Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Skeletal System

C h a p t e r 4 2

Osteocyte Canaliculi Lamella Lacuna Haversian canal

Inner circumferential lamellae Spongy bone

B

Compact bone

Outer circumferential lamellae

Haversian system

Blood vessel into marrow

FIGURE 42-4. (A) Haversian systems as seen in a wedge of compact bone tissue.The periosteum has been peeled back to show a blood vessel entering a Volkmann canal. (B) Osteocytes lying within lacunae; canaliculi permit interstitial fluid to reach each lacuna.

Periosteum

Volkmann canal Haversian canal Vessel of haversian canal

A

When appropriately stimulated by bone morphogenic proteins (BMPs) and transforming growth factors, they undergo cell division and differentiate into osteoblasts. Osteoblasts. Osteoblasts are cells that secrete bone matrix. Bone formation occurs in two stages: ossification and calcification. Ossification involves the formation of the initial unmineralized bone, or osteoid (also called prebone). Osteoblasts secrete both type I collagen (which constitutes 90% of the protein of bone) and bone matrix proteins, forming the osteoid. They also are responsible for calcification of the bone matrix. The calcification process appears to be initiated by the osteoblast through the secretion of the enzyme alkaline phosphatase , which is thought to act locally in bone tissue to raise calcium and phosphate levels to the point at which precipitation occurs. The activity of the osteoblasts undoubtedly con- tributes to the increase in serum levels of alkaline phos- phatase that follows bone injury and fractures. Osteocytes. Osteocytes are mature bone cells, derived from osteoblasts, which are housed in lacunae within the calcified bony matrix (see Fig. 42-4B). Extracellular

fluid-filled passageways permeate the calcified matrix and connect with the lacunae of adjacent osteocytes. These passageways are called canaliculi. Because diffusion does not occur through the calcified matrix of bone, the cana- liculi serve as communicating channels for the exchange of nutrients and metabolites between the osteocytes and the blood vessels on the surface of the bone layer. Osteocytes are the cells responsible for maintain- ing the bone matrix. They can synthesize new matrix, as well as participate in matrix degradation. Death of osteocytes through trauma, cell senescence, or apopto- sis, results in resorption of the bone matrix by osteo- clasts, followed by repair or remodeling of the bone tissue by osteoblasts. Osteoclasts. Osteoclasts are large multinucleated “bone-chewing” cells that function in the resorption of bone, removing its mineral content and organic matrix. They are derived from the same mononuclear hemato- poietic progenitor cells that give rise to blood monocytes and tissue macrophages. Osteoclast formation occurs in close association with the stromal cells of the bone mar- row. These cells secrete essential cytokines that promote