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Skeletal System. Chapter 5. Skeletal System. Two divisions: Axial Skeleton (“axis”) Head & trunk Appendicular Skeleton (“appendages”) limbs & their attachments to trunk Also includes: Joints Cartilages Ligaments. Functions of Bones. Support Internal framework, cradles soft organs
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Skeletal System Chapter 5
Skeletal System • Two divisions: • Axial Skeleton (“axis”) • Head & trunk • Appendicular Skeleton (“appendages”) • limbs & their attachments to trunk • Also includes: • Joints • Cartilages • Ligaments
Functions of Bones • Support • Internal framework, cradles soft organs • Protection • i.e. skull, vertebrae, rib cage • Movement • Used as levers by muscles • Storage • Fat is stored in yellow marrow (internal cavities) • Minerals stored in bone (calcium & phosphorus) • Blood Cell Formation • Hematopoiesis occurs in red marrow
Classification of Bone • 206 bones in adult skeleton • Two types of osseous tissue: • Compact bone – dense, looks smooth & homogenous • Spongy bone – small needlelike pieces of bone & lots of open space
Classification of Bones • Classified according to shape • Long bones – longer than they are wide; contain mostly compact bone • i.e. Bones of the limbs • Short bones – cube-shaped, contain mostly spongy bone • i.e. Bones of wrist and ankle, patella, sesamoid bones (within tendons) • Flat bones – thin, flattened, usually curved; compact bone sandwiches layer of spongy bones • i.e. Bones of the skull, ribs, sternum • Irregular bones – do not fit into one of the preceding categories • i.e. Vertebrae and hip bones
Gross Anatomy of a Long Bone (Hyaline) metaphysis (disc) (DCT) bone labeling exercise
Microscopic Anatomy of the Bone • Osteocytes (mature bone cells) • Lacunae (cavities in matrix) • Lemellae (concentric circles around central canal) • Central (Haversian) canal (carry blood vessels and nerves to all areas of the bone) • Osteon (Haversian System) • Canaliculi (radiate outward from central canal to all lacunae) • Volkmann’s canal (communication from exterior to interior of bone)
Bone Tissue • Connective Tissue: calcified matrix with abundant collagen fibers • Four types of cells: • Osteogenic cells: unspecialized from stem cells; found along periosteum and endosteum • Make osteoblasts • Osteoblast: bone-building cells – synthesize and secrete collagen fibers • Osteocyte: mature cells – maintain metabolism • Osteoclasts: huge collection of white blood cells found in endosteum • release acids and enzymes that digest bone matrix (resorption)
Anatomical pumpkins!!
Bone Formation • Ossification (osteogenesis): formation of bone • Begins in 6th week of pregnancy • Two patterns: • Intramembraneous ossification: flat bones • i.e. skull & clavicles • Endochondral ossification: most bones • i.e. long bones
Intramembraneous Ossification • Flat bones form on fibrous membranes • Mesenchymal cells cluster and form osteoblasts which harden • Form ossification center in membrane • Osteoblasts secrete bone matrix which mineralizes and traps cells in bones (become osteocytes) • Trabeculae then form followed by the periosteum forming around bone • Trabeculae then thicken to form bone collar and deeper remain distinct as spongy bone and eventually red marrow
Endochondral Bone Development • Most bones develop using a hyaline cartilage model • Fetal skeleton = hyaline cartilage (formed from mesenchyme) • Two phases (Fig 5.5, page 140): 1. hyaline cartilage model covered with bone matrix (bone “collar”) by osteoblasts • Primary ossification: develops inward from outer surface & forms spongy bone (eventually compact bone) • Secondary ossification: develops outward (from center of epiphysis) 2. enclosed hyaline cartilage model digested away, opening up medullary cavity within newly formed bone • Osteoclasts break down center to form medullary cavity • By birth or shortly after, all cartilage converted to bone except articular cartilages (cover bone ends) and epiphyseal plates • Adult skeleton = cartilage exists in nose, parts of ribs, and joints
Bone Growth • Bones increase in length (interstitial growth) & width (appositional growth) • Epiphyseal plate: layer of hyaline cartilage in metaphysis of growing bone; new bone forms on diaphysis side • Interstitial growth (length): from epiphyseal plate • Four zones; close around 18-21 (18-females, 21-males) • Resting cartilage: nearest epiphysis, anchor plate to bone (not part of growing bone) • Proliferating cartilage: cells divide & replace dying cells • Hypertrophic cartilage: matruingchondrocytes; arranged in columns • Calcified cartilage: cells harden & die; replaced by bone tissue • Osteoclasts dissolve and osteoblasts and blood vessels enter area
Appositional Bone Growth • Growth in thickness of the bone • Cells in periosteum differentiate into osteoblasts while osteoclasts increase medullary cavity • Osteoblasts in periosteum add bone tissue to external face of diaphysis as osteoclasts in endosteum remove bone from inner surface • Occur at about the same rate
Bone Remodeling • Bone resorption & deposition used to renew and replace injured bone • Affected by minerals, vitamins, and hormones • Resorption: removal of minerals and collagen fibers by osteoclasts • Deposition: addition of minerals and collagen fibers by osteoblasts
Bone Remodeling • Bones are remodeled continually in response to changes in two factors: • Calcium levels in the blood • Parathyroid gland releases PTH when blood calcium levels drop • PTH activates osteoclasts to break down bone matrix and release Ca2+ into blood • Hypercalcemia: Ca2+ is deposited into bone matrix as hard calcium salts • Pull of gravity and muscles on the skeleton • Shape of bone altered for stress • Osteoblasts lay down new matrix & become trapped within it (become osteocytes) where bulky muscles attach (due to stress) • Bedridden/inactive: lose mass & atrophy (no stress) • Ongoing replacement of old bone tissue by new bone tissue • PTH determines when/if bone broken down or formed in response to need for more or fewer Ca2+ ions in the blood; stress determines where bone matrix is to be broken down or formed • Helps maintain skeletal strength
Bone Fracture Repair • Repair involves four major events: • Hematoma forms • From ruptured blood vessels, cells die that are deprived of oxygen • Fibrocartilage callus forms • growth of new capillaries (granulation tissue) into clotted blood at site of damage and disposal of dead tissue by phagocytes. • CT cells form mass of repair tissue which splints broken bone and closes gap (made up of bony matrix, collagen, collagen matrix)
Bone Fracture Repair 3. Bony callus forms • Fibrocartilage callus is gradually replaced by the bony callus (made of spongy bone) as more osteoblasts and osteoclasts migrate to area and multiply 4. Bone remodeling occurs • Bony callus is remodeled in response to mechanical stresses placed on it
Skeletal System Anatomy Bone Markings
Skeletal System Anatomy 206 total bones
Axial Skeleton • Three parts: • Skull • Vertebral column • Thoracic cage
Skull: 22 bones • Cranial (8), Facial (13), Mandible (1) • Protects brain • Many paired bones • Page 148-149
Skull • Infants have fontanels (soft spots) • Bones of the skull are not fused yet • Adults: skull bones fused with sutures • Immoveable joints • Mandible: freely moving joint
Middle Ear: 6 bones • Three in each ear • Stapes (stirrup) • Incus (anvil) • Malleus (hammer)
Hyoid Bone • Does not articulate with any other bone • Often broken during strangulation • CT attaches to larynx & trachea
Vertebral Column: 26 bones • Strong, flexible, rotates • Protects spinal cord & supports head • Intervertebral discs between made of fibrocartilage • Shock absorption & spine flexibility • High in water content when young (spongy, compressible); discs harden with age
Vertebral Column: 26 bones • Born with 33 - fuse to 26 • 7 cervical (neck)- smaller, bifed clef • C1: atlas (articulates with occipital condyle) • C2: axis • 12 thoracic (chest)- stronger, long spine • 5 lumbar (lower back)- strongest, short spine • Sacrum (5 fused by mid 20s) • Coccyx (4 fused by 30)- tail bone
Curvatures of the Spine • Four normal curvatures • Fetus has 2 primary curvatures (thoracic and sacral) • Secondary curvatures develop after birth • Cervical (3 months) and lumbar (6 months) • Cervical: concave (anterior) • Thoracic: convex (posterior) • Lumbar: concave (anterior) • Sacrum: convex (posterior)
Thoracic Cage: 25 Bones • Sternum: fusion of 3 bones, attached to first 7 ribs • 12 pairs of ribs (1-7 increase in size, 8-12 decrease in size) • Attached by costal cartilage • True ribs (1-7)- direct attachment • False ribs (8-12)- no anterior attachment • Floating ribs (11-12)- no anterior attachment • All have posterior attachment to vertebrae
Appendicular Skeleton 126 bones: limbs, pectoral & pelvic girdles (attach limbs to axial skeleton)
Pectoral Girdle: 4 bones • Attach arms; two of each • Clavicle: collar bones • s-shaped- weak juncture • Attaches to manubrium and sternum medially and scapula posteriorly to form shoulder joint • Prevents dislocation • Scapula: shoulder bones
Upper Limbs: 60 bones • 8 carpals each • 5 metacarpals • 14 phalanges • 2 in thumb • Radius (thumb side) • Ulna (pinky side)
Pelvic Girdle: 2 bones • Function: bearing weight (total weight of upper body) & protects reproductive and urinary systems • United at pubic symphysis (fibrocartilage; some flexibility) • Attached to axial skeleton via sacral attachment to lower lumbar vertebrae • Three parts fuse at birth • Ilium • Pubis • Ischium
Lower Limbs: 60 bones • Femur, tibia, fibula, patella • 7 tarsals, 5 metatarsals, 14 phalanges • Carry our total body weight when erect • much thicker/stronger than upper limbs
Male vs. Female • Males bones larger and heavier • Angle of pubic symphysis less for men • Page 163 lists differences
Fetal Skeleton • Arises from mesenchymal cells (derived from mesoderm) • Intramembraneous and endochondral ossification • 1st long bones = hyaline cartilage • 1st flat bones of skull = fibrous membranes