In males, Mullerian inhibiting substance (MIS) mRNA was first detected on the medial aspect of the urogenital ridge early on the morning of day 13 of gestation before testicular differentiation was evident, and localized to the more obvious Sertoli cells later on embryonic day 13. MIS transcripts remained at maximal levels between 14.5 and 17.5 days gestation, while the Mullerian duct involutes, and remained high until birth. MIS gene expression decreased progressively after birth and, as germ cell meiosis increased, became barely detectable in the Sertoli cells of the seminiferous tubules. In female rats, MIS mRNA was first detected in the single layer of cuboidal granulosa cells surrounding larger primary follicles 3 days after birth, coincident with the initiation of follicular growth. As follicular growth progressed, MIS mRNA expression was high in preantral and small antral follicles, especially in those granulosa cells closest to the oocyte. MIS mRNA expression decreased gradually in larger antral follicles, remaining prominent only in the cumulus cells and the dividing population of granulosa cells closest to the lumen. MIS gene expression was absent in follicles with features of atresia and in the larger antral follicles. The expression of MIS mRNA in actively dividing Sertoli and granulosa cells correlates with the stages of germ cell division. These findings are suggestive of a role for MIS in the control of germ cell maturation.
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