The influence of immobilization on osteocyte morphology: osteocyte differential count and electron microscopical studies.

Differential counts and electron microscopical studies of osteocytes were performed on rats immobilized by spinal cord severing, plaster cast and ischiatic nerve dissection. In undecalcified ground sections of tibia and femur (100 micron) stained with basic fuchsin, osteocytes were differentiated into small (metabolically inactive) osteocy es enlarged (metabolically activated) osteocytes and empty lacunae. In rats (immobilizedfor' three weeks) with functioning parathyroid glands, but not after parathyroidectomy, the number of activated cells is markedly increased, whereas the number of small osteocytes is reduced. In animals with spinal cord severing the number of empty lacunae is also increased. Electron microscopical studies of undecalcified tibiae taken from rats immobilized for ten days showed a periosteocytic osteolysis with destruction of the lacunar wall, fragmentation of collagen fibres and loss of mineral crystals. The cytoplasmic seams of osteocytes were broadened, mitochondria were enlarged, and the cytoplasma showed vacuoles containing amorphous material which could be found in the pericellular space. Deep invaginations of the cytoplasma and an increase of the cell processes were typical findings. The results of the investigation point to an activation of osteocyte metabolism by immobilization. The osteocytes thus play an important part at the onset of immobilization osteoporosis. Periosteocytic osteolysis can be inhibited by parathyroidectomy. Therefore, the response of osteocytes to endogenous parathyroid hormone must be altered under conditions of immobilization.