Figure 87

Electron micrograph of bone-lining cells, a. The cytoplasm of a bone-lining cell located on the surface of a spicule of mature bone is very attenuated and contains small amounts of rER and free ribosomes. A gap junction is seen between the two adjacent bone-lining cells. In addition, cytoplasmic processes are clearly seen where they pass through the matrix of unmineralized bone (osteoid). A fat cell of the marrow is also present. x8,900. (From Miller SC, et al. Anat Rec 1980;198:163-173.) Inset. High-magnification photomicrograph of a similar bone spicule stained with H8-E, included for orientation purposes. The bone-lining cells on the surface of the spicule are indicated by the arrows. x350. b. Electron micrograph of the cytoplasm of two bone-lining cells observed at higher magnification. The gap junction is clearly seen where the two cells are in apposition. The edge of a fat cell is seen at the top of the electron micrograph; its lipid, thin rim of cytoplasm, plasma membrane, and external lamina are also evident, x27,000.

plasma membrane lipid form a complete cellular lining on the bone surface; gap junctions are present where the lining cell processes contact one another (Fig. 8.7b). These cells, designated simply as bone-lining cells, are analogous to the osteoprog-enitor cells but are probably in a more quiescent state than those located at sites of bone growth. They are also thought to function in the maintenance and nutritional support of the osteocytes embedded in the underlying bone matrix. This suggested role is based on the observation that the cell processes of bone-lining cells extend into the canalicular channels of the adjacent bone (see Fig. 8.7b) and communicate by means of gap junctions with processes of osteocytes.

The degree of differentiation of the osteoprogenitor cell is not entirely clear. Their derivation from mesenchymal cells and their apparent ability to differentiate into three kinds of cells other than osteoblasts (adipose cells, chondroblasts, and fibroblasts) suggest that they, like fibroblasts, can modify their morphologic and functional characteristics in response to specific stimuli. This question is important because the healing of bone fractures involves the formation of new connective tissue and cartilage in the callus that develops around the bone during the repair process (see "Fractures and Bone Repair," page 201). Although the precise origin of the cells in healing bone may still be unclear, some evidence indicates that periosteal cells and endosteal cells participate in all stages of the healing process.

Osteoblasts

The osteoblast is the differentiated bone-forming cell that secretes bone matrix

Like its close relatives, the fibroblast and the chondro-blast, the osteoblast is a versatile secretory cell that retains the ability to divide. It secretes both the collagen and the ground substance that constitute the initial unmineralized bone, or osteoid. The osteoblast is also responsible for the calcification of the matrix. The calcification process appears to be initiated by the osteoblast through the secretion into the matrix of small, 50- to 250-nm, membrane-limited matrix vesicles. The vesicles are rich in alkaline phosphatase and are actively secreted only during the period in which the cell produces the bone matrix. The role of these vesicles is discussed on page 200 ("Biologic Mineralization and Matrix Vesicles").

Osteoblasts are recognized in the light microscope by their cuboidal or polygonal shape and their aggregation into a single layer of cells lying in apposition to the forming bone (Fig. 8.8). Because the newly deposited matrix is not immediately calcified, it stains lightly or not at all, compared with mature mineralized matrix, which stains heavily with eosin. Because of this staining property of the newly formed matrix, osteoblasts appear to be separated from the bone by a light band. This band represents the osteoid; it is nonmineralized matrix.

The cytoplasm of the osteoblast is markedly basophilic, and the Golgi apparatus, because of its size, is sometimes observed as a clear area adjacent to the nucleus. Small, periodic acid-Schiff (PAS)-positive granules are observed in the cytoplasm, and a strong alkaline phosphatase reaction associated with the cell membrane can be detected by appropriate histochemical staining.

In contrast to the secreting osteoblasts found in active matrix deposition, inactive osteoblasts are flat or attenuated cells that cover the bone surface. These cells resemble osteoprogenitor cells. Osteoblasts respond to mechanical stimuli to mediate the changes in bone growth

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