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, M ax, and the repressors of its transactivation activity, Mad1 and Mxi1, in t he epiphyseal-plate cartilage matrix of growing rats by immunocytochemistry in the electron microscope. c-Myc and Mxi1 immunoreactivities were found i n the calcifying areas of the cartilage matrix only. There was no immunolab eling in response to anti-Max or anti-Mad1 antibodies. Mxi1 immunoreactivit y was mainly in the early calcifying areas, in the calcification front and ahead of it, whereas c-Myc immunoreactivity was essentially in the incomple tely calcified regions of the matrix. The two immunolabelings occurred main ly 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 asso ciated with the matrix vesicles, or in their immediate surroundings. The da ta suggest that the protooncogene proteins, c-Myc and Mxi1, could be implic ated in the calcification involving type II collagen fibrils of the epiphys eal-plate cartilage. The absence of Max immunoreactivity from the calcifyin g cartilage matrix raises the question of whether there are other c-Myc- an d Mxi1-dimerization partners. (Bone 24:555-563; 1999) (C) 1999 by Elsevier Science Inc. All rights reserved.