The osteoblast cytoplasm is characterized by abundant rER and free ribosomes (Fig. 8.9). These features are consistent with its basophilia observed in the light microscope as well as with its role in the production of collagen and proteoglycans
FIGURE 8.9 • Electron micrograph showing active bone formation. This electron micrograph is similar to the growing surface of the bone spicule in the preceding light micrograph (Fig. 8.8). The marrow cavity (M) with its developing blood cells is seen in the lower right corner. Osteoprogenitor cells (Opc) are evident between the marrow and the osteoblasts (Ob). They exhibit elongated or ovoid nuclei. The osteoblasts are aligned along the growing portion of the bone, which is covered by a layer of osteoid (Os). In this same region, one of the cells (upper right corner) embedded within the osteoid exhibits a small process (arrow). This cell, because of its location within the osteoid, can now be called an osteocyte (Oc). The remainder of the micrograph (upper left) is composed of calcified bone matrix (CB). Within the matrix are canaliculi (C) containing osteocyte processes. The boundary between two adjacent lamellae (L) of previously formed bone is evident as an irregular dark line. 9,000.
for the extracellular matrix. The Golgi apparatus and surrounding regions of the cytoplasm contain numerous vesicles with a flocculent content that is presumed to consist of matrix precursors. These vesicles are the PAS-staining granules seen in light microscopy. The matrix vesicles, also produced by the osteoblast, appear to arise by a different pathway, originating as spherelike outgrowths that pinch off from the plasma membrane to become free in the matrix. Other cell organelles include numerous rod-shaped mitochondria and occasional dense bodies and lysosomes.
The osteocyte is the mature bone cell enclosed by bone matrix that it previously secreted as an osteoblast.
When completely surrounded by osteoid or bone matrix, the osteoblast is referred to as an osteocyte (see Fig. 8.8). Osteocytes are the cells responsible for maintaining the bone matrix. One of the roles of osteocytes is mechanotransduction, the process by which the osteocyte responds to mechanical forces applied to the bone. Different mechanical stimuli (e.g., weightlessness or increased mechanical loading) alter not only gene expression but also the cell’s apoptotic mechanism. Osteocytes can synthesize new matrix, as well as participate in matrix degradation. Such activities help to maintain calcium homeostasis.
The death of osteocytes through trauma (e.g., a fracture), cell senescence, or apoptosis results in resorption of the bone matrix by osteoclast activity, followed by repair or remodeling of the bone tissue by osteoblast activity.