Possible Mechanism of Sex Determination in Mammals

It is known that undifferentiated embryonic gonads (UEG) in embryos of mammals have a dual nature. They consist of an outer layer of cortex, from which in the process of differentiation develop female sex cells, and from the inner layer – medulla – develop male gametes. During the determination of sex, one of the layers of the gonad develops and the other is suppressed. Therefore, the sex of the future fetus depends on which of the tissue cells - medulla or cortex - survives in the UEG. However, almost nothing is known about the causes for the survival of the medulla or cortex.We believe that, perhaps, the main cause for the development of one of the layers of the gonad and the suppression of the other is cell thermoregulation. The survival of the tissue cells of medulla depends on whether they can avoid heat death, which is determined by their ability to effectively remove excess metabolic heat from the cell nucleus. Since excess thermal energy is released into the cytoplasm through a dense layer of condensed chromatin around the nucleus, consisting predominantly of chromosomal constitutive heterochromatin regions (cHR), the fate of the medulla directly depends on the cHR in its cells.Seemingly, the sex in mammals and human is determined by the cHR on Y chromosomes. It is important not so much the amount of cHR on Y chromosome as its localization in the interphase cell around the nucleoli. Thanks to this localization, excess heat is more effectively removed from the "hottest" point of the nucleus, where ribosomes are formed, which is also necessary for the production of proteins that inhibit the development of cortex cells. Otherwise medulla doomed to degeneration and a cortex tissue will remain in the UEG.