REGULATION OF MEIOSIS IN MAMMALS

Culture media which contain either substances that can induce meiosis (meiosis-inducing substances, MIS) or prevent meiosis (meiosis-preventing substances, MPS) can be obtained from different experiments. MIS is present in culture media in which tissues of ovaries with germ cells in meiosis, or fetal epididymis, or puberal edpididymis and testes, have grown. MIS is apparently species-nonspecific since media containing MIS obtained from puberal bulls induce meiosis in different fetal mouse testes. MPS is secreted into the culrure media of minced fetal mouse testes. Meiosis is prevented or greatly inhibited in fetal mouse ovaries, when they are cultured in such media. It was concluded that MIS was present in mesonephric derived tissues in gonads in which meiosis occurred. Moreover, MPS within the testicular tissue (within the tetsicular cords?) might participate in preventing meiosis in the fetal testis. However, a negative 'MIS-activity-test' result was obtained in an experiment with rabbit gonads: meiosis was not induced in fetal gonads by infant ovarian tissue containing germ cells in meiosis. This led to comparative investigations of gonads from species in which the onset of meiosis and gondal sex differentiation coincide in time, 'simultaneous meiosis', and those, like the rabbit, in which the onset of meiosis in the female is delayed, 'delayed meiosis'. In species with 'delayed meiosis' the germ cells were confirmed to cord-like structures during the period between differentiation and the onset of meiosis. Within that period these species also produced sex steroids. In species with 'simultaneous meiosis' the female germ cells were distributed freely with the cortex, and no sex steroid hormones were produced in the period following sex differentiation. The significance of these features are discussed in relation to regulation of meiosis.