrix

bone remodeling

continual addition of new bone tissue (bone deposition) and removal of old bone tissue (bone resorption)

• occurs at different rates, depending on bone

• compact bone remodels slower than spongy bone

achondroplasia dwarfism

abnormal conversion of hyaline cartilage to bone

• long bones of limbs stop growing in childhood

• bones of head and neck grow as expected

hormones that promote bone growth

• growth hormone - released from anterior pituitary

  • stimulates liver to produce insulin-like growth factors (IGFs)

  • IGFs cause cartilage proliferation at epiphyseal plate

• thyroid hormone - secreted from thyroid gland

  • stimulates metabolic rate of osteoblasts

• calcitonin - promotes calcium deposition in bone, inhibits osteoclast activity

  • primarily in children, minimal effects in adulthood

• sex hormones - stimulate osteoblasts

  • promote cartilage and bone growth at epiphyseal plate

  • also signals closure of epiphyseal plate

hormones that inhibit bone growth

• parathyroid hormone - increases blood calcium levels

  • stimulate bone resorption by osteoclasts

• calcitriol - increases blood calcium levels

  • stimulates bone resorption by osteoclasts

  • stimulates absorption of calcium ions from small intestine into blood

• glucocorticoids - steroid hormones released from adrenal cortex

  • increases bone loss in when present high amounts

  • impair growth at epiphyseal plate in children

• serotonin - prevents osteoprogenitor cells from differentiating into osteoblasts

enzymatic pathway of calcitriol (vitamin D) activation

1. UV light converts precursor molecules (in keratinocytes of skin) to cholecalciferol (vitamin D3) OR cholecalciferol (vitamin D3) from meal is absorbed from small intestine

   • cholecalciferol (vitamin D3) enters bloodstream

2. cholecalciferol (Vitamin D3) passes through liver

   • converted to calcidiol

3. Calcidiol passes through kidneys

   • covered to calcitriol hormone (active vitamin D)

effects of parathyroid hormone and calcitriol on blood calcium levels

1. blood calcium levels decrease (stimulus)

2. receptors of parathyroid gland detect drop in calcium levels

3. parathyroid gland release parathyroid hormone (PTH)

   • increases conversion of rate of calcidiol to calcitriol (vitamin D hormone)

4. PTH and calcitriol (vitamin D hormone) effect major organs

   • bone - increase osteoclast activity (bone resorption) to release of calcium from bone into blood

   • kidneys - triggers retention of calcium

   • small intestine - increases absorption of calcium into blood (calcitriol only)

5. net effect - more calcium in blood

   • removal of calcium from bone

   • increase of calcium resorption in kidneys

   • increase of calcium absorption from GI tract

6. Negative feedback - as blood calcium levels increase, PTH hormone secretion decreases

calcitonin

hormone that aids in regulating blood calcium in children

• released from parafollicular cells in thyroid gland in response to high blood calcium levels or exercise stress

• lows blood calcium levels

  • inhibits osteoclast activity

  • stimulates increase of calcium loss in urine

bone aging

• bone strength decreases

  • osteoblasts produce less osteiod

• bones become brittle

  • relative about of inorganic molecules (crystals salts) increases as protein decreases

• bones become thinner

  • osteopenia - insufficient ossification as osteoblast activity declines

stress fracture

thin break in bone caused by physical activity

pathological fracture

fracture in bone that is weakened by disease

simple fracture

broken bone does not penetrate skin

compound fracture

one or both ends of broken bone pierce overlying skin

comminuted fracture

bone is splintered into several small pieces

transverse fracture

fracture is perpendicular to long axis of bone

spiral fracture

fracture spirals around axis of long bone, as a result of twisting stress

linear fracture

fracture is parallel to long axis of bone

avulsion fracture

bone fragment pulled off from rest of bone by soft tissue attachment

colles fracture

fracture of distal end of radius

complete fracture

bone is broken in 2 or more places

compression fracture

bone is squashed

depressed fracture

broken part of bone forms concavity (i.e. skull fracture)

displaced fracture

fractured bone parts are out of anatomical alignment

epiphyseal fracture

epiphysis is separated from diaphysis at epiphyseal plate

greenstick fracture

partial fracture

• one side of bone breaks, other side is bent

• more common in children

hairline fracture

fine crack in which sections for bone remain aligned

impacted fracture

one fragment of bone is driven into another

incomplete fracture

partial fracture extends only partway across bone

pott fracture

fracture at distal ends of fibula and fibula

bone repair

1. fracture hematoma forms as blood vessels inside bone are torn

2. fibrocartilaginous (soft) callus forms

   • blood capillaries infiltrate fracture hematoma

   • fracture hematoma reorganizes into growing connective tissue (procallus)

   • fibroblasts produce collagen fibers to connect broken ends of bones

   • chondroblasts form fibrocartilage and collagen fibers

   • procallus becomes soft callus

3. bony (hard) callus forms

   • osteoprogenitor cells in soft callus become osteoblasts

   • osteoblasts produce trabeculae of primary bone to replace soft callus

   • bony callus continues to grow and thicken for several months

4. bone is remodeled

   • osteoclasts remove excess bony material

   • compact bone replaced primary bone