Chapter 7

Bone Structure

Bone Classification• Long Bones

– Have longitudinal axes and expanded ends• Ex: forearms, thigh bones

• Short Bones– Have lengths and widths that are roughly equal

• Ex: wrist and ankle bones

• Flat Bones– Have broad surfaces

• Ex: ribs, scapulae, some skull bones

• Irregular Bones– Have a variety of shapes and are usually connected to several other bones

• Ex: vertebrae, many facial bones

• Sesamoid (Round) Bones – Usually small and nodular and are embedded within tendons adjacent to

joints where the tendons are compressed.• Ex: patella

Bone Classification

Parts of Long Bones• Epiphysis

– Expanded portion on the end of the bone– Forms joints with other bones

• The proximal epiphysis is closest to the torso• The distal epiphysis is farthest from the torso

• Articular cartilage– Layer of hyaline cartilage that covers the epiphysis where it articulates with other bones

• Diaphysis– Long part of the bone between the epiphyses

• Periosteum– Membrane made of tough, vascular fibrous tissue that encloses all the parts of the bone

that are not covered by articular cartilage.• Periosteal fibers are continuous with ligaments and tendons that are connected to the membrane

– Helps form and repair bone tissue• Processes

– Bony projections that provide sites for attachment of ligaments and tendons• Grooves, openings, and depressions

– Provide spots for the passages of nerves, blood vessels, and other bones

Parts of Long Bones

Structure of Long Bones• Compact (or cortical) bone– Makes up the wall of the diaphysis– Has a continuous extracellular matrix with no gaps– Forms a semi-rigid tube with a hollow chamber called the medullary

cavity that is continuous with the spaces of the spongy bone• A thin membrane containing bone-forming cells, called endosteum, lines

these areas and a specialized type of soft connective tissue called marrow fills them

• Spongy (or cancellous) bone– Makes up the majority of epiphyses (covered by a thin layer of

compact bone)– Consists of many branching bony plates called trabeculae

• Irregular connecting spaces between these plates help reduce the weight of the bone

• Most highly developed in regions of epiphyses that are subjected to compressive forces

Structure of Long Bones

Structure of Other Bones

• Bones usually have compact bone overlying spongy bone, with the relative amounts of each varying in differently shaped bones.– Short, flat, and irregular bones typically consist of

a mass of spongy bone that is either covered by a layer of compact bone or sandwiched between plates of compact bone.

Structure of Other Bones

Microscopic Structure• Bone cells are called osteocytes.– Osteocytes are located in tiny, bony chambers called

lacunae– Lacunae form concentric circles around central canals

(Haversian canals)– Osteocytes transport nutrients and wastes to and

from nearby cells by means of cellular processes passing through canaliculi

• The extracellular matrix of bone tissue is largely collagen and inorganic salts– Collagen gives bone its strength and resilience– Inorganic salts make bone hard and resistant to

crushing

Microscopic Structure• Compact Bone– Contain many osteons (or Haversian systems) cemented

together• An osteon is a cylindar-shaped unit formed by osteocytes and layers of

extracellular matrix that are concentrically clustered around a central canal

• The orientation of the osteons resists compressive forces

– Central canals extend longitudinally through the bone tissue and are connected to one another through transverse perforating canals (or Volkmann’s canals)• Each central canal contains blood vessels and nerve fibers surrounded

by loose connective tissue– Blood in the vessels nourishes bone cells associated with the central canal via

gap junctions between osteocytes

• Perforating canals contain larger blood vessels and nerves by which the smaller blood vessels and nerve fibers in central canals communicate with the surface of the bone and the medullary cavity

Microscopic Structure

Microscopic Structure

Microscopic Structure

• Spongy Bone– Composed of osteocytes and extracellular matrix– However, the bone cells do not aggregate around

central canals, but instead lie within the trabeculae and get nutrients from substances diffusing into the canaliculi that lead to the surface of the tiny, bony plates