BONES AND BONE TISSUE

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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

ƒ hyaline cartilage – precursor of endochondral bones; flexible and resilient ƒ elastic cartilage – cartilage subjected to repeated bending

ƒ 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

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• 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

ƒ consist of thin plates of periosteum covered compact bone on outside ƒ endosteum covered spongy bone is internal

ƒ no shaft or epiphyses

ƒ 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

ƒ 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

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Bone Formation and Remodeling

A. endochondral ossification – all bones from the brain case down except the clavicles

ƒ 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

ƒ 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

ƒ 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

ƒ most of the cartilage is replaced by bone except in a band on either end of the shaft facing the epiphyses (called the metaphysis)

ƒ cartilage in the center of forming bone dies and forms the marrow cavity; invading bone cells form spongy areas under the joint surfaces

ƒ as diaphysis enlarges osteoclasts erode the central portion (filled with spongy bone) and create the marrow cavity

ƒ 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

ƒ 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) ƒ embryonic cells within the periosteum form osteoblasts within the connective tissue

ƒ osteoblasts cluster together and secrete organic components of matrix including collagen fibers that form the scaffolding or framework for bone formation and osteoid

ƒ ossification occurs in the eighth week of development via a process of crystallizing calcium salts and forms an ossification center

ƒ developing bone grows outward in small struts or spicules and osteoblasts become entrapped and entombed within the bone; they are then called osteocytes

ƒ new osteoblasts continue to be formed from embryonic cells to continue process; are supplied by blood vessels that grow between the spicules

ƒ this forms spongy bone which is then remodeled into compact bone as marrow cavities are formed ƒ examples of these bones are clavicles, mandible, patella and the skull roofing bones such as the

frontal, parietal and zygomatic C. bone growth

ƒ post-natal growth:

ƒ long bones lengthen by interstitial growth of epiphyseal plates ƒ all bones grow in thickness by appositional growth

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ƒ length increases in bone mimics endochondral ossification

ƒ cartilages stack up at the epiphyseal plate; those cells on top undergo rapid mitosis and push epiphyses away from diaphysis

ƒ those on bottom, hypertrophy (lacunae enlarge and then erode); ossification occurs and leaves long spicules of calcified cartilage at epiphysis/diaphysis junction

ƒ 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 ƒ chondroblasts divide less often in plate region at close of adolescence (18 for females;

21 for males)

ƒ 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

ƒ too much growth hormone = giantism ƒ 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

• Ð blood calcium causes release of parathyroid hormone

• stimulates osteoclasts to reabsorb bone and release clacium into blood • turns off calcitonin production

• Ï 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

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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

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