Action of 1,25-dihydroxycholecalciferol on cartilage mineralization and on endosteal lining cells of bone. Experimental in vitro studies.

The aim of our study was to answer the following question: 1) Does 1,25-dihydroxycholecalciferol influence the mineralization process directly? 2) How does 1,25-D3 act on the endosteal cell envelope of bone, which is thought to be engaged in the moment to moment regulation of calcium homeostasis? We studied the endosteal surface of calvaria and the epiphyseal cartilage of rachitic rats in a tissue culture system in order to avoid the secondary effects of endogenous hormones or mineral sources. Investigations were carried out using transmission and scanning electron microscopy. Compared with untreated rachitic control specimens, 1,25-D3 (4 X 10(-8) M) did not enhance the mineralization process. We therefore conclude that the established in vivo effect of 1,25-D3 on the mineralization process must be an indirect one. Within 2 h the flat-shaped surface lining cells of calvaria showed a rapid transformation into globiform cells and a loss of cell adhesion. Within two days a second slower transformation into spindle-shaped fibroblast-like proliferating cells occurred. All changes of cell shape were reversible within several days in the absence of 1,25-D3. Obviously, bone matrix fiber texture influences the spatial movement of proliferating cells. The endosteal cell envelope is considered to act as a functional membrane of bone. Starting from this consideration we conclude that the changes of cellular shape described are the morphological basis for the action of 1,25-D3 in the regulation of calcium homeostasis in bone at the cellular level.