Bone - Normal anatomy

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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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