Electron micrographs of three different functional stages of an osteocyte. a. Relatively quiescent osteocyte that contains only a few profiles of rER and a few mitochondria (M). The cell virtually fills the lacuna that it occupies; the arrows indicate where cytoplasmic processes extend into canaliculi. Hydroxyapatite crystals have been lost from the matrix, which is ordinarily mineralized (MM), but some hydroxyapatite crystals fill the pericellular space. The hydroxyapatite crystals obscure the other substances within the pericellular space. The dark band marking the boundary of the lacuna is the osmiophilic lamina (OL). x25,000. b. A formative osteocyte containing larger amounts of rER and a large Golgi apparatus (G). Of equal importance is the presence of a small amount of osteoid in the pericellular space within the lacuna. The osteoid shows profiles of collagen fibrils (arrows) not yet mineralized. The lacuna of a formative osteocyte is not bounded by an osmiophilic lamina. x25,000. c. A resorptive osteocyte containing a substantial amount of rER, a large Golgi apparatus, mitochondria (M), and lysosomes (L). The pericellular space is devoid of collagen fibrils and may contain some flocculent material. The lacuna containing a resorptive osteocyte is bounded by a less conspicuous osmiophilic lamina (OL). x25,000.
released into the extracellular space in the clefts between the cytoplasmic processes of the ruffled border, a clear example of lysosomal hydrolases functioning outside the cell. Once liberated, these hydrolytic enzymes, which include collagenase, digest the organic components of the bone matrix. Before digestion can occur, however, the bone matrix must be decalcified. Current evidence indicates that the dissolution of the calcium salts occurs through secretion of organic acids by the membranes of the ruffled border. Moreover, a low pH favors the action of acid hydrolases. Accordingly, a local acidic environment is created in the extracellular space between the bone and the osteoclast. The clear zone adjacent to the ruffled border seems to form a seal against the bone, thus creating a compartment at the site of the ruffled border where focal decalcification and degradation of the matrix occur. Support for the concept of acid secretion by the osteoclast comes, in part, from the finding that carbonic anhydrase, an enzyme associated with carbonic acid production, is present in the region of the ruffled border.
Numerous coated pits and coated vesicles are also present at the ruffled border, suggesting endocytotic activity. Osteoclasts are observed at sites where bone remodeling is in progress. (The process of remodeling is described in more detail shortly.) Thus, in sites where osteons are being altered or where a bone is undergoing change during the growth process, osteoclasts are relatively numerous. As noted, an increase in PTH level promotes bone resorption and has a demonstrable effect on osteoclast activity, in addition to its ef-
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