MULLERIAN-INHIBITING SUBSTANCE PRODUCTION ASSOCIATED WITH LOSS OF OOCYTES AND TESTICULAR-DIFFERENTIATION IN THE TRANSPLANTED MOUSE XX GONADAL PRIMORDIUM

The mouse XX gonadal primordium develops seminiferous-like tubules after transplantation into the renal subcapsular site of the adult male or female mouse. We examined the ontogeny of Sertoli cell differentiation in XX gonadal grafts by immunocytochemical staining and organ culture bioassay for Mullerian Inhibiting Substance (MIS). During normal in situ development of the XY gonad, MIS staining was first detected in fetal Sertoli cells at 12 days of gestation (d.g.) and remained intense until 4 days postpartum (d.pp.), after which it gradually diminished with progressive testicular development. In the normal in situ XX gonad, MIS was detected in granulosa cells of growing follicles at 7 d.pp. and thereafter. When the XX gonad at 12 d.g. was grafted beneath the renal capsule, a few testicular cords composed of MIS-positive cells appeared on Day 7 post-transplantation (equivalent to 19 d.g.), much earlier than the normal appearance of MIS production in the intact XX ovary. The ovarian region containing germ cells at the meiotic prophase was unstained for MIS in the same sections. The incidence of XX gonadal grafts containing MIS-positive testicular cords and the number of such cords per gonadal graft steadily increased from Day 7 to Day 14 post-transplantation. Germ cells were absent or scarce inside the MIS-positive testicular cords. The MIS bioactivity in both control gonads and gonadal grafts coincided with the immunocytochemical staining for MIS. These results support the hypothesis that XX cells differentiate into Sertoli cells as a consequence of oocyte loss in the gonadal graft.