Sex determination and differentiation in mammalian germ cells

Abstract

The germinal and somatic elements of the gonad are initially independent of one another and under separate genetic control mechanisms. They subsequently acquire a high degree of functional interdependence so that a defect in one cell type may have marked repercussions on the other. The behavior of the germ cell is determined in part by its genotype, and in part by the phenotypic environment in which it finds itself. Two X chromosomes are normally essential for oocyte formation, but an XX oocyte cannot survive in a testis. A Y chromosome seems to be essential in the spermatogonium if normal spermatozoa are to be produced. The somatic cells of the testis can develop normally and function as an endocrine organ in the absence of any germ cells and also apparently in the absence of a Y chromosome. However, the development of the graafian follicle, the endocrine apparatus of the ovary, is dependent on the presence of normal oocytes. A number of specific genetic loci have recently been discovered which regulate sex determination and differentiation. These include the autosomal dominant Sxr in the mouse, the autosomal recessive Polled in the goat, and the X-linked Tfm and the Y-linked histocompatibility antigen in a wide variety of species. At the moment, all the evidence points to these genes as regulators of somatic rather than germinal sex differentiation.