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Ultrastructural immunocytochemical localization of endogenous 1,25-dihydroxyvitamin D3 and its receptors in osteoblasts and osteocytes from neonatal mouse and rat calvaria.

Immunoreactivity to 1,25-dihydroxyvitamin D3 receptors and endogenous 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) were studied in osteoblasts and osteocytes from calvaria of neonatal mice and rats by immunocytochemistry with the use of ultrathin sections obtained by cryo-ultramicrotomy. Tissue samples were fixed in glutaraldehyde, postfixed in osmium tetroxide and frozen under liquid nitrogen. 1,25(OH)2D3 and 1,25(OH)2D3 receptor-like immunoreactivities were observed in osteoblasts and osteocytes. In both types of cell, 1,25(OH)2D3 and its receptors were similarly located in the cytoplasmic matrix but not in organelles, and mainly in the nucleus (primarily in the chromatin and sometimes near the nuclear membrane or in the nucleolus). Reaction products, however, were never seen at the plasma membrane level. These results provide immunocytological evidence for the presence of 1,25(OH)2D3 and its receptors in osteoblasts and osteocytes. The similar localization of the hormone and its receptors in osteoblasts and osteocytes supports the hypothesis of a direct action of 1,25(OH)2D3 in these bone cells. The fact that the main localization of 1,25(OH)2D3 receptors was nuclear, implies, as postulated for other steroid receptors, that 1,25(OH)2D3 receptors occur primarily in the nucleus.