Figure 89

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. Osteoprogen-itor cells (Opc) are evident between the marrow and the osteoblasts (Ob). They exhibit elongate 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 canali-culi (C) containing osteocyte processes. The boundary between two adjacent lamellae (L) of previously formed bone is evident as an irregular dark line. x9,000.

granules seen in light microscopy. The matrix vesicles, also produced by the osteoblast, appear to arise by a different pathway, originating as sphere-like 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.

Osteocytes

The osteocyte is the mature bone cell and is 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, the cell now responsible for maintaining the bone matrix. Osteocytes can synthesize new matrix, as well as resorb it, at least to a limited extent. Such activities help to maintain the homeostasis of blood calcium. Death of osteocytes, either through trauma, e.g., a fracture, or cell senescence, results in resorption of the bone matrix by osteoclast activity, followed by repair or remodeling of the bone tissue by osteoblast activity.

Each osteocyte occupies a space, or lacuna, that conforms to the shape of the cell. Osteocytes extend cytoplasmic processes through the canaliculi in the matrix to contact processes of neighboring cells by means of gap junctions. In hematoxylin and eosin (H&E)-stained sections, the canaliculi and the processes they contain are not discernable. In ground sections, the canaliculi are readily evident (see Fig. 8.11). Osteocytes are typically smaller than their precursors because of their reduced perinuclear cytoplasm. Often, in routinely prepared microscopic specimens, the cell is highly distorted by shrinkage and other artifacts that result from decalcifying the matrix prior to sectioning the bone. In such instances, the nucleus may be the only prominent feature. In well-preserved specimens, osteocytes exhibit less cytoplasmic basophilia than osteoblasts, but little additional cytoplasmic detail can be seen.

Electron microscopy has revealed osteocytes in various functional states. Indeed, there is evidence that the osteocyte can modify the surrounding bone matrix through synthetic and reabsorptive activities. Three functional states, each with a characteristic morphology, have been described:

• Quiescent osteocytes exhibit a paucity of rER and a markedly diminished Golgi apparatus (Fig. 8.10a). An osmiophilic lamina representing mature calcified matrix is seen in close apposition to the cell membrane.

• Formative osteocytes show evidence of matrix deposition and exhibit certain characteristics similar to those of osteoblasts. Thus, the rER and Golgi apparatus are more abundant, and there is evidence of osteoid in the pericellular space within the lacuna (Fig. 8.10b).

• Resorptive osteocytes, like formative osteocytes, contain numerous profiles of endoplasmic reticulum and a well-

developed Golgi apparatus. Moreover, secondary lysosomes are conspicuous (Fig. 8.10c).

That the resorptive osteocyte removes matrix is supported by the observation that the pericellular space is devoid of collagen fibrils and may contain a flocculent material suggestive of a breakdown product. The more peripheral, nonresorbed matrix is bounded by an osmiophilic lamina, which presumably represents the boundary of the intact mature calcified matrix. Resorption of bone by this mechanism, called osteocytic osteolysis, with the concomitant release of calcium ions, allows increases in blood calcium to maintain appropriate levels. The stimulus for the resorption of bone is increased secretion of parathyroid hormone (PTH).

Osteoclasts

The osteoclast is responsible for bone resorption

Osteoclasts are large, multinucleated cells found at sites where bone is being removed. They rest directly on the bone tissue where resorption is taking place (Fig. 8.11). As a result of osteoclast activity, a shallow bay called a resorption bay (Howship's lacuna) can be observed in the bone directly under the osteoclast. The cell is conspicuous not only because of its large size but also because of its marked acidophilia. It also exhibits a strong histochemical reaction for acid phosphatase because of the numerous lysosomes that it contains.

The portion of the cell in direct contact with the bone can be divided into two parts: a central region containing numerous plasma membrane infoldings forming microvil-lous-type structures, called the ruffled border; and a ringlike perimeter of cytoplasm, the clear zone, that demarcates the bone area being resorbed. The clear zone contains abundant microfilaments but essentially lacks other organelles. The ruffled border stains less intensely than the remainder of the cell and often appears as a light band adjacent to the bone at the resorption site (see Fig. 8.11).

At the electron microscopic level, hydroxyapatite crystals from the bone substance are observed between the processes of the ruffled border (Fig. 8.12). Internal to the ruffled border and in close proximity are numerous mitochondria and lysosomes. The nuclei are typically located in the part of the cell more removed from the bone surface. In this same region are profiles of rER, multiple stacks of Golgi saccules, and many vesicles.

Osteoclasts resorb bone by releasing lysosomal hydrolases into the extracellular space

Some, if not most, of the vesicles in the osteoclast are lysosomes that develop from late endosomes. They are

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