BIO 2401 BONES & BONE TISSUE page 1 BONES AND BONE TISSUE Organization of the Skeletal System components: 1) bone 2) skeletal cartilage: surrounded by dense irregular connective tissue which acts to girdle the cartilage to prevent it from deforming too much under stress hyaline cartilage precursor of endochondral bones; flexible and resilient elastic cartilage cartilage subjected to repeated bending fibrocartilage highly compressible with great tensile strength Major Functions of skeletal system 1. support provides hard framework to support body and cradle its soft organs 2. protection provides protective framework encasing for body structures/organs 3. movement used as a lever system to move body and its parts; arrangement of bones and design of joints determines types of movement possible 4. mineral storage retrievable storage for calcium and phosphate for release into blood 5. blood cell formation most occurs within marrow cavities of certain bones (ribs, sternum, long bones) Ways to Classify Bones: a. Based on Shape (1) long bones longer than wide; has shaft plus two ends (ex. limb bones) (2) short bones cube shaped bones (ex. carpal and tarsal elements and sesamoid bones that form within tendons) (3) flat bones are thin flattened and usually a bit curved (ex. skull roofing bones, sternum and scapula) (4) irregular bones bones that have complicated shapes (ex. includes vertebrae and brain case) b. Based on Formation (1) membrane or dermal bones bones that form within a collagen membrane (ex. skull roofing bones frontals and parietals) (2) endochondral bones bones that are preformed in hyaline cartilage and then transformed into bone (ex. long bones, vertebrae) Structure and Histology of a Long bone structure: diaphysis shaft of bone cross section shows from outside toward inside the following layers: (1) periosteum (2) compact bone (3) medullary cavity for marrow epiphyses ends of bones that form articular surfaces; have thin layer of compact bone that is underlain by cancellous or spongy bone
BIO 2401 BONES & BONE TISSUE page 2 periosteum outer wrapping of bone made up of collagen (dense irregular connective tissue); inner layers are osteogenic and contain osteoblasts (bone forming cells), osteoclasts (bone destroying cells; this layer is richly supplied by blood vessels, nerves and lymphatic vessels articular cartilage covers joint surfaces of epiphyses; made up of hyaline cartilage; acts to cushion stresses during joint movement epiphyseal line remnants of epiphyseal growth plate, a band of actively dividing hyaline cartilage that acts to lengthen bone medullary cavity marrow cavity that contains blood forming tissue (red marrow) or yellow or fat marrow; lined with an endosteal membrane histology: outer layer = periosteum layers of collagen that surround bone; during growth will have bone forming cells and fibrocytes inside epiphyses = membrane that lines medullary cavity and trabecular system inside bone = endosteum; contains osteoblasts and osteocytes Structure of Short, Irregular, and Flat Bones consist of thin plates of periosteum covered compact bone on outside endosteum covered spongy bone is internal no shaft or epiphyses contain some marrow but no marrow cavity Microscopic Anatomy of Bone Tissue 1. compact bone composed of lamellar and Haversian bone osteon concentric cylinders of bone that usually run in the long axis of bone and support stress and weight of bone osteocyte ameboid bone cells that maintain bone matrix lacunae small cavities in which bones cells reside lamellae each layer of concentric tube of an osteon (Haversian system) Haversian canal central canal of Haverian system that contains blood vessels and nerves canaliculi canals of radiating out from lacunae and housing pseudopods of osteocytes; mechanism of nutrient transfer from one osteocyte to another Volkmann's canal tranversely arranged canals that bring blood vessels into the Haversian canals 2. spongy bone trabeculae system of plates and spicules supporting epiphyses of bones; plates and spicules are arranged along lines of stress and are only a few layers or lamellae thick Types of Bone Cells osteoblasts embryonic bone cells that lay down bone matrix osteocytes mature bone cells that are derived from osteoblasts and are trapped in bony matrix osteoclasts bone cells that break down and remodel bone; derived from hemopoietic stem cells and macrophages; use acids to destroy bone; are able to phagocytize demineralized and dead osteocytes
BIO 2401 BONES & BONE TISSUE page 3 Bone Formation and Remodeling A. endochondral ossification all bones from the brain case down except the clavicles bone is preformed in hyaline cartilage; ossification begins in shaft at a primary ossification center (other centers occur in the epiphyses) when chondrocytes near shaft center enlarge and their surrounding matrix calcifies; this kills the chondryocytes which then disintegrate perichondrium becomes infiltrated with blood vessels that break into the eroding cartilage; cells in the lower layers of the periosteal membrane (formerly the perichondrium) are differentiated into osteoblasts this converts the perichondrium into a periosteum and its inner layer is called the osteogenic layer osteoblasts in osteogenic layer form a bony collar around cartilage; chondrocytes hypertrophy (enlarge) and signal other cartilage cells to secrete osteoid (calcium phosphate matrix); spaces left by disintegrating chondrocytes are invaded by blood vessels most of the cartilage is replaced by bone except in a band on either end of the shaft facing the epiphyses (called the metaphysis) cartilage in the center of forming bone dies and forms the marrow cavity; invading bone cells form spongy areas under the joint surfaces as diaphysis enlarges osteoclasts erode the central portion (filled with spongy bone) and create the marrow cavity length increases then occur at metaphyses; at shaft end of metaphyses, osteoblasts are continually invading cartilage and replacing it with bone; at epiphyseal end of metaphyses, new cartilage is produced at same rate at time of birth, centers of epiphyses begin to calcify and capillaries and osteoblasts migrate into these areas (called secondary ossification centers); this fills epiphyses with spongy bone, but at the proximal or distal-most end of the bone, cartilage remains to form articulating cartilage protecting bones from grinding against each other; at metaphyses, cartilage band remains to permit bone length growth (called epiphyseal plate) with ossification near shaft and cartilage growth near epiphysis B. intramembranous (dermal) ossification bone is formed within a fibrous membrane (the periosteum) embryonic cells within the periosteum form osteoblasts within the connective tissue osteoblasts cluster together and secrete organic components of matrix including collagen fibers that form the scaffolding or framework for bone formation and osteoid ossification occurs in the eighth week of development via a process of crystallizing calcium salts and forms an ossification center developing bone grows outward in small struts or spicules and osteoblasts become entrapped and entombed within the bone; they are then called osteocytes new osteoblasts continue to be formed from embryonic cells to continue process; are supplied by blood vessels that grow between the spicules this forms spongy bone which is then remodeled into compact bone as marrow cavities are formed examples of these bones are clavicles, mandible, patella and the skull roofing bones such as the frontal, parietal and zygomatic C. bone growth post-natal growth: long bones lengthen by interstitial growth of epiphyseal plates all bones grow in thickness by appositional growth all bones except facial bones stop growth in early adulthood
BIO 2401 BONES & BONE TISSUE page 4 length increases in bone mimics endochondral ossification cartilages stack up at the epiphyseal plate; those cells on top undergo rapid mitosis and push epiphyses away from diaphysis those on bottom, hypertrophy (lacunae enlarge and then erode); ossification occurs and leaves long spicules of calcified cartilage at epiphysis/diaphysis junction spicules are invaded by marrow elements from medullary cavity; osteoclasts erode them and they are then covered with bone matrix by osteoblasts to form spongy bone chondroblasts divide less often in plate region at close of adolescence (18 for females; 21 for males) when bone of epiphysis and bone of diaphysis fuse ( epiphyseal closure ), growth ends effect of hormones on bone growth: 1. growth hormone released by anterior pituitary gland and modulated by thyroid hormone; acts to stimulate epiphyseal plate activity too much growth hormone = giantism too little growth hormone = dwarfism 2. sex hormones initially promotes growth spurts and masculinization or feminization of specific parts of the skeleton D. bone remodeling occurs continually in response to: 1. Ca +2 levels in the blood blood calcium causes release of parathyroid hormone stimulates osteoclasts to reabsorb bone and release clacium into blood turns off calcitonin production blood calcium shuts off release of parathyroid hormone stimulates secretion of calcitonin inhibits bone reabsorption and stimulates calcium deposition in bone matrix 2. stress on bones from gravity and muscles bone grows or remodels in response to forces placed upon it weight is put on bones in an assymetrical way so that bone is stretched on one side and compressed on other in long bones bending stresses are midway down shaft the neck regions also bear the most stress cancellous bone best supports compression under joints Types of Bone Fractures a. classified by position of bone ends after fracture: non displaced bone ends are in natural position displaced bone ends are out of alignment b. by completeness of break complete fracture or incomplete fracture. c. by orientation of break to long axis of bone: linear = parallel to long axis or transverse = perpendicular to long axis d. whether bone ends penetrate the skin: open (compound fracture), closed (simple)
BIO 2401 BONES & BONE TISSUE page 5 Healing of fractures a. fracture hematoma occurs because blood vessels in bone, periosteum, and surrounding tissues are torn and hemorrhage hematoma is the mass of clotted blood that forms at fracture site bone cells deprived of nutrition die and site becomes swollen and painful b. fibrocartilage callus granulation tissue forms capillaries grow into the hematoma; phagocytocic cells clean up debris fibroblasts and osteoblasts migrate into fracture site from periosteum and endosteum and begin reconstructing bone fibroblasts form collagen fibers that connect broken end osteoblasts form woven bone c. bony callus osteoblasts lay down new bone, trabeculae in the fibrocartilage callus gets converted into bony callus takes 3-4 weeks continues for 2-3 months before stopping Effects of aging on skeletal system a. estrogen estrogen levels drop during menopause; affects calcium absorption b. insufficient exercise no stress leads to bone reabsorption c. diet poor in calcium and protein leads to osteomalacia bones inadequately mineralized d. vitamin D and calcitonin metabolism also leads to osteomalacia e. smoking reduces estrogen levels f. hormone related conditions (corticosteroid drugs) Osteoporosis bone reabsorption outpaces bone deposition bone mass is reduced estrogen and testosterone help restrain osteoclast activity and promote bone growth peak density is reached between 35-40 years occurs more frequently in menopausal females because males secrete testosterone throughout life