Bone Tissue PDF

Bone tissue Osteology: study of osseous structures. Functions: Support Protection Movement Mineral homeostasis Hemopoiesis: blood cell formation Storage of adipose tissue: yellow marrow  Long bone anatomy n Diaphysis: long shaft of bone n Epiphysis: ends of bone n...

Bone tissue Osteology: study of osseous structures. Functions: Support Protection Movement Mineral homeostasis Hemopoiesis: blood cell formation Storage of adipose tissue: yellow marrow  Long bone anatomy n Diaphysis: long shaft of bone n Epiphysis: ends of bone n Epiphyseal plate: a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length n Metaphysis: b/w epiphysis and diaphysis n Articular cartilage: thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. This reduces friction and absorbs shock at freely movable joints. Periosteum: bone covering (pain sensitive) composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. periosteum cells enable bone to grow in thickness, but not in length. It protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. Sharpey’s fibers: periosteum attaches to underlying bone through thick large bundles of collagen fibres Medullary cavity: Hollow chamber in bone - red marrow produces blood cells - yellow marrow is adipose. Endosteum: thin layer lining the medullary cavity Long bone Diaphysis Epiphysis Metaphysis Epiphyseal (growth) plate Medullary cavity n Histology of bone tissue Cells are surrounded by matrix. - 25% water - 25% protein - 50% mineral salts n The most abundant mineral salt is calcium phosphate [Ca3(PO4)2]. n A bone’s hardness depends on the crystallized inorganic mineral salts and bone’s flexibility depends on its collagen fibers.  Calcification/ Mineralization Calcium Phosphate combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapatite [Ca10(PO4)6 (OH)2]. As the crystals form, they combine with still other mineral salts, such as calcium carbonate (CaCO3), and ions such as magnesium, fluoride, potassium, and sulfate. As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. This process is initiated by bone-building cells called osteoblasts  Different types of cells in bone tissue nOsteoprogenitor cells: - derived from mesenchyme - all connective tissue is derived - unspecialized stem cells - undergo mitosis and develop into osteoblasts - found on inner surface of periosteum and endosteum. n Osteoblasts: - bone forming cells - found on surface of bone - no ability to mitotically divide - collagen and organic component secretors - initiate calcification n - As they surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes  Osteocytes - mature bone cells - derived form osteoblasts - do not secrete matrix material - cellular duties include exchange of nutrients and waste with blood. - do not undergo cell division  Osteoclasts -huge cells derived from the fusion of as many as 50 monocytes -concentrated in the endosteum -bone resorbing cells, the osteoclast’s plasma membrane on the side of the cell that faces the bone surface, is deeply folded into a ruffled border. Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix. - bone surface - growth, maintenance and bone repair n Compact bone: (Osteon) external layer - called lamellar bone (groups of elongated tubules called lamella) - contains few spaces - majority of all long bones - protection and strength (wt. bearing) - concentric ring structure - blood vessels and nerves penetrate periosteum through horizontal openings called perforating (Volkmann’s) canals and connect periosteum, medullary cavity and central canal. - central (Haversian) canals run longitudinally. - around canals are concentric lamella,ring of calcified extra cellular matrix - osteocytes occupy lacunae (“little lakes”) which are between the lamella - radiating from the lacunea are channels called canaliculi. (finger like processes of osteocytes) n Lacunae are connected to one another by canaliculi and provides many routes for nutrients and oxygen to reach the osteocytes and for the removal of wastes. - Osteon contains: - central canal - surrounding lamellae - lacunae - osteocytes - canaliculi Compact bone n Spongy bone (cancellous bone): internal layer - latticework of bone tissue (haphazard arrangement). n - made of trabeculae (“little beams”) lamellae arranged in an irregular lattice of thin columns called trabeculae - filled with red and yellow bone marrow - osteocytes get nutrients directly from circulating blood. - short, flat and irregular bone is made up of mostly spongy bone q spongy bone tissue is light, which reduces the overall weight of a bone so that it moves more readily when pulled by a skeletal muscle. q the trabeculae of spongy bone tissue support and protect the red bone marrow. q The spongy bone tissue in the hip bones, ribs, sternum (breastbone), vertebrae (backbones), and the ends of long bones is where red bone marrow is stored and, thus, where hemopoiesis (blood cell production) occurs in adults. Cancellous (spongy) bone  Ossification/ Osteogenesis The process of bone formation is ossification. Bone formation occurs in four principal situations: (1) the initial formation of bones in an embryo and fetus, (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, (3) the remodeling of bone (replacement of old bone by new bone tissue throughout life), and (4) the repair of fractures (breaks in bones) throughout life. n Bone formation (ossification) occurs in two ways 1- Intramembranous ossification 2- Endochondral ossification Both methods above lead to the same bone formation but are different methods of getting there. Ossification (osteogenesis) begins around the 6th -7th week of embryonic life. At this time the embryonic skeleton is made of fibrous membranes and hyaline cartilage. Hyaline Cartilage Review a. Most abundant b. Provides support, flexibility and resilience c. Located: a. forming nearly all the fetal skeleton b. articular cartilage: ends of moving bones c. costal cartilage: ribs to sternum d. tip of nose e. respiratory cartilage n Skeletal Cartilage: 1. Chondrocytes: cartilage producing cells. 2. Lacunae: small cavities where the chondrocytes are encased. 3. Extracellular matrix: jellylike ground substance. 4. Perichondrium: layer of dense irregular connective tissue that surrounds the cartilage. 5. No blood vessels or nerves n Intramembranous (within the membrane) ossification: Bone develops from a fibrous membrane. - flat bones of skull - mandible - clavicles -mesenchymal cells become vascularized and become osteoprogenitor cells and then osteoblasts. - organic matrix of bone is secreted - osteocytes are formed - calcium and mineral salts are deposited and bone tissue hardens. - trabeculae develop and spongy bone is formed - red marrow fills spaces Replacement of hyaline cartilage with bone is called. Endochondral (intracartilaginous) ossification Most bones are formed this way (i.e. long bones). Where bone is going to form: 1- mesenchymal cells differentiate into chondroblasts (immature cartilage cells) which produces hyaline cartilage. Perichondrium develop around new cartilage 2- Chondrocytes (mature) mitotically divide increasing in length This pattern of growth: interstitial growth. - growth from within Growth of cartilage in thickness occurs from the deposition of new matrix to the periphery formed by chondroblasts within the perichondrium. Appositional growth. Chondrocytes undergo hypertrophy, swell and burst. pH of the matrix changes and calcification is triggered. Ultimately, cartilage cells die. Lacunae are now empty. Nutrients are supplied by way of the nutrient artery passing through the perichondrium through the nutrient foramen. Osteoprogenitor cells are stimulated in the perichondrium to produce osteoblasts. A thin layer of compact bone is laid down under the perichondrium called the periosteal bone collar. Perichondrium becomes periosteum Osteoblasts begin to deposit bone matrix forming spongy bone trabeculae. In the middle of the bone, osteoclasts break down spongy bone trabeculae and form a hollowed out cavity called the medullary cavity. This cavity will be filled with red bone marrow for hemopoiesis. The shaft of the bone is replaced (was hyaline cartilage) with compact bone. n Physiology of bone growth: - epiphyseal plate (bone length) - 4 zones of bone growth under hGH. 1- Zone of resting cartilage: - no bone growth - located near the epiphyseal plate - scattered chondrocytes - anchors plate to bone 2- Zone of proliferating cartilage - chondrocytes stacked like coins - chondrocytes divide 3- Zone of hypertrophic (maturing) cartilage - large chondrocytes arranged in columns - lengthwise expansion of epiphyseal plate 4- Zone of calcified cartilage - few cell layers thick - occupied by osteoblasts and osteoclasts and capillaries from the diaphysis - cells lay down bone - dead chondrocytes surrounded by a calcified matrix. Matrix resembles long spicules of calcified cartilage. Spicules are partly eroded by osteoclasts and then covered in bone matrix from osteoblasts: spongy bone is formed. Age 18-21: Longitudinal bone growth ends when epiphysis fuses with the diaphysis. - epiphyseal plate closure - epiphyseal line is remnant of this - last bone to stop growing: clavicle n Bone width: increase in diameter of bone occurs through appositional growth. - Osteoblasts located beneath the periosteum secrete bone matrix and build bone on the surface - Osteoclasts located in the endosteum resorbs (breakdown) bone. n Bone Remodeling: - bone continually renews itself - never metabolically at rest - enables Ca to be pulled from bone when blood levels are low - osteoclasts are responsible for matrix destruction - produce lysosomal enzymes and acids - spongy bone replaced every 3-4 years - compact bone every 10 years - Blood calcium levels signal release of either parathyroid hormone (PTH, secreted by parathyroid gland) and calcitonin (secreted by thyroid). PTH causes calcium release from bone matrix by stimulating osteoclast activity and bone resorption. Calcitonin inhibits bone resorption and causes calcium salts to be deposited in bone matrix. Vitamins A, C, D and B12 help in bone remodeling n Fractures: Any bone break. - blood clot will form around break - fracture hematoma - inflammatory process begins - blood capillaries grow into clot - phagocytes and osteoclasts remove damaged tissue - procallus forms and is invaded by osteoprogenitor cells and fibroblasts - collagen and fibrocartilage turns procallus to fibrocartilagenous (soft) callus - broken ends of bone are bridged by callus - Osteoprogenitor cells are replaced by osteoblasts and spongy bone is formed - bony (hard) callus is formed - callus is resorbed by osteoclasts and compact bone replaces spongy bone. Remodeling : the shaft is reconstructed to resemble original unbroken bone. Closed reduction - bone ends coaxed back into place by manipulation Open reduction - surgery, bone ends secured together with pins or wires n Simple/Closed: bone breaks cleanly, but does not penetrate skin n Compound/Open: broken ends of bone protrude through tissue and skin n Comminuted bone fragments into many pieces n Compression: bone is crushed ( due to porous bone) n Depressed: broken bone is pressed inward (e.g. in skull) n Colles’: posterior displacement of distal end of radius (extension) n Smith’s: anterior displacement of distal end of radius (flexion) n Transverse: break occurs across the long axis of a bone n Impacted: broken bone ends are forced into each other n Spiral: ragged break as a result of excessive twisting of bone n Epiphyseal: break occurring along epiphyseal line/plate n Greenstick: bone breaks incompletely n Pott's: malleolus of tibia and fibula break