Gonadal sex reversal of the developing marsupial ovary in vivo and in vitro.

Undifferentiated tammar wallaby ovaries were transplanted under the skin of male pouch young during the period of mitotic division of the XX germ cells. After 25 days, all the germ cells had disappeared and the ovaries contained seminiferous-like cords. Similarly, undifferentiated ovaries cultured for 4 days with recombinant human Müllerian-inhibiting substance (rhMIS) also contained well-differentiated seminiferous-like cords and few or no surviving germ cells. The majority of controls cultured without rhMIS developed as normal ovaries. However, in a few control ovaries seminiferous-like cords developed in those regions of the ovaries that were partially necrotic and contained few germ cells. These results strongly suggest that sex-reversal of the tammar ovary is the direct result of loss of mitotic germ cells, rather than an effect of MIS on female somatic cells. MIS is apparently toxic to these female germ cells in mitosis, but not to male germ cells in mitosis. Thus, in normal development in the tammar, the presence of XX germ cells in the ovary inhibits the formation of seminiferous cords so that the gonad develops as an ovary.