1,25-DIHYDROXYVITAMIN D-3 AND 22-OXA-1,25-DIHYDROXYVITAMIN D-3 IN-VIVO NUCLEAR RECEPTOR-BINDING IN DEVELOPING BONE DURING ENDOCHONDRAL AND INTRAMEMBRANOUS OSSIFICATION

Target cells for H-3-labeled 1 alpha, 25(OH)(2) vitamin D-3 [1,25(OH)( 2)D-3, vitamin D] and its analog H-3-labeled 22-oxa-1 alpha, 25(OH)(2) vitamin D-3 (OCT) have been identified during endochondral and intram embranous ossification in developing, undecalcified, unembedded bone, using thaw-mount autoradiography. Two-day-old neonatal rats were injec ted with [H-3]1,25(OH)(2)D-3 or [H-3]OCT; after 2 h leg, spine, and he ad were frozen and sectioned. In the epiphyseal-metaphyseal region spe cific nuclear concentrations of [H-3]1,25(OH)(2)D-3 and [H-3]OCT were observed in identical cell populations, being low in cells of the arti cular and resting zone, intermediate in the proliferating zone, and hi ghest in hypertrophic chondrocytes and in osteoblasts and precursor ce lls. In the primary spongiosa intertrabecular spaces there were a larg e number of cells with nuclear labeling - probably osteoblasts and pre cursor cells. In contrast, in the secondary spongiosa intertrabecular spaces, apparent blood-forming cells were mostly unlabeled. Osteoblast s along bone spicules and compact bone in long bones, vertebrae, and h ead also showed strong nuclear labeling, as did cells of the periosteu m. These data suggest that 1,25(OH)(2)D-3 and OCT regulate development , differentiation, and activities of chondrocytes and osteoblasts, inc luding differentiation of resting chondrocytes into proliferating and hypertrophic chondrocytes that involve ''chondroclastic'' enlargement of lacunae and ''trans-differentiation'' of surviving hypertrophic cho ndrocytes; differentiation of stroma cells into osteoblasts; and in pe riosteum and other regions of intramembranous ossification differentia tion of precursor cells and osteoblasts. Nuclear receptor binding and their selective and hierarchical distribution during cell differentiat ion appear to correspond to multiple genomic effects toward growth, re generation and repair. The findings indicate a physiological significa nce and therapeutic potential of 1,25(OH)(2)D-3 and in particular of i ts less hypercalcemic analog OCT.