Chapter 7

Phosphate

- ATP utilization - plasma membrane

Nerves that Supply Bone

- accompany blood vessels through foramen - innervate bone, periosteum, endosteum, and marrow cavity - mainly sensory nerves

Yellow Bone Marrow

- Product of red bone marrow degeneration as children mature - fatty substance - may convert back to red bone marrow during severe anemia - facilitates production of additional erythrocytes

Activation of Vitamin D to Calcitrol

- UV light concerts 7-dehydrocholesterol to Vitamin D3: cholecalciferol; released into blood; absorbed from small intestine from diet - Vitamin D3 circulates throughout the body: converted to calcidiol by liver enzymes; adds an -OH group - calcidiol circulates in the blood: converted to calcidiol by kidney enzymes; an -OH group added; active form of Vitamin D3: calcitriol; parathyroid hormone increases rate, so more calcitriol is formed - calcitriol stimulates absorption of calcium ions from small intestine into the blood

PTH and Calcitriol Interactions

- act synergistically to increase release of calcium from the bone into the blood; increase osteoclast activity - stimulate the kidney to excrete less calcium in urine; increasing calcium reabsorption in the kidney tubules - only calcitriol increases absorption of calcium from small intestine into the blood

Calcitonin

- aids in regulating blood calcium levels - less significant role than PTH or calcitriol - released from the thyroid gland in response to high blood calcium levels - secreted in response to exercise - inhibits osteoclast activity - stimulates kidneys to increase loss of calcium in the urine to reduce blood calcium levels - greatest effect during greatest bone turnover (ex: growing children) - therapeutic injections: not able to provide long-term decrease in blood calcium - entire function still unclear

Hyaline Cartilage

- attaches ribs to sternum - covers ends of some bones - cartilage within growth plates - provides model for formation of most bones in body

Osteons

- basic functional and structural unit of mature compact bone - oriented parallel to bone diaphysis - appears as bull's-eye target - composed of central canal, concentric lamellae, osteocytes, canaliculi

Process of Cartilage Growth

- begins during embryonic development - grows in length during interstitial growth: occurs within regions of cartilage - growth in width by appositional growth: occurs on cartilage's outside edge

Bone Growth and Remodeling

- begins during embryonic development - interstitial growth depends upon cartilage growth in epiphyseal plate - 5 zones of epiphyseal plate: nearest epiphysis, small chondrocytes distributed throughout matrix, resembles mature hyaline cartilage, secures epiphysis to epiphyseal plate

Endochonral Ossification

- begins with a hyaline cartilage model - produces most bones of skeleton, including bones of upper and lower limbs, pelvis, vertebrae, ends of clavicle - ex: long bone development

Bone Formation

- begins with secretion of osteoid - calcification occurs, deposition of hydroxyapatite crystals (calcium and phosphate ions precipitate out, form crystals - requires Vitamin D (enhances calcium absorption from GI tract, Vitamin C (required for collagen formation, calcium and phosphate (for calcification)

Hemopoiesis

- blood cell production - occurs in red bone marrow connective tissue - stem cells form blood cells and platelets

Intramembranous Ossification

- bone growth within a membrane - dermal ossification - produces: flat bones of skull, some facial bones, mandible, central part of clavicle - begins when mesenchyme thickens with capillaries

Blood Supply

- bone highly vascularized in regions of spongy bone - vessels enter from periosteum - nutrient foramen: small opening or hole in bone; artery entrance and vein exit here

Red Bone Marrow Transplant

- bone marrow destroyed by radiation / chemo - abnormally functioning marrow - harvested cells injected into bloodstream of recipient - migrate to normal locations for red bone marrow - must be a "match" between donor and recipient so immune system doesn't attack

Parallel Lamellae

- bone matrix - osteocytes between lamellae - canaliculi radiate from lacunae

Bone Resorption

- bone matrix is destroyed by substances released from osteoclasts - proteolytic enzymes released from lysosomes within osteoclasts (chemically digest organic matrix components) - calcium and phosphate dissolved by hydrochloric acid - occurs when blood calcium levels are low

Fractures

- breaks in bone - result of unusual stress or sudden impact - increased incidence with age due to normal thinning and weakening of bone

Components of Skeletal System

- cartilage - ligaments - other connective tissue

Hyaline Cartilage

- cells scattered through matrix of protein fibers - embedded in a gel-like ground substance (includes proteoglycans but not calcium) - resistant and flexible - high percentage of water - highly compressible and a good shock absorber - avascular and contains no nerves - includes chondroblasts, chondrocytes, perichondrium

Achondroplasia

- characterized by abnormal conversion of hyaline cartilage to bone - most common: achondroplastic dwarfism: long bones of limbs stop growing in childhood; other bones continue normal growth - short in stature but large head - failure of chondrocytes in epiphyseal plate to grow and enlarge - inadequate endochondral ossification

Chondrocytes

- chondroblasts encased within the matrix - occupy small spaces: lacunae - maintain the matrix

Zone of Hypertrophic Cartilage (of Epiphyseal Plate)

- chondrocytes cease dividing - begin to hypertrophy - walls of lacunae become thin

Zone of Proliferating Cartilage (of Epiphyseal Plate)

- chondrocytes undergo rapid mitotic division - align into longitudinal columns of flattened lacunae - columns parallel to diaphysis

Steps of Interstitial Growth

- chondrocytes within lacuna are stimulated to mitotically divide - two cells, chondroblasts, occupy a single lacuna - chondroblasts secrete new material and are pushed apart; each is now called a chondrocyte - cartilage continues to grow internally

Interstitial Lamellae

- components of compact bone between osteons or partially resorbed osteons - look like they have a "bite" taken out - incomplete: no central canal - not a part of osteon; part of long bone

Zone of Calcified Cartilage (of Epiphyseal Plate)

- composed of 2 to 3 layers of chondrocytes - minerals are deposited between columns of lacunae - destroys chondrocytes

Bone Remodeling

- continues throughout adulthood - occurs at different rates: distal part of femur replaced every 4-6 months; diaphysis of femur not completely replaced over a lifetime - 20% of skeleton replaced yearly - dependent upon the coordinated activities of osteoblasts, osteocytes, osteoclasts - influenced by hormones and mechanical stress

Endosteum

- covers all internal surfaces of bone within medullary cavity - incomplete layer of cells - contains osteoprogenitor cells, osteoblasts, osteoclasts

Articular Cartilage

- covers the joint surface - thin layer of hyaline cartilage - reduces friction - absorbs shock in moveable joints

Central Canal

- cylindrical channel at center of osteon and parallel to it - blood vessels and nerves extend through channel

Aging Bone

- decreased tensile strength of bone: reduced rate of protein synthesis by osteoblasts; relative amount of inorganic material increases; become brittle and susceptible to fracture - bone loss of calcium and other minerals: bones thinner and weaker; insufficient ossification: osteopenia

Perichondrium

- dense regular connective tissue - covers cartilage and helps maintain its shape

Regions of Long Bone

- diaphysis - medullary cavity - epiphysis - metaphysis - epiphyseal plate

Rickets

- disease caused by Vitamin D deficiency in childhood - characterized by deficient calcification of osteoid tissue - bowlegged appearance - disturbances in growth, hypocalcemia, tetany (cramps and twitches) - occurs in some developing nations - incidence increasing in urban US children

Steps of Cartilage Growth

- during early embryonic development: interstitial and appositional growth occur simultaneously - as cartilage matures: interstitial growth declines rapidly, cartilage becomes semirigid, future growth primarily appositonal - after cartilage is fully mature: new cartilage growth stops, growth occurs only after injury

Irregular Bones

- elaborate, sometimes complex shapes - do not fit any other category - vertebrae, ossa coxae (hip bones), several bones in skull (ethmoid, sphenoid, sutureal bones)

Diaphysis

- elongated, usually cylindrical shaft - provides leverage and weight support - compact bone with thin spicules of spongy bone extending inward

Sex Hormones

- estrogen and testosterone - secreted in large amounts at puberty: dramatically accelerate bone growth - increases rate of cartilage growth and bone formation in epiphyseal plate - bone formation rate greater than cartilage growth: eventually cartilage replaced with bone (growth stops)

Steps of Long Bone Development (w/ Endochondral Ossification)

- fetal hyaline cartilage model develops: chondroblasts secrete cartilage matrix during 8th to 12th week of development - cartilage calcifies, a periosteal bone collar forms: chondrocytes in the cartilage model produce holes in the matrix; matrix calcifies and chondrocytes die (produces calcified cartilage shaft with large holes); blood vessels grow toward cartilage; osteoblasts develop and secrete osteoid (form a layer of osteoid around calcified cartilage shaft and periosteal bone formed) - primary ossification center forms in diaphysis: periosteal bud extends from periosteum into cartilage shaft (growth of capillaries and osteoblasts); osteoids produce osteoid on calcified cartilage template: primary ossification center of bone formation, most formed by 12th week of development - bone development extends in both directions towards epiphyses - bone connective tissue displaces calcified, degenerating cartilage - secondary ossification centers form in epiphyses: hyaline cartilage calcifies and degenerates; blood vessels and osteoprogenitor cells enter; secondary ossification centers form (bone displaces cartilage, not all form at birth); osteoclasts resorb some bone matrix (creates hollow medullary cavity) - bone replaces cartilage, except articular cartilage and epiphyseal plates - epiphyseal plates ossify and form epiphyseal lines: lengthwise bone growth continues into puberty; growth continues until epiphyseal plate is converted into epiphyseal line (indicates bone has reached adult length); occurs between ages 10-25

Flat Bones

- flat, thin surfaces, may be slightly curved - provide surfaces for muscle attachment - protect underlying soft tissues - located in roof of skull scapulae, sternum, ribs

Ossification / Osteogenesis

- formation and development of bone connective tissue - begins in the embryo - continues through childhood and adolescence - by 8th through 12th weeks of embryonic development: skeleton begins forming from intramembranous ossification or endochondral ossification

Steps of Bone Fracture Repair

- fracture hematoma forms from clotted blood: blood vessels torn within periosteum - fibrocartilaginous callus forms: regenerated blood capillaries infiltrate hematoma; fracture hematoma reorganized into a connective tissue procallus; fibroblasts produce collagen fibers; chondroblasts form dense regular connective tissue; procallus becomes fibrocartilaginous (soft) callus - hard bony callus forms: osteoblasts adjacent to callus produce trabeculae; replaces callus; forms a hard (bony) callus; continues to grow and thicken - bone is remodeled: final phase of fracture repair; osteoclasts remove excess bony material; compact bone replaces primary bone; usually leaves a slight thickening of bone

Osteoblasts

- from osteoprogenitor stem cells - side-by-side on bone surfaces - cuboidal shape - abundant rough ER and Golgi apparatus - synthesize and secrete osteoid - become entrapped within the matrix - differentiate into osteocytes

Long Bones

- greater in length than width - elongated, cylindrical shaft (diaphysis) - most common bone shape - found in upper and lower limbs - vary in size

Glucocorticoids

- group of steroid hormones - released from adrenal cortex - regulate blood glucose level - high amounts increase bone loss: impairs growth at epiphyseal plate in children; must monitor if child receiving high doses of glucocorticoids (ex: asthma)

Red Bone Marrow

- hemopoietic: blood cell forming - reticular connective tissue, immature blood cells, fat - in children: located in spongy bone and medullary cavity of long bones - in adults: located only in selected areas of axial skeleton (skull, vertebrae, ribs, sternum, ossa coxae, proximal epiphyses of humerus and femur)

Medullary Cavity

- hollow, cylindrical space within the diaphysis - contains red bone marrow in children - contains yellow bone marrow in adults

Epiphyseal Plate

- in metaphysis - growth plate - thin layer of hyaline cartilage - provides for lengthwise bone growth - in adults, the epiphyseal line, is the remnant of of the epiphyseal plate

Osteoid

- initial semisolid organic form of bone matrix - later calcifies

Forensic Anthropology

- juvenile skeleton with separate diaphysis and epiphysis - adult skeleton with whole fused bones - this helps determine age of skeletal remains - open: no union between epiphysis and other bone end - partial union: some fusion between epiphysis and rest of bone - complete union: all visible aspects united to rest of bone - female epiphyseal plates fuse about 1-2 years earlier than males

Epiphysis

- knobby region at each end of long bone - proximal: closest to body trunk - distal: farthest from trunk - composed of outer layer of compact bone and inner region of spongy bone - articulate cartilage covers the joint surface

Osteoclasts

- large, multinuclear, phagocytic cells - derived from fused bone marrow cells - ruffled border increases surface area exposed to bone - located within / adjacent to a depression / pit on bone surface: resorption lacuna - involved in bone resorption

Short Bones

- length nearly equal to width - carpal bones (wrist bones), tarsals (foot bones), sesamoid bones (bones along tendons of some muscles), patella (knee cap)

Spongy Bone (Cancellous / Trabecular)

- located internal to compact bone - appears porous - 20% of bone mass

Four Classes of Bones

- long bones - flat bones - short bones - irregular bones

Epiphyseal Plate

- maintains thickness during childhood - at maturity, rate of cartilage production slows - osteoblastic activity increases - plate narrows until it disappears: interstitial growth stops - remnant is an internal thin line of compact bone: epiphyseal line

Osteocyte

- mature bone cells derived from osteoblasts - lost bone-forming ability - maintain bone matrix - detect mechanical stress on bone - trigger deposition of new bone matrix - found in small spaces between concentric lamellae: lacunae

Hormones

- molecules released from one cell into the blood - travel throughout the body to affect cells - bind to cellular receptors of specific cells - initiate specific cellular changes - some alter rates of chondrocyte, osteoblast, osteoclast activity - affect bone composition and growth patterns

Serotonin

- neurotransmitter and hormone - most bones with serotonin receptors - role in rate and regulation of normal bone remodeling - if levels are too high: osteoprogenitor cells are prevented from differentiating into osteoblasts; could be linked to low bone density disorders

Mechanical Stress

- occurs in weight-bearing movement and exercise - required for normal bone remodeling - detected by osteocytes and communicated to osteoblasts: increase synthesis of osteoid - causes increase in bone strength - results from skeletal contraction and gravitational forces - can increase bone mass with weight-bearing activities and can increase total bone mass - can decrease bone mass from removal of mechanical stress, reduced collagen formation, demineralization (decreased strength of unstressed bone in immobilized fracture)

Osteopenia

- occurs slightly in all people with age - begins age 35-40 - osteoblast activity declines; osteoclast activity at previous levels - vertebrae, jaw bones, epiphyses lose large amounts of mass - women lose more of their skeletal mass every decade than men

Interstitial Bone Growth in Length

- occurs specifically within Zone 2 (Proliferating Cartilage) and Zone 3 (Hypertrophic Cartilage) - pushes zone of resting cartilage toward epiphysis - flexible hyaline cartilage permitting growth - new hyaline cartilage replaced by bone - similar to endochondral ossification during bone development

Appositional Growth

- occurs within the periosteum - bone matrix deposited within layers parallel to surface - layers termed external circumferential lamellae: as they increase in number, structure increases in diameter - osteoclasts resorb bone matrix along medullary cavity - transforms infant bone into a larger adult version

Trabeculae

- open lattice of marrow rods and plates of bones - bone marrow fills spaces - meshwork of crisscrossing bars - resistance to stresses

Steps of Intramembranous Ossification

- ossification centers form within thickened regions of mesenchyme: some cells become osteoprogenitor cells, some cells become osteoblasts secreting osteoid - osteoid undergoes calcification: calcium salts deposit onto osteoid and crystallize - entrapped cells become osteocytes - woven bone and surrounding periosteum form: at first, bone is immature and poorly organized; woven bone: primary bone; mesenchyme surrounding woven bone forms periosteum - lamellar bone (secondary bone) replaces woven bone; compact and spongy bone form trabeculae: typical structure of a flat cranial bone, composed of 2 external layers of compact bone, layer of sponge bone in between

Organic Components of Bone Matrix

- osteoid produced by osteoblasts containing collagen protein and semisolid ground substance of proteoglycans and glycoproteins - gives bones tensile strength by resisting stretching - contributes to bone flexibility

Bone Connective Tissue

- primary component of bone - osseous connective tissue - bone is composed of cells and extracellular matrix - includes osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts

Bones

- primary organ of skeletal system - support - protection - movement (attachment sights for muscles, tissues, organs; muscle contraction) - hemopoiesis - storage of mineral and energy reserves (calcium and phosphate released from bone into blood when needed)

Growth Hormone: Somatotropin

- produced by anterior pituitary gland - stimulates liver to produce somatotropin hormone: directly stimulates growth of cartilage in epiphyseal plate

Osteoporosis

- reduced bone mass sufficient to compromise normal function - occurs in a significant percentage of older women - occurs in a smaller percentage of older men - reduced hormones with age - results in decreased bone mass, weakened bones prone to fracture - postmenopausal women at most risk - increased incidence of fracture especially at wrist, hip, vertebral column - best treatment is prevention with diet and physical activity in young adults - medical treatments involve slowing rate of bone loss and attempting to stimulate new bone growth

Calcium

- regulating calcium concentration in blood is essential - required for initiation of muscle contraction, exocytosis of molecules from cells and neurons, stimulation of the heart by pacemaker cells, blood clotting - regulated by calcitriol and parathyroid hormone

Compact Bone (Dense / Cortical)

- relatively dense connective tissue - appears white, smooth, solid - 80% of bone mass

Perforating / Volkmann Canals

- resemble central canals - blood vessels and nerves travel through - perpendicular to central canals - connect central canals within different osteons - not a part of osteon; part of long bone

Osteitis Deformans

- results from disruption between osteoclast and osteoblast function - characterized by excessive bone resorption followed by excessive bone deposition - larger osteoclasts resorb bone at higher rate - newly deposited bone poorly formed - most commonly affected bones: pelvis, skull, vertebrae, femur, tibia - symptoms: bone deformity, pain

Circumferential Lamellae

- rings of bone - external: run immediately internal to bone periosteum - internal: run internal to the endosteum - both run the entire circumference of the bone - not a part of osteon; part of long bone

Concentric Lamellae

- rings of bone connective tissue - surround central canal - collagen fibers: 90° from previous and next lamellae; gives bone strength and resilience

Inorganic Components of Bone Matrix

- salt crystals, calcium phosphate - interacts with calcium hydroxide: forms crystals, hydroxyapatite - other substances incorporated into crystals: calcium carbonate, sodium, magnesium, sulfate, fluoride - crystals deposit around collagen fibers - harden matrix and account for rigidity of bones

Parathyroid Hormone (PTH)

- secreted and released by parathyroid glands in response to blood calcium levels - accelerates conversion to calcitriol by kidney

Thyroid Hormone

- secreted by thyroid gland - influences basal metabolic rate of bone cells - regulates normal activity at epiphyseal plates

Fracture Healing

- simple fracture: 2-3 months to heal - compound fracture: longer to heal - healing slows with age - some require surgical intervention to heal correctly

Bone Marrow

- soft connective tissue of bone - red and yellow bone marrow

Steps of Appositonal Growth

- stem cells at edge of perichondrium begin to divide - new undifferentiated stem cells and new chondroblasts are formed - chondroblasts push apart and become chondrocytes

Osteoprogenitor Cells

- stem cells derived from mesenchyme - cellular division yields another stem cell and a "committed cell" (matures to become an osteoblast) - located in periosteum and endosteum

Stress Fractures

- thin break caused by increased physical activity - bone experiences repetitive loads (ex: runners)

Canaliculi

- tiny, interconnecting channels within bone connective tissue - extend from each lacuna, travel through lamellae, and connect to lacunae and central canal - house osteocyte projections that allow intercellular contact - allow exchange of nutrients, minerals, gases, and wastes between blood vessels and osteocytes

Periosteum

- tough sheath covering outer surface of bone - outer fibrous layer of dense irregular connective tissue - protects bone from surrounding structures - anchors blood vessels and nerves to bone surface - attachment site for ligaments and tendons - inner cellular layer includes osteoprogenitor cells, osteoblasts, osteoclasts - attached to bone by numerous collagen fibers: perforating fibers

Components of Spongy Bone

- trabeculae - parallel lamellae

Zone of Ossification (of Epiphyseal Plate)

- walls break down between lacunae in columns - spaces invaded by capillaries and osteoprogenitor cells - new bone matrix deposited on the calcified cartilage matrix

Fibrocartilage

- weight bearing cartilage that withstands compression - located in invertebral discs, pubic symphysis, cartilage pads of knee

Ligaments

Anchor bone to bone

Simple Fracture

Broken bone not penetrating skin

Compact Bone

Composed of small, cylindrical structures: osteons (haversian systems)

Anemia

Condition with reduced erythrocytes (red blood cells)

Tendons

Connect muscle to bone

Calcium

Essential for muscle contraction, blood clotting, nerve impulse transmission

Pathologic Fracture

Occurs in bone weakened by disease

Compound Fracture

One or both ends of the bone pierce overlying skin

Chondroblasts

Produce cartilage matrix

Metaphysis

Region of mature bone between diaphysis and epiphysis

Diploë

Spongy bone in flat bones of skull

Yellow Bone Marrow

Stores lipids in adult bones