Like its close relatives, the fibroblast and the chondroblast, the osteoblast is a versatile secretory cell that retains the ability to divide. It secretes both type I collagen (which constitutes 90% of the protein in bone) and bone matrix proteins (BMPs), which constitute the initial unmineralized bone, or osteoid. The bone matrix proteins produced by the osteoblast include calcium-binding proteins such as osteocalcin and osteonectin; multiadhesive glycoproteins such as bone sialoproteins I and II, osteopontin, and thrombospondin, various proteoglycans and their aggregates, and alkaline phosphatase (ALP). Circulating levels of ALP and osteocalcin are used clinically as markers of osteoblast activity.
The osteoblast is also responsible for the calcification of bone matrix. The calcification process appears to be initiated by the osteoblast through the secretion into the matrix of small, 50to 250-nm, membrane-limited matrix vesicles. The vesicles are rich in ALP and are actively secreted only during the period in which the cell produces the bone matrix. The role of these vesicles is discussed later in this chapter (page 241).
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). The newly deposited matrix is not immediately calcified. It stains lightly or not at all compared with the 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, the nonmineralized matrix.
FIGURE 8.8 • Photomicrograph of a growing bone spicule stained with Mallory-Azan. Osteocytes are embedded within the bone matrix of the spicule, which is stained dark blue. These cells are metabolically active, laying down the unmineralized bone matrix (osteoid). A number of osteoblasts are aligned on the right side of the spicule. Between these cells and the calcified bone spicule is a thin, light-blue–stained layer of osteoid. This is the uncalcified matrix material produced by the osteoblasts. One of the cells (arrow) has virtually surrounded itself by its osteoid product; thus it can now be called an osteocyte. On the left side of the spicule, the nongrowing part, are inactive osteoblasts. The cells exhibit flattened nuclei and attenuated cytoplasm. 550.
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 ALP 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 and bone remodeling. As osteoid deposition occurs, the osteoblast is eventually surrounded by osteoid matrix and then becomes an osteocyte.
Osteoblast processes communicate with other osteoblasts and with osteocytes by gap junctions.
At the electron microscope level, osteoblasts exhibit thin cytoplasmic processes that penetrate the adjacent osteoid produced by the cell and are joined to similar processes of adjacent osteocytes by gap junctions. This early establishment of junctions between an osteoblast and adjacent osteocytes (as well as between adjacent osteoblasts) allows neighboring cells within the bone tissue to communicate.