The In Vivo Function of Müllerian-Inhibiting Substance During Mammalian Sexual Development

Abstract

In mammals, both XX and XY individuals develop one pair of undifferentiated gonads and two pairs of genital ducts, the Müllerian ducts and the Wolffian ducts, associated with the mesonephroi during development (Fig. 1). The undifferentiated gonads are bipotent, and will give rise to either testes or ovaries depending on the sex-chromosome genotype. The Müllerian ducts have the potential to give rise to female reproductive organs, including the uterus, oviducts, and upper portion of the vagina. The Wolffian ducts are the primordia of male reproductive organs, which are the vas deferens, epididymis, and seminal vesicles. Therefore, regardless of sex-chromosome genotype, each individual has the potential to develop male and female reproductive systems. The presence of the Y chromosome determines that an individual becomes a male because of a gene termed Sry. There must be a mechanism to select only one duct system, depending on the presence or absence of Sry. Two gonadal hormones—Müllerianinhibiting substance (MIS)—and testosterone, play essential roles during this selection process (Fig. 1) (1–3) This chapter, examines the results of in vivo approaches using transgenic and targeted mutant mice to study MIS function.