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 aft er transplantation into the renal subcapsular site of the adult male o r female mouse. We examined the ontogeny of Sertoli cell differentiati on in XX gonadal grafts by immunocytochemical staining and organ cultu re 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 intens e until 4 days postpartum (d.pp.), after which it gradually diminished with progressive testicular development. In the normal in situ XX gon ad, 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 e arlier than the normal appearance of MIS production in the intact XX o vary. The ovarian region containing germ cells at the meiotic prophase was unstained for MIS in the same sections. The incidence of XX gonad al grafts containing MIS-positive testicular cords and the number of s uch cords per gonadal graft steadily increased from Day 7 to Day 14 po st-transplantation. Germ cells were absent or scarce inside the MIS-po sitive testicular cords. The MIS bioactivity in both control gonads an d gonadal grafts coincided with the immunocytochemical staining for MI S. These results support the hypothesis that XX cells differentiate in to Sertoli cells as a consequence of oocyte loss in the gonadal graft.