In sites where remodeling is not occurring, the bone surface is covered by a layer of flat cells with attenuated cytoplasm and a paucity of organelles beyond the perinuclear region (see Fig. 8.11a). These cells are designated simply as bone-lining cells. Bone-lining cells on external bone surfaces are called periosteal cells, and those lining internal bone surfaces are often called endosteal cells (see Fig. 8.7) Gap junctions are present where the bone-lining cell processes contact one another (Fig. 8.11b). Bone-lining cells represent a population of cells that are derived from osteoblasts. They are thought to function in the maintenance and nutritional support of the osteocytes embedded in the underlying bone matrix and regulate the movement of calcium and phosphate into and out of the bone. These suggested roles are based on the observation that the cell processes of bone-lining cells extend into the canalicular channels of the adjacent bone (see Fig. 8.11b) and communicate by means of gap junctions with osteocytic processes. In these respects, bone-lining cells are somewhat comparable to osteocytes.
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. 8.12). As a result of osteoclast activity, a shallow bay called a resorption bay
FIGURE 8.10 • 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). 25,000.
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. 25,000. 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). 25,000.
(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. One of these enzymes, the 35-kilodalton iron-containing tartrate-resistant acid phosphatase (TRAP), is used clinically as a marker of osteoclast activity and differentiation.
Osteoclasts are derived from the fusion of mononuclear hemopoietic progenitor cells under the influence of multiple cytokines.