DISTRIBUTION AND SUBCELLULAR IMMUNOLOCALIZATION OF 1,25-DIHYDROXYVITAMIN-D(3) RECEPTORS IN RAT EPIPHYSEAL CARTILAGE

The distribution and subcellular localization of the 1,25 dihydroxyvit amin D3 receptor (VDR) in the epiphyseal cartilage of normal weaning r ats were examined immunocytochemically at the light and electron micro scope level using a monoclonal anti-VDR antibody (9A7gamma). VDR immun oreactivity was detected in the nuclei of chondrocytes in all zones of the epiphyseal plate cartilage from the resting to calcifying chondro cytes, and at much lower concentrations, in the cytoplasms. Perichondr ial mesenchymal cells contained no VDR immunoreactivity. VDR immunorea ctivity developed in the nuclei of cells in the lateral margin area as they acquired the chondroblast phenotype. VDR immunoreactivity was al so found over the nucleoli of chondrocytes in all cells zones of the e piphyseal plate and appeared in the nucleoli of the cells in the later al margin area before immunostaining of the nuclei, as the mesenchymal cells differentiated into chondroblasts. Electron microscopy showed t hat the immunoreactivity for 1,25(OH)2D3 receptor, indicated by gold p articles, was associated with scattered clumps of compact chromatin an d small clumps of dispersed chromatin. But the nuclei immunostaining p atterns before and after mitosis were different in proliferative chond rocytes. The heterochromatin along the nuclear envelope was immunonega tive in interphase chondrocytes, but there was VDR immunostaining over the rim of the perinuclear chromatin just after mitosis. In the nucle oli, the dense fibrillar component was immunostained, but the fibrilla r centers and the perinuclear chromatin were not. This distribution of VDR immunoreactivity suggests that the hormone is directly involved i n differentiation, proliferation and maturation of cartilage cells, an d also with extracellular calcification in epiphyseal cartilage. The p resence of immunoreactive VDR receptors in nucleoli of chondrocytes, p articularly the fibrillar component, suggests that 1,25 (OH)2D3 may be involved in regulation of ribosomal genes.