Distribution and subcellular immunolocalization of 1,25-dihydroxyvitamin D3 receptors in rat epiphyseal cartilage

The distribution and subcellular localization of the 1,25-dihydroxyvitamin D3 receptor (VDR) in the epiphyseal cartilage of normal weaning rats were examined immunocytochemically at the light and electron microscope level using a monoclonal anti-VDR antibody (9A7 gamma). VDR immunoreactivity was detected in the nuclei of chondrocytes in all zones of the epiphyseal plate cartilage from the resting to calcifying chondrocytes, and at much lower concentrations, in the cytoplasms. Perichondrial mesenchymal cells contained no VDR immunoreactivity. VDR immunoreactivity developed in the nuclei of cells in the lateral margin area as they acquired the chondroblast phenotype. VDR immunoreactivity was also found over the nucleoli of chondrocytes in all cells zones of the epiphyseal plate and appeared in the nucleoli of the cells in the lateral margin area before immunostaining of the nuclei, as the mesenchymal cells differentiated into chondroblasts. Electron microscopy showed that the immunoreactivity for 1,25(OH)2D3 receptor, indicated by gold particles, was associated with scattered clumps of compact chromatin and small clumps of dispersed chromatin. But the nuclei immunostaining patterns before and after mitosis were different in proliferative chondrocytes. The heterochromatin along the nuclear envelope was immunonegative in interphase chondrocytes, but there was VDR immunostaining over the rim of the perinuclear chromatin just after mitosis. In the nucleoli, the dense fibrillar component was immunostained, but the fibrillar centers and the perinuclear chromatin were not. This distribution of VDR immunoreactivity suggests that the hormone is directly involved in differentiation, proliferation and maturation of cartilage cells, and also with extracellular calcification in epiphyseal cartilage. The presence of immunoreactive VDR receptors in nucleoli of chondrocytes, particularly the fibrillar component, suggests that 1,25(OH)2D3 may be involved in regulation of ribosomal genes.