c-Myc and Mxi1 immunoreactivities in the calcifying areas of the epiphyseal-plate cartilage matrix of growing rats.

We looked for the protooncogene protein, c-Myc, its dimerization partner, Max, and the repressors of its transactivation activity, Mad1 and Mxi1, in the epiphyseal-plate cartilage matrix of growing rats by immunocytochemistry in the electron microscope. c-Myc and Mxi1 immunoreactivities were found in the calcifying areas of the cartilage matrix only. There was no immunolabeling in response to anti-Max or anti-Mad1 antibodies. Mxi1 immunoreactivity was mainly in the early calcifying areas, in the calcification front and ahead of it, whereas c-Myc immunoreactivity was essentially in the incompletely calcified regions of the matrix. The two immunolabelings occurred mainly over the large type II collagen fibrils of the cartilage matrix and over the thin filaments connecting them. c-Myc and Mxi1 immunoreactivities were rarely found along the dark cristallites. There was no immunolabeling associated with the matrix vesicles, or in their immediate surroundings. The data suggest that the protooncogene proteins, c-Myc and Mxi1, could be implicated in the calcification involving type II collagen fibrils of the epiphyseal-plate cartilage. The absence of Max immunoreactivity from the calcifying cartilage matrix raises the question of whether there are other c-Myc- and Mxi1-dimerization partners.