Figure

Photomicrographs of decalcified immature and mature bone.

a. Decalcified immature bone, stained with H&E, showing the relationships of cells to the extracellular matrix. The immature bone has more cells, and the matrix is not layered in osteonal arrays. x130. b. This cross section of decalcified mature compact bone stained with H&E shows several osteons (O) with concentric lamellae. The Haversian canals contain blood vessels and connective tissue. Osteocytes undergo considerable shrinkage during routine slide preparation, revealing empty lacunae with a small nucleus attached to their walls. Mature bone has fewer osteocytes per unit area than immature bone. Note the presence of interstitial lamellae between neighboring osteons. x160.

(Haversian) canals, and die perforating (Volkmann's) canals. Osteoprogenitor cells can divide and proliferate, as shown by autoradiographic studies. In growing bones, osteoprogenitor cells appear as flattened cells with lightly staining, elongate or ovoid nuclei and inconspicuous acidophilic or slightly basophilic cytoplasm. Electron micrographs reveal profiles of rough endoplasmic reticulum (rER) and free ribosomes as well as a small Golgi apparatus and other organelles. The morphology of the osteoprogenitor cell is consistent with the finding that its stimu lation leads to differentiation into a more active secretory cell, the osteoblast.

Bone-lining cells cover bone that is not remodeling but are analogous to osteoprogenitor cells at sites of bone growth

In sites where remodeling is not occurring in mature bone, the bone surfaces are covered by a layer of flat cells with attenuated cytoplasm and a parity of organelles beyond the perinuclear region (see Fig. 8.7a). They do not

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