6 PART 1
Bones and Skeletal Tissues
Cartilage
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Location and basic structure of cartilages
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Found throughout adult body
• Cartilage in the external ear
• Cartilage of the nose
• Articular cartilages and costal cartilage
• Cartilages in the larynx and trachea
• Intervertebral discs, pubic symphysis, and articular discs
Cartilage
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Perichondrium
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Surrounds cartilages
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Resists outward pressure
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Functions in growth and repair of cartilage
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Consists primarily of water
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Is a resilient tissue
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Springs back to original shape
Types of Cartilage
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All cartilages share some similarities
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Cell type is the chondrocyte
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Chondrocytes are located within lacunae
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Matrix contains
• Fibers
• Jellylike ground substance
Types of Cartilage
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Hyaline cartilage
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Most abundant cartilage
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Chondrocytes appear spherical
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Collagen unit fibril is the only type of fiber in the matrix
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Ground substance holds a large amount of water
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Provides support through flexibility
Types of Cartilage
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Elastic cartilage
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Contains many elastic fibers
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Able to tolerate repeated bending
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Locations—epiglottis and cartilage of external ear
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Fibrocartilage
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Resists strong compression and strong tension
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An intermediate between hyaline and elastic cartilage
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Locations—pubic symphysis, menisci of knee, anulus fibrosus
Growth of Cartilage
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Appositional growth
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Chondroblasts in surrounding perichondrium produce new cartilage
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Interstitial growth
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Chondrocytes within cartilage divide and secrete new matrix
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Cartilage stops growing when the skeleton stops growing
Tissues in Bone
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Bones contain several types of tissues
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Dominated by bone connective tissue
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Contain nervous tissue and blood connective tissue
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Contain cartilage in articular cartilages
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Contain epithelial tissue lining blood vessels
Function of Bones
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Support—provide hard framework
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Movement—skeletal muscles use bones as levers
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Protection of underlying organs
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Mineral storage—reservoir for important minerals
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Blood cell formation—bone contains red marrow
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Energy metabolism—osteoblasts secrete osteocalcin
Bone Tissue
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Bone tissue
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Organic components
• Cells, fibers, and ground substance
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Inorganic components
• Mineral salts that invade bony matrix
Extracellular Matrix
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Unique composition of matrix
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Gives bone exceptional properties
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35%—organic components
• Contribute to flexibility and tensile strength
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65%—inorganic components
• Provide exceptional hardness, resist compression
Cells
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Three types of cells in bone produce or maintain bone
Osteogenic cells—stem cells that differentiate into osteoblasts
Osteoblasts—actively produce and secrete bone matrix
• Bone matrix is osteoid
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Osteocytes—keep bone matrix healthy
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Cells
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Osteoclasts
Found within bone tissue
Responsible for resorption of bone
Are derived from a line of white blood cells
Secrete hydrochloric acid and lysosomal enzymes
Classification of Bones
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Long bones—longer than wide; a shaft plus ends
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Short bones—roughly cube-shaped
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Flat bones—thin and flattened, usually curved
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Irregular bones—various shapes, do not fit into other categories
Gross Anatomy of Bones
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Compact bone
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Dense outer layer of bone
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Spongy (cancellous) bone
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Internal network of bone
• Trabeculae—little “beams” of bone
• Open spaces between trabeculae are filled with marrow
Structure of a Typical Long Bone
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Diaphysis—“shaft” of a bone
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Epiphyses—ends of a bone
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Blood vessels—well vascularized
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Medullary cavity—hollow cavity filled with yellow marrow
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Membranes
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Periosteum, perforating collagen fiber bundles (Sharpey’s fibers), and endosteum
Structure of Short, Irregular, and Flat Bones
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Flat bones, short bones, and irregular bones
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Contain bone marrow but no marrow cavity
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Diploë
• Internal spongy bone of flat bones
Gross Anatomy of Bones
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Bone design and stress
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Anatomy of a bone reflects stresses
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Compression and tension greatest at external surfaces
Bone Markings
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Superficial surfaces of bones reflect stresses on them
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There are three broad categories of bone markings:
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Projections for muscle attachment
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Surfaces that form joints
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Depressions and openings
Microscopic Structure of Compact Bones
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Compact bone
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Contains passage ways for blood vessels, lymph vessels, and nerves
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Osteons
• Long cylindrical structures
• Function in support
• Structurally resemble rings of a tree in cross section
Microscopic Structure of Compact Bones
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Osteons contain:
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Lamellae
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Central canal
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Perforating canals
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Canaliculi
Microscopic Structure of Compact Bones
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Spongy bone
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Is less complex than compact bone
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Trabeculae contain layers of lamellae and osteocytes
• Are too small to contain osteons
6 PART 2
Bones and Skeletal Tissues
Bone Development
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Ossification (osteogenesis)—bone tissue formation
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Membrane bones—formed directly from mesenchyme
• Intramembranous ossification
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Other bones—develop initially from hyaline cartilage
• Endochondral ossification
Endochondral Ossification
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All bones except some bones of the skull and clavicles
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Bones are modeled in hyaline cartilage
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Begins forming late in the second month of embryonic development
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Continues forming until early adulthood
Anatomy of Epiphyseal Growth Areas
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In epiphyseal plates of growing bones:
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Cartilage is organized for quick, efficient growth
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Cartilage cells form tall stacks
• Chondroblasts at the top of stacks divide quickly
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Pushes the epiphysis away from the diaphysis
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Lengthens entire long bone
Anatomy of Epiphyseal Growth Areas
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Older chondrocytes signal surrounding matrix to calcify
Older chondrocytes then die and disintegrate
Leaves long trabeculae (spicules) of calcified cartilage on diaphysis side
Trabeculae are partly eroded by osteoclasts
Osteoblasts then cover trabeculae with bone tissue
Trabeculae finally eaten away from their tips by osteoclasts
Postnatal Growth of Endochondral Bones
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During childhood and adolescence:
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Bones lengthen entirely by growth of the epiphyseal plates
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Cartilage is replaced with bone connective tissue as quickly as it grows
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Epiphyseal plate maintains constant thickness
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Whole bone lengthens
Postnatal Growth of Endochondral Bones
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As adolescence draws to an end:
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Chondroblasts divide less often
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Epiphyseal plates become thinner
• Cartilage stops growing
• Replaced by bone tissue
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Long bones stop lengthening when diaphysis and epiphysis fuse
Postnatal Growth of Endochondral Bones
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Growing bones widen as they lengthen
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Osteoblasts—add bone tissue to the external surface of the diaphysis
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Osteoclasts—remove bone from the internal surface of the diaphysis
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Appositional growth—growth of a bone by addition of bone tissue to its surface
Hormonal Regulation of Bone Growth
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Growth hormone—produced by the pituitary gland
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Stimulates epiphyseal plates
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Thyroid hormone—ensures that the skeleton retains proper proportions
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Sex hormones (estrogen and testosterone)
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Promote bone growth
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Later induces closure of epiphyseal plates
Bone Remodeling
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Bone is a dynamic living tissue
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500 mg of calcium may enter or leave the adult skeleton each day
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Bone matrix and osteocytes are continually removed and replaced
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Cancellous bone of the skeleton is replaced every 3–4 years
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Compact bone is replaced every 10 years
Bone Remodeling
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Bone deposit and removal
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Occurs at periosteal and endosteal surfaces
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Bone remodeling
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Bone deposition—accomplished by osteoblasts
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Bone reabsorption—accomplished by osteoclasts
Osteoclast—A Bone-Degrading Cell
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A giant cell with many nuclei
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Crawls along bone surfaces
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Breaks down bone tissue
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Secretes concentrated HCl
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Lysosomal enzymes are released
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Derived from hematopoietic stem cells
Repair of Bone Fractures
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Simple and compound fractures
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Treatment by reduction
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Closed reduction
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Open reduction
Disorders of Bones
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Osteoporosis
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Characterized by low bone mass
• Bone reabsorption outpaces bone deposition
• Occurs most often in women after menopause
Disorders of Bones
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Osteomalacia
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Occurs in adults—bones are inadequately mineralized
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Rickets
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Occurs in children—analogous to osteomalacia
Disorders of Bones
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Osteosarcoma
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A form of bone cancer
The Skeleton Throughout Life
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Cartilage grows quickly in youth
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Skeleton shows fewer chondrocytes in the elderly
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Bones are a timetable
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Mesoderm
• Gives rise to embryonic mesenchyme cells
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Mesenchyme
• Produces membranes and cartilage
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Membranes and cartilage ossify
The Skeleton Throughout Life
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Skeleton grows until the age of 18–21 years
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In children and adolescents, bone formation exceeds rate of bone reabsorption
In young adults, bone formation and bone reabsorption are in balance
In old age, reabsorption predominates
Bone mass declines with age