In vivo effect of 1,25-dihydroxycholecalciferol on the proliferation and differentiation of avian chondrocytes

A combination of immunocytochemistry and in situ biochemistry has been used to determine the in vivo effects of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] on the proliferation and differentiation of chondrocytes. Chicks were fed a diet supplemented with 1,25-(OH)2D3 (2.5, 5, or 10 micrograms/kg diet) for 3 weeks, and measurements were made in sections of growth plate of chondrocyte proliferation and rate of maturation through the growth plate [using bromodeoxyuridine (BrdUrd) labeling] and also chondrocyte differentiation [assessed by alkaline phosphatase (ALP) activity]. The labeling indices of the control and supplemented chicks were similar (23.1 +/- 1.3 versus 23.2 +/- 1.6%); however, within a 21 h period the BrdUrd-positive cells of the supplemented chicks had moved down the growth plate significantly farther than in the control chicks (71.0 +/- 2.8 versus 52.6 +/- 1.8%). Greater ALP (mean integrated absorbance) activity higher up the growth plate of the supplemented chicks indicated a more differentiated phenotype in cells closer to the epiphyseal junction. Within individual transitional chondrocytes ALP activity in the 10 micrograms/kg supplemented chicks was 26.6 +/- 0.85, which was significantly higher (p <0.01) than that of the control chicks (19.2 +/- 0.9). These results suggest that 1,25-(OH)2D3 in vivo does not increase the rate of chondrocyte proliferation but accelerates the onset of maturation.