FIGalpha, a germ cell-specific transcription factor required for ovarian follicle formation.

Primordial follicles are formed perinatally in mammalian ovaries and at birth represent the lifetime complement of germ cells. With cyclic periodicity, cohorts enter into a growth phase that culminates in ovulation of mature eggs, but little is known about the regulatory cascades that govern these events. FIGalpha, a transcription factor implicated in postnatal oocyte-specific gene expression, is detected as early as embryonic day 13. Mouse lines lacking FIGalpha were established by targeted mutagenesis in embryonic stem cells. Although embryonic gonadogenesis appeared normal, primordial follicles were not formed at birth, and massive depletion of oocytes resulted in shrunken ovaries and female sterility. Fig(&agr;) (the gene for FIGalpha null males have normal fertility. The additional observation that null females do not express Zp1, Zp2 or Zp3 indicates that FIGalpha plays a key regulatory role in the expression of multiple oocyte-specific genes, including those that initiate folliculogenesis and those that encode the zona pellucida required for fertilization and early embryonic survival. The persistence of FIGalpha in adult females suggests that it may regulate additional pathways that are essential for normal ovarian development.

Both nuclear and cytoplasmic components are defective in oocytes of the B6.Y(TIR) sex-reversed female mouse.

In the mammalian gonadal primordium, activation of the Sry gene on the Y chromosome initiates a cascade of genetic events leading to testicular organization whereas its absence results in ovarian differentiation. An exception occurs when the Y chromosome of Mus musculus domesticus from Tirano, Italy (Y(TIR)), is placed on the C57BL/6J (B6) genetic background. The B6.Y(TIR) progeny develop only ovaries or ovotestes despite Sry transcription in fetal life. Consequently, the XY offspring with bilateral ovaries develop into apparently normal females, but their eggs fail to develop after fertilization. Our previous studies have shown that the primary cause of infertility can be attributed to oocytes rather than their surrounding somatic cells in the XY ovary. This study attempted to identify the defects in oocytes from the B6.Y(TIR) female mouse. We examined the developmental potential of embryos from XY and XX females after exchanging their nuclear components by microsurgery following in vitro maturation and fertilization. The results suggest that both nuclear and cytoplasmic components are defective in oocytes from XY females. In the XY fetal ovary, most germ cells entered meiosis and their autosomes appeared to synapse normally while the X and Y chromosomes remained unpaired during meiotic prophase. This lack of X-Y pairing probably caused aneuploidy in some secondary oocytes following in vitro maturation. However, normal numbers of chromosomes in the rest of the secondary oocytes indicate that aneuploidy alone can not explain the nuclear defect in oocytes.

Live-borns from XX but not XY oocytes in the chimeric mouse ovary composed of B6.Y(TIR) and XX cells.

When the Y chromosome of some Mus musculus domesticus subspecies is placed onto a C57BL/6J mouse background, the XY (B6.Y(TIR)) progeny develop only ovaries or ovotestes during fetal life. The XY sex-reversed female is infertile mainly because of death of embryos during preimplantation development. In the present study, we constructed female mouse chimera composed of B6.Y(TIR) and XX BALB/c cells to determine whether developmental incompetence of XY oocytes can be attributed to defects in the oocytes themselves or in the surrounding XY somatic cells. Distribution of XY cells in chimeric ovaries was examined by in situ hybridization. Of nine XX <--> XY chimeric females born, eight were composed of B6.Y(TIR) and XX BALB/c cells with a wide range of XY contribution (16-95%), whereas one had 12% XY components of the BALB/c strain. All these females produced progeny exclusively derived from XX oocytes. By comparison, most XX <--> XX chimeric females produced progeny derived from oocytes of either strain. Two XY <--> XY males also produced progeny of both strains. In conclusion, the XY chromosomal composition in the oocyte appears to be responsible for programming its incompetence for postfertilization development. On the other hand, the presence of XY somatic cells in the chimeric ovary allows development of fertile XX oocytes.

Interactions between the oocyte and cumulus cells in the ovary of the B6.Y(TIR) sex-reversed female mouse.

The XY (B6.Y(TIR)) sex-reversed female mouse is infertile, primarily because of the early death of its embryos. We have previously determined that the XY oocyte itself, not the surrounding somatic cells, is responsible for its failure in postfertilization development. In the present study, we assessed the ability of the XY oocyte to regulate granulosa cell differentiation and functions. Oocyte-cumulus complexes (OCC) were isolated from antral follicles and were cultured in the presence of FSH and testosterone. Microsurgical removal of oocytes prevented cumulus cell expansion and suppressed estradiol production while it promoted progesterone production. Coculture with denuded oocytes from either XX or XY ovaries restored cumulus expansion and the endocrine profile observed in intact OCC. Morphology of oocytes and OCC in the preantral and antral follicles in situ as well as after isolation was compared for XX and XY ovaries. The average area of XY oocytes was smaller by 20% only at the preantral stage, whereas the zona pellucida layer was thinner by 20% at all stages. Furthermore, the XY oocyte was found to be attached to fewer cumulus cells (60% of XX control) in antral follicles and isolated OCC. In conclusion, the XY oocyte develops the normal ability of regulating granulosa cell differentiation despite its inferiority with respect to some morphometric parameters when compared to the XX oocyte.

Competence of oocytes from the B6.YDOM sex-reversed female mouse for maturation, fertilization, and embryonic development in vitro.

When the Y chromosome of a Mus musculus domesticus mouse strain is placed onto the C57BL/6J (B6) inbred genetic background, the XY (B6.YDOM) progeny develop ovaries or ovotestes, but not normal testes, during fetal life. At puberty, while some of the hermaphroditic males become fertile, none of the XY sex-reversed females produce litters. We have previously demonstrated that the eggs ovulated from the B6.YDOM ovary undergo fertilization efficiently, but cannot develop beyond the 2-cell stage either in vivo or in vitro. In the present study, we collected oocytes directly from the XY ovary, and examined their maturation, fertilization, and embryonic development in vitro. The results show that the juvenile XY ovary yielded far more fertilizable oocytes by direct collection and in vitro maturation than through in vivo ovulation, but the majority of fertilized eggs failed to reach the blastocyst stage. Hence, developmental incompetence of oocytes in the XY ovary appears to be programmed during oocyte differentiation or growth. Nonetheless, in vitro matured oocytes showed a higher potential of embryonic development than the ovulated eggs, suggesting that fertility of the XY female may be impaired by multiple factors. We hypothesized that poor responsiveness of the XY ovary to gonadotropins, as we have previously demonstrated in testosterone production, may impair follicular development or proper recruitment of oocytes for ovulation. In the present study, we compared 125I-hCG binding in XX and XY ovaries, but did not find a significant difference. Hence, LH activity appears to be impaired after receptor binding in the XY ovary. On the other hand, the pattern of 125I-hCG binding indicated that the majority of antral follicles in the XY ovary failed to undergo normal preovulatory phases, which may explain the lower developmental capacity of eggs after ovulation.