Cartilage

Vitamin D is found circulating throughout the body and is a known regulator of calcium homeostasis. Vitamin D deficiency is associated with rickets in the developing skeleton. This chapter examines the roles of two specific vitamin D metabolites (1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3]) in growth plate development and signaling. Both 1,25(OH)2D3 and 24,25(OH)2D3 induce genomic and nongenomic responses in growth plate chondrocytes and nongenomic responses by growth plate matrix vesicles. The effects of these metabolites are largely zonal with 24,25(OH)2D3 acting on the resting zone and 1,25(OH)2D3 on the growth zone. These metabolites affect numerous behaviors including proliferation, maturation, enzyme activity, extracellular matrix component production, calcium flux, and vitamin D metabolite production. The avascular nature of cartilage coupled with the ability of chondrocytes to produce both 1,25(OH)2D3 and 24,25(OH)2D3 metabolites allows local regulation of both cells and matrix vesicles.