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Bone

Normal anatomy


Reviewers: Dariusz Borys, M.D., (see Reviewers page)
Revised: 5 February 2012, last major update January 2012
Copyright: (c) 2003-2012, PathologyOutlines.com, Inc.

General
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Basic function of bone:
● Bone is the basic unit of the skeletal system and provides shape and support for the body, as well as protection for some organs
● There are 206 bones in the human skeleton: 80 axial skeletal bones (e.g. skull, vertebral column and sacrum) and 120 appendicular skeletal bones (e.g. bones of extremities, scapula, pelvis)

Gross structure of bone
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Epiphysis region between the growth plate or growth plate scar and the extended end of bone, covered by articular cartilage
Metaphysis region between the growth plate and diaphysis; contains abundant trabecular bone, but the cortical bone thins here comparing to diaphysis
Diaphysis or shaft region between metaphyses, composed mainly of compact cortical bone
Physis (epiphyseal plate, growth plate) region of bone that separates the epiphysis from metaphysis
     ● Zone of endochondral ossification in actively growing bone or the epiphyseal scar in a full grown bone

● Cross section: periosteum, cortex (composed of cortical bone or compact bone), medullary space (composed of cancellous or spongy bone)
● Bone composition: 35% organic (cells, proteins), 65% calcium hydroxyapatite (contains 99% of body's calcium, 85% of phosphorus, 65% of sodium, also magnesium)
● Hydroxyapatite crystal is formed via phase transition; 12 day lag between matrix deposition and mineralization
● Collagen resists tension, hydroxyapatite and proteoglycans in cartilage resist compression
● Thicker cortex in middle of long bones resists bending; cancellous bone at ends of long bones resists compression

Classification
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● Bones are divided on the basis of their location, shape, size and structure

Based on location, bones can be classified as:
● Axial skeleton bones of the skull, scapula, vertebral column
● Appendicular skeleton bones of the pectoral girdle, pelvis and limbs

Based on shape, bones can be classified as:
● Flat bone bones of the skull, sternum, pelvis and ribs
● Tubular bone long tubular bones are bones of the extremities (e.g. femur, humerus); short tubular bones are bones of hands and feet
● Irregular bone bones of the face and vertebrae
● Sesamoid bones patella

Based on size, bones can be classified as:
● Long bone tubular bones of extremities (e.g. femur, humerus)
● Short bone cuboidal in shape, in the foot (tarsal bones) and wrist (carpal bones)

Blood supply
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● The blood supply of bone varies with different types of bone, but vascular supply is especially rich in bones rich in red bone marrow

Long bones:
● Diaphyseal nutrient artery most important arterial supply, passes obliquely through cortical bone
● Metaphyseal and epiphyseal arteries numerous small arteries supply the ends of bones; these blood vessels arise from arteries that supply adjacent joints, anastomose with the diaphyseal capillaries and terminate in bone marrow
● Periosteal arterioles - these vessels supply the outer layers of cortical bone

Large irregular bones, short bones and flat bones:
● These bones are supplied by superficial periosteal arterioles

Venous and lymphatic drainage:
● Blood is drained from the bone via venous and lymphatic vessels that accompany arteries and frequently leave through foramina near the articular end of the bones

Nerve supply of bone:
● Nerves are most rich in articular extremities of long bones, vertebrae and larger flat bones
● Nerves accompany the blood vessels to the interior of the bone and to the perivascular spaces of the haversian canals
● The periosteal nerves are sensory, causing periosteum to be particularly sensitive to tearing or tension

Bone tissue types and structure
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● Bone tissue can be classified based on texture, matrix arrangement; also maturity and developmental origin (see Histology topic)

Based on texture, bone can be classified as:
Compact bone (dense bone, cortical bone) dense bone that surrounds trabecular bone in the center, contains Haversian system and secondary osteons
Sponge bone (trabecular bone, cancellous bone) sponge-like with numerous cavities, located in the center of bone cavity, consists of connected bony trabeculae

Based on matrix arrangement bone can be classified as:
Lamellar bone mature bone with collagen fibers arranged in lamellae
     ● Lamellae of sponge bone are arranged parallel to each other
     ● In contrast, lamellae of compact bone are organized concentrically to around vascular canal (haversian canal)
Woven bone immature bone; collagen fibers in woven bone are arranged in irregular random arrays and contain smaller amounts of mineral substance and a higher proportion of osteocytes to lamellar component
     ● Woven bone is eventually converted to lamellar bone

Drawings
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Skeleton with major bones labeled

               
Femur

       
Humerus

   
Patella

   
Pelvis: left-male, right-female

   
Radius and ulna

       
Tibia and fibula

End of Bone > Normal anatomy


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