A&P Chapter 6

Epiphyses

Proximal and distal ends of bone

Assistance in movement

Most skeletal muscles attach to bones. When they contract, they pull on bones to produce movement

Diaphysis

Bone shaft/body

Mineral homeostasis (storage and release)

Bone tissue stores minerals like calcium and phosphorus, which contribute to strength of bone. Bone tissue stores about 99% of body's calcium.

Support

structural framework for body.

Osteoclasts

Huge cells derived from fusion of many monocytes. Concentrated in endosteum. On side of cell that faces bone surface, osteoclast's PM deeply folded into ruffled border. Here cell releases powerful lysosomal enzymes and acids that digest protein and mineral components of underlying extracellular bone matrix. Breakdown of extracellular matrix termed resorption.

Veins that carry blood away from long bones

In 3 places: 1) 1-2 nutrient veins accompany nutrient artery and exit through diaphysis. 2) Numerous epiphyseal veins and metaphyseal veins accompany their respective arteries and exit through epiphyses. 3) Many small periosteal veins accompany their respective arteries and exit through periosteum.

Minerals

Large amounts of calcium and phosphorus are needed while bones are growing, as are smaller amounts of magnesium, fluoride, and manganese.

Zone of hypertrophic cartilage

Layer consists of large, maturing chondrocytes arranged in columns.

Zone of resting cartilage

Layer nearest to epiphysis and conceits of scattered chondrocytes. Cells do not function in bone growth, but anchor epiphyseal plate to epiphysis of bone.

Spongy bone tissue locations

Makes up most of interior bone tissue of short, flat, sesamoid, and irregularly shaped bones. In long bones, forms core of epiphyses beneath layer of compact-bone, and forms variable narrow rim bordering medullary cavity of diaphysis. Spongy bone ALWAYS covered by layer of compact bone for protection. Tends to be located where bones are not heavily stressed or where stresses are applied from many directions.

Osteocyte

Mature bone cells, are main cells in bone tissue and maintain its daily metabolism, such as exchange of nutrients and wastes with blood. Do not undergo cell division.

Demineralization

Old age loss of bone mass. Loss of calcium from bones is one of problems in osteoporosis.

Bone formation

Ossification or osteogenesis. 4 principle situations: 1) initial formation of bones in embryo and fetus. 2) Growth of bones during infancy, childhood, and adolescence until their adult sizes are reached. 3) Remodeling of bone. 4) Repair of fractures throughout life.

Articular cartilage

Thin layer of hyaline cartilage covering part of epiphysis where bone forms an articulation with another bone. Reduces friction and absorbs shock. Repair of damage is limited.

Endosteum

Thin membrane that lines medullary cavity. Contains single layer of bone-forming cells and small amount of connective tissue.

Epiphyseal line

When epiphyseal cartilage cells stops dividing and people stop growing.

Triglyceride storage

Yellow bone marrow consists mainly of adipose cells, which store triglycerides. Potential chemical energy reserve.

Long bone

one that has greater length than width. Consists of diaphysis, epiphyses, metaphyses, articular cartilage, periosteum, medullary cavity, and endosteum.

Epiphyseal plate 4 zones

zone of resting cartilage, zone of proliferating cartilage, zone of hypertrophic cartilage, and zone of calcified cartilage

Repair of bone fracture

- Formation of fracture hematoma - Fibrocartilaginous callus formation - Bony callus formation - Bone remodeling

Factors affecting bone growth and bone remodeling

- Minerals - Vitamins - Hormones

Initial bone formation in embryo and fetus

1) Intramembranous ossification: bone forms directly within mesenchyme, which is arranged in sheetlike layers that resemble membranes. 2) Endochondral ossification: bone forms within hyaline cartilage that develops from mesenchyme.

Nerve supply of bone

Accompany blood vessels that supply bones. Periosteum rich in sensory nerves, some of which carry pain sensations.

Medullary cavity

Aka marrow cavity. Hollow, cylindrical space within diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. Minimizes weight of bone by reducing dense bony material.

Spongy bone tissue

Aka trabecular or cancellous bone tissue. Does not contain osteons. Located in interior of bone, protected by covering compact bone. Consists of lamellae that are arranged in irregular pattern of thin columns called trabeculae. Macroscopic spaces between trabeculae are filled with red bone marrow in bones that produce blood cells, and yellow bone marrow in other bones. Each trabecular consists of concentric lamellae, osteocytes that lie in lacunae, and canaliculi that radiate outward from lacunae.

Compact bone composition 2

Areas between neighboring osteons contain lamellae called interstitial lamellae, which also have lacunae with osteocytes and canaliculi. These are fragments of older osteons. Blood vessels, lymphatic vessels, and nerves from periosteum penetrate compact bone through transverse perforating canals. Connect with those of medullary cavity, periosteum, and central canals. Arranged around entire outer and inner circumference of shaft of long bone are lamellae called circumferential lamellae. Develop during initial bone formation. Circumferential lamellae directly deep to periosteum are called outer circumferential lamellae. Connected to periosteum by perforating fibers. Circumferential lamellae that line medullary cavity are called inner circumferential lamellae.

Formation of fracture hematoma

Blood vessels crossing fracture line are broken. Mass of clotted blood forms around site of fx. This is called fracture hematoma. Nearby bone cells die. Swelling and inflammation occur. Phagocytes and osteoclasts begin to remove dead or damaged tissue in and around fx hematoma. several weeks.

Osteoblasts

Bone-building cells. Synthesize and secrete collagen fibers and other organic components needed to build extracellular matrix of bone tissue, and initiate calcification. As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. No cell division.

Compact bone composition 1

Composed of repeating structural units called osteons (haversian systems). Each osteon consists of concentric lamellae arranged around central canal. Resembling growth rings of tree, concentric lamellae are circular plates of mineralized extracellular matrix of increasing diameter, surrounding small network of blood vessels, lymphatics, and nerves located in central canal. Between concentric lamellae are small spaces called lacunae, which contain osteocytes. Radiating in all directions from lacunae are tiny canaliculi, which are filled with extracellular fluid. Inside canaliculi are processes of osteocytes. Neighboring osteocytes communicate via gap junctions. Canaliculi connect lacunae with one another and with central canals, forming intricate miniature system of interconnected canals throughout bone. Provides many routes for nutrients and oxygen to reach osteocytes and removal of wastes.

Blood cell production

Connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets. This is known as hemopoiesis. Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within network of reticular fibers. Present in developing bones of fetus and in some adult bones. In newborn, all bone marrow is red and involved in hemopoiesis. Much changes to yellow as age increases.

Bone tissue

Contains abundant extracellular matrix is about 15% water, 30% collagen fibers, and 55% crystallized mineral salts. Most abundant mineral salt is calcium phosphate, and combines with calcium hydroxide to form crystals of hydroxyapatite. These combine with other mineral salts. As they are deposited into framework, tissue hardens. This is called calcification, and is initiated by bone-building cells called osteoblasts. Requires presence of collagen fibers. Mineral salts first crystallize in microscopic spaces between collagen fibers. After spaces filled, mineral crystals accumulate around collagen fibers. Combination of crystallized salts and collagen fibers responsible for characteristics of bone. Determines bone's "hardness".

Compact bone

Contains few spaces. Strongest form of bone tissue. Found beneath periosteum of all bones and makes up bulk of diaphysis of long bones. Protection and supports.

Bone tissue flexibility

Depends on collagen fibers. Collagen fibers provide tensile strength. Soaking bone in acidic solution dissolves mineral salts causing bone to become rubbery and flexible.

Fibrocartilaginous callus formation

Fibroblasts from periosteum invade fx site and produce collagen fibers. Cells from periosteum develop into chondroblasts and begin to produce fibrocartilage in this region. Lead to development of fibrocartilaginous callus, mass of repair tissue consisting of collagen fibers and cartilage that bridges broken ends of bone. 3 weeks.

Bone remodeling (fx repair)

Final phase. Dead portions of original fragments of broken bone are gradually resorbed by osteoclasts. Compact bone replaces spongy bone around periphery of fx.

Zone of calcified cartilage

Final zone of epiphyseal plate. Only few cells thick. and mostly chondrocytes that are dead because extracellular matrix around them is calcified. Osteoclasts dissolve calcified cartilage and osteoblasts and capillaries from diaphysis invade the area. Osteoblasts lay down bone extracellular matrix, replacing calcified cartilage by process of endochondral ossification. Zone of calcified cartilage becomes "new diaphysis" that is firmly cemented to rest of diaphysis of bone.

Intramembranous ossification

Forms flat bones of skull, most of facial bones, mandible, and medial part of clavicle are formed in this way. 1) Osteoblasts secrete organic extracellular matrix 2) Calcium and other mineral salts are deposited and extracellular matrix calcified 3) Extracellular matrix develops into trabeculae that fuse to form spongy bone. 4) Mesenchyme at periphery of bone develops into periosteum.

Bony callus formation

In areas closer to well-vascularized healthy bone tissue, osteogenic cells develop into osteoblasts, which begin to produce spongy bone trabecular. Trabeculae join living and dead portions of original bone fragments. In time, fibrocartilage is converted to spongy bone, and callus is referred to as bony callus. 3-4 months.

Hormones

In childhood, insulin like growth factors (IGFs) are important. Stimulate osteoblasts, promote cell division at epiphyseal plate, and enhance synthesis of proteins needed to build new bone. IGFs produced in response to secretion of hGH. T3 and T4 from thyroid gland also promote bone growth by stimulating osteoblasts. Sex hormones affect growth

Growth in length

Involves following two events - 1) Interstitial growth of cartilage on epiphyseal side of epiphyseal plate and 2) replacement of cartilage on diaphyseal side of epiphyseal plate with bone by endochondral ossification.

Bone's role in calcium homeostasis

One way to maintain level of calcium in blood is to contral rates of calcium resoption from bone into blood and calcium deposition from blood into bone. Blood plasma level of Ca is very closely regulated between 9-11 mg/100mL. Role of bone in calcium homeostasis it to help "buffer" blood Ca level, releasing Ca into blood plasma using osteoclasts when level decreases, and absorbing Ca using osteoblasts when level rises.

Bone remodeling

Ongoing replacement of old bone tissue by new bone tissue. Involves bone resorption (removal of minerals and collagen fibers from bone by osteoclasts) and bone deposition (addition of minerals and collagen fibers to bone by osteoblasts). If newly formed bone is subjected to heavy loads, it will grow thicker and be stronger than old bone. Shape can be altered for proper support based on stress patterns experienced during remodeling process. New bone is more resistant to fracture than old bone.

Bone resorption

Osteoclast attaches tightly to bone surface at endosteum or periosteum and forms leakproof seal at edges of its ruffled border. Releases protein-digesting lysosomal enzymes and several acids into sealed pocket. Working together, several osteoclasts carve out small tunnel in old bone. Degraded bone proteins and extracellular matrix minerals enter an osteoclast by endocytosis, cross cell in vesicles, and undergo exocytosis on side opposite ruffled border. Now in interstitial fluid, products of bone resorption diffuse into nearby blood capillaries. Once small area of bone has been resorbed, osteoclasts depart and osteoblasts move in to rebuild bone in that area.

Four types of cells present in bone

Osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

Parathyroid hormone

PTH. Regulated Ca exchange. Increases blood Ca level. Operates as negative feedback. If Ca levels decrease, increases production of cyclic adenosine monophosphate (cAMP). PTH gene detects intercellular increase of cAMP and PTH synthesis speeds up. Highter levels of PTH increases number and activity of osteoclasts which step up pace of bone resorption. PTH also acts on kidneys to decrease loss of Ca in urine, so more is retained in blood. Stimulates formation of calcitriol, hormone that promotes absorption of Ca from foods in Gi tract into blood.

Blood supply of bone

Periosteal arteries enter diaphysis through many perforating canals and supply periosteum and outer part of compact bone. Near center of diaphysis, a large nutrient artery passes through hole in compact bone called nutrient foramen. On entering medullary cavity, nutrient artery divides into proximal and distal branches that course toward each end of bone. These supply both inner part of compact bone tissue of diaphysis and spongy bone tissue and red bone marrow as far as the epiphyseal plates. Metaphyseal arteries enter metaphyses of long bone and together with nutrient artery, supply red bone marrow and bone tissue of metaphyses. Epiphyseal arteries enter epiphyses of long bone and supply red bone marrow and bone tissue of epiphyses.

Metaphyes

Regions between diaphysis and epiphyses. In growing bone, each metaphysis contains epiphyseal growth plate. When bone ceases to grow in length, cartilage in epiphyseal plate is replaced by bone, resulting in epiphyseal line.

Calcitonin

Released by parafollicular cells when blood Ca level increases. CT inhibits activity of osteoclasts, speeds blood Ca uptake by bone, and accelerates Ca deposition into bones.

Endochondral ossification

Replacement of cartilage by bone is called endochondral ossification. Most bones of body formed this way. 2) Interstitial growth results increase in length. Appositional growth Growth of cartilage in thickness due to deposition of extracellular matrix material on cartilage surface of model by new chondroblasts that develop from perichondrium.

Stress fracture

Series of microscopic fissures in bone that forms without any evidence of injury to other tissues. About 25% of stress fractures involve tibia.

Protection

Skeleton protects most important internal organs fro injury.

Zone of proliferating cartilage

Slightly larger chondrocytes in this zone arranges like stacks of coins. Undergo interstitial growth as they divide and secrete extracellular matrix. Divide to replace those that die at diaphyseal side of epiphyseal plate.

Differences between spongy and compact bone

Spongy bone is light. This allows bone to move more readily when pulled by skeletal muscle. Trabeculae of spongy bone tissue support and protect red bone marrow, and is only site where red bone marrow is stored.

6 Main functions of skeletal system

Support, protection, assistance in movement, mineral homeostasis (storage and release), blood cell production, and triglyceride storage

Periosteum

Tough connective tissue sheath and its associated blood supply that surrounds bone surface wherever it is not covered by articular cartilage. Composed of outer fibrous layer and inner osteogenic layer. Some of cells of inner helps bone grow in thickness. Also protects bone, assists in fracture repair, helps nourish bone tissue, and serves as attachment point for ligaments and tendons. Periosteum attached to underlying bone by perforating (Sharpey's) fibers.

Osteogenic cells

Unspecialized bone stem cells derived from mesenchyme, tissue from which almost all connective tissues are formed. Only bone cells to undergo division, resulting cell osteoblast. Osteogenic cells found in inner portion of periosteum, in endosteum, and canals within bone that contain blood vessels.

Vitamins

Vitamin A stimulates activity of osteoblasts. Vitamin C is needed for synthesis of collagen. Vitamin D helps build bone by increase absorption of calcium from foods in GI tract into blood. Vitamins K and B12 also needed for synthesis of bone proteins.