Gonads-Mullerian Ducts

The embryonic gonad is unique and valuable in terms of the study of cell-type specification, because it has the capacity to develop into one of the two very different organs, the testis or the ovary. For this reason, it is known, before the point of sex determination, as the "indifferent" or "bipotential" gonad. This chapter describes the origin of urogenital ridge and the morphological and cellular changes that occur once a sexual fate has been determined. The various cell types in the indifferent gonad, their likely origins, and the means by which they are induced to adopt male- or female-appropriate fates are also described. The sex of a mammalian individual is ultimately determined by whether or not the Y-chromosome, and hence the SRY gene, is present. The initiation of expression of SRY is yet to be known, and it is possible that this varies among species. It is hypothesized that the Y-chromosomal male-determining factor must act solely in the presupporting cell lineage to induce Sertoli cell differentiation. In addition, it was predicted that its action is cell autonomous, although recruitment of some presupporting cells must be possible through a paracrine mechanism