A&P Lecture 6 Part 2

Aging and Bone Tissue

-2 principal effects of aging on bone tissue: Loss of bone mass Brittleness -A decrease in bone mass occurs as the level of sex hormones diminishes during middle age (especially in women after menopause). --Bone resorption by osteoclasts outpaces bone deposition by osteoblasts. --Since female bones are generally smaller and less massive than males to begin with, old age has a greater adverse effect in females.

Bone Growth and Remodeling

-A balance must exist between the actions of osteoclasts and osteoblasts. --If too much new osseous tissue is formed, the bones become abnormally thick and heavy, as seen with acromegaly. --Excessive loss of calcium weakens the bones, as occurs in osteoporosis. --Bones may also become too "soft", as seen in the bone diseases rickets (children) and osteomalacia (adults).

Postnatal Bone Growth

-After initial bone formation, bones grow by via two methods: 1. Interstitial (longitudinal) growth -Increase in length of long bones 2. Appositional growth -Increase in bone thickness

Appositional Growth

-Allows lengthening bone to widen Occurs throughout life --Majority of osteoblast contribution to appositional growth occurs in the periosteum --secretes bone matrix on external bone -Majority of osteoclasts contribution to appositional growth occurs in the endosteum --removes bone on endosteal surface -Usually more building up than breaking down --Thicker, stronger bone but not too heavy

Classification of Bone Fractures

-Also described by location of fracture -External appearance -Nature of break -Eponym (someone's name)

Greenstick Fracture

-Bone breaks incompletely, much in the way a green twig breaks. Only one side of the shaft breaks: the other side bends. -Common in children, whose bones have relatively more organic matrix and are more flexible than those of adults.

Endochondral Ossification Stage 1

-Bone collar forms around the diaphysis of the hyaline cartilage model -week 9

Communited Fracture

-Bone fragments into 3 or more pieces -Particularly common in the aged, whose bones are more brittle

Results of Mechanical Stressors: Wolff's Law

-Bone grows or remodels in response to demands placed on it -Handedness (right or left handed) results in thicker and stronger bone of that upper limb -Curved bones thickest where most likely to buckle -Trabeculae of spongy bone form trusses along lines of stress -Large, bony projections occur where heavy, active muscles attach --Even more pronounced on professional weight lifters -Bones of fetus and bedridden featureless

Compression Fracture

-Bone is crushed -common in porous bones (osteoporotic bones subjected to extreme trauma, as in a fall)

Responses to mechanical and gravitational forces

-Bones stressed when bearing weight or pulled on by muscle --Usually stress is off center so tends to bend bones --Bending compresses on one side; stretches on other -Bones reflect stresses they encounter --Long bones thickest midway along diaphysis where bending stresses greatest

Fractures

-Breaks in the bone tissue --Fractures in youth ~Most result from trauma --Fractures in old age ~Most result of weakness from bone thinning

Depressed Fracture

-Broken bone portion is pressed inward -Typical of skull fracture

Vitamin D Deficiency

-Can happen in places with low sun exposure or low calcium content in diet --Rickets ~Primarily caused by deficiency in Vitamin D ~More common in children than adults ~~Called osteomalacia in adults ~Weakening of bone hardness due to insufficient absorption of dietary calcium from lack of vitamin D or prolonged diets deficient in calcium

Endochondral Ossification Stage 2

-Cartilage in the center of the diaphysis calcifies and then develops cavities

Proliferation (growth) Zone

-Cartilage on diaphysis side of epiphyseal plate -Rapidly divide pushing epiphysis away from diaphysis -> lengthening

Resting (quiescent) Zone

-Cartilage on epiphyseal side of epiphyseal plate -Relatively inactive

Ossification (osteogenic) Zone

-Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction -Covered with new bone by osteoblasts -Ultimately replaced with spongy bone

Brittleness

-Collagen fibers give bone its tensile strength, and protein synthesis decreases with age. -The loss of tensile strength causes the bones to become very brittle and susceptible to fracture.

Completeness of break

-Complete: broken all the way through -Incomplete: not broken all the way through

Negative feedback hormonal loop for Ca2+ homeostasis

-Controls blood Ca2+ levels; Not bone integrity -Serum Ca2+ concentrations are very important for proper nervous and muscle function -Even minute changes in blood calcium are dangerous -Parathyroid hormone (PTH) --Produced by parathyroid glands --Removes calcium from bone regardless of bone integrity -Calcitonin may be involved --Produced by parafollicular cells of thyroid gland --In humans, high doses lowers blood calcium levels temporarily ~Normal human physiological serum levels not high enough to cause the above effect

Epiphyseal Fracture

-Epiphysis separates from the diaphysis along the epiphyseal plate -Tends to occur where cartilage cells are dying & calcification of the matrix is occuring

Results of Hormonal and Mechanical Influences

-Hormonal controls determine whether and when remodeling occurs to changing blood calcium levels -Mechanical/gravitational stress determines where remodeling occurs

Intramembranous Ossification Stage 4

-Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. --Trabeculae just deep to the periosteum thicken. Mature lamellar bone replaces them, forming compact bone plates. --Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.

Risk Factors for Osteoporosis

-Most often aged, postmenopausal women --30% 60 - 70 years of age; 70% by age 80 --30% Caucasian women will fracture bone because of it -Men to lesser degree -Petite body form -Insufficient exercise to stress bones -Diet poor in calcium and protein -Smoking -Hormone-related conditions --Hyperthyroidism --Low blood levels of thyroid-stimulating hormone --Diabetes mellitus -Immobility -Males with prostate cancer taking androgen-suppressing drugs

Position of bone ends after fracture

-Nondisplaced: ends retain normal position -Displaced: ends out of normal alignment

Control of Bone Remodeling

-Occurs continuously but regulated by genetic factors and 2 control loops 1. Negative feedback hormonal 2. Responses to mechanical and gravitational forces

Whether skin is penetrated

-Open (compound): skin is penetrated -Closed (simple): skin is not penetrated

Intramembranous Ossification Stage 1

-Ossification centers appear in the fibrous connective tissue membrane. --Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center that produces the first trabeculae of spongy bone.

Intramembranous Ossification Stage 2

-Osteoid is secreted within the fibrous membrane and calcifies. --Osteoblasts begin to secrete osteoid, which calcifies in a few days. --Trapped osteoblasts become osteocytes.

Day to day control of calcium regulation

-PTH stimulates osteoclastic activity and raises blood serum calcium level. Stimulates reabsorption of calcium ions in the kidneys -To a small extent, calcitonin - maybe -, hGH, and the sex hormones (estrogen and testosterone) stimulate osteoblastic activity and lower serum calcium level. -Vitamin D is produced for absorption of the Ca2+ and PO4- ions from the small intestine

Preventing Osteoporosis

-Plenty of calcium in diet in early adulthood --Can help to increase bone deposition -Reduce carbonated cola beverage and alcohol consumption --Leaches minerals from bone so decreases bone density -Plenty of weight-bearing exercise --Increases bone mass above normal for buffer against age-related bone loss

Fracture Classification

-Position of bone ends after fracture -Completeness of break -Whether skin is penetrated

Spiral Fracture

-Ragged break occurs when excessive twisting forces are applied to a bone -common sports fracture

Reduction

-Realignment of broken bone ends -Closed reduction: physician manipulates to correct position -Open reduction: surgical pins or wires secure ends

Fracture Treatment

-Reduction -Immobilization

Interstitial (longitudinal) Growth

-Requires presence of epiphyseal cartilage -Epiphyseal growth plate maintains constant thickness --Rate of cartilage growth on one side balanced by bone replacement on other -Concurrent remodeling of epiphyseal ends to maintain proportion -Result of five zones within cartilage: 1. Resting (quiescent) zone 2. Proliferation (growth) zone 3. Hypertrophic zone 4. Calcification zone 5. Ossification (osteogenic) zone -Ossification contributing to bone length (Interstitial growth) occurs throughout childhood and adolescence --Near end of adolescence chondroblasts divide less often --Epiphyseal plate thins then is replaced by bone ---Epiphyseal plate closure ---Bone lengthening ceases ---Bone of epiphysis and diaphysis fuses ---Usually complete by 18-21 years of age. ~18 in females ~21 in males -Fractures (breaks) to the epiphyseal growth plate can accelerate it's closure. --The fractured bone may be shorter than normal when adulthood is reached --Inhibits length wise growth of bone

Calcification Zone

-Surrounding cartilage matrix calcifies, chondrocytes die and deteriorate

Endochondral Ossification Stage 4

-The diaphysis elongates and a medullary cavity forms. Secondary ossification centers appear in the epiphyses -birth

Endochondral Ossification Stage 5

-The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages -childhood to adolescence

Step 4: Bone remodeling occurs

-The final step takes several months and is called remodeling : --Spongy bone is replaced by compact bone. --The fracture line disappears and little to no evidence of the break remains once complete -Final structure resembles original because responds to same mechanical stressors

Loss of bone mass

-The loss of calcium from bones is one of the symptoms in osteoporosis.

Endochondral Ossification Stage 3

-The periosteal bud invades the internal cavities and spongy bone forms -month 3

Exercise and Bone Tissue

-Under mechanical stress, bone tissue becomes stronger through deposition of mineral salts and production of collagen fibers by osteoblasts. Unstressed bones, on the other hand, become weaker. -Astronauts in space suffer rapid loss of bone density. --As much a 1% a week

Step 3: Bony callus forms

-Within one week new trabeculae appear in fibrocartilaginous callus -Callus converted to bony (hard) callus of spongy bone ~2 months later firm union form

Intramembranous Ossification Stage 3

-Woven bone and periosteum form. --Accumulating osteoid is laid down between embryonic blood vessels in a manner that results in a network (instead of concentric lamellae) of trabeculae called woven bone. --Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum.

Immobilization

-by cast or traction for healing -Depends on break severity, bone broken, and age of patient

Control of Bone Growth

-hormones key contributors -during childhood, most important hormones human growth hormone (hGH) & growth factors called IGFs (produced by liver) --stimulate osteoblasts, promote cell division at epiphyseal plate & enhance protein synthesis -Thyroid hormone's contribution to bone growth involves the modulation of the activity of growth hormone --Ensures proper bone proportions -Sex hormones (estrogen and testosterone) cause a dramatic effect on bone growth, such as the sudden "growth spurt" that occurs during adolescence --The female sex hormones also promote widening of the pelvis in the female skeleton --Sex hormones are responsible for closing the epiphyseal plates at the end of puberty --Also important in bone density maintenance during adulthood

Vitamins

-necessary for normal bone growth: --Vitamin A is important for the activity of osteoblasts. --Vitamin C is needed for synthesis of collagen. --Vitamin D is essential to healthy bones because it promotes the absorption of calcium from foods in the gastrointestinal tract into the blood. --Vitamins K and B12 are needed for synthesis of bone proteins

Ossification (osteogenesis)

-process of forming new bone -Bone formation occurs in four situations: 1. Formation of bone in a late stage embryo 2. Growth of bones until adulthood 3. Remodeling of bone 4. Repair of fractures -occurs by two different methods, beginning about the 8th week of embryonic development 1. Intra-membranous ossification 2. Endochondral ossification is a

Endochondral Ossification

-process whereby cartilage is replaced by bone -Forms both compact and spongy bone -the method used in the formation of most bones, especially long bones. -Involves replacement of a cartilage model by bone. -Begins at the primary ossification center in center of shaft --Blood vessel infiltration of perichondrium converts it to periosteum ~Underlying cells change to osteoblasts -Bone collar forms around diaphysis of cartilage model -Central cartilage in diaphysis calcifies, then develops cavities -Periosteal bud invades cavities --Leads to the formation of spongy bone -Diaphysis elongates & medullary cavity forms -2ndary ossification centers form in the epiphyses -Epiphyses ossify

Intra-membranous Ossification

-produces spongy bone -This bone may subsequently be remodeled to form compact bone -simpler of the two methods. -It is used in formation of the flat bones of the skull, mandible, and clavicle. -Bone forms from mesenchymal cells that develop into osteoblasts within a fibrous membrane --Recall that mesenchyme is the tissue from which almost all other C.T. develop. -Many ossification centers. --Centers of bone formation --The bone that is produced does not go through a cartilaginous stage -Woven bone and periosteum form -Lamellar bone replaces woven bone & red marrow appears

Osteoporosis

-refers to a group of diseases where bone resorption outpaces bone deposition. -Often due to depletion of calcium from the body or inadequate intake -Sex hormones maintain normal bone health and density --As secretion wanes with age osteoporosis can develop -Spongy bone of spine and neck of femur most susceptible --Vertebral and hip fractures common

Fracture and Repair

-repair proceeds in 4 steps

Step 2: Fibrocartilaginous callus forms

-the formation of a callus -Capillaries grow into hematoma -Phagocytic cells clear debris -Fibroblasts secrete collagen fibers to span break and connect broken ends -Fibroblasts, cartilage, and osteogenic cells begin reconstruction of bone -Create cartilage matrix of repair tissue -Osteoblasts form spongy bone within matrix -Mass of repair tissue called fibrocartilaginous callus

Step 1: Hematoma forms

-the formation of a fracture hematoma (clot) as a result of blood vessels breaking in the periosteum and in osteons. -Site swollen, painful, and inflamed

Negative Feedback Hormonal Loop for blood Ca2+ Homeostasis

Controlled by parathyroid hormone (PTH) <(down)Blood Ca2+ levels -> Increase PTH release -> PTH stimulates osteoclasts to degrade bone matrix, releasing Ca2+ -> ^Blood Ca2+ levels -> Decrease PTH release

Hypertrophic Zone

Older chondrocytes closer to diaphysis and their lacunae enlarge and erode interconnecting spaces