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 osteocytes and bone-lining cells by means of gap junctions. Osteocytes can also communicate indirectly with distant osteoblasts, pericytes of blood vessels, and other bone cells through the expression of various signaling molecules such as nitric oxide and glutamate transporters. In hematoxylin and eosin (H&E)–stained sections, the canaliculi and the processes they contain are not discernible. In ground sections, the canaliculi are readily evident (Plate 11, page 244). 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 before sectioning the bone. In such instances, the nucleus may be the only prominent feature. In wellpreserved specimens, osteocytes exhibit less cytoplasmic basophilia than osteoblasts, but little additional cytoplasmic detail can be seen (Plate 12, page 246).
Electron microscopy has revealed osteocytes in various functional states. Indeed, there is histologic and microradiologic evidence (i.e., enlarged lacunae and reduced radiodensity) that the osteocyte can modify the surrounding bone matrix. 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 deposi tion 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 pericel lular space within the lacuna (Fig. 8.10b).  Resorptive osteocytes, like formative osteocytes, con tain numerous profiles of endoplasmic reticulum and a well-developed Golgi apparatus. Moreover, lysosomes are conspicuous (Fig. 8.10c).
The “resorptive” function of the osteocyte is not precisely defined and is supported mainly by the observation that the pericellular space is devoid of collagen fibrils and contains a flocculent material suggestive of a breakdown product. These observed changes could be explained by enzymatic degradation of collagen by osteocyte-secreted matrix metalloproteinases (MMPs). It has been shown under experimental conditions that a reduced load on bone initiates expression of MMP mRNA in the osteocyte. Degradation of bone by MMPs is called osteocytic osteolysis. The current concept of osteocytic osteolysis is that the lytic role of osteocytes is not related to bone matrix remodeling but functions to maintain blood calcium level.
Bone-lining cells are derived from osteoblasts and cover bone that is not remodeling.