Competence of Oocytes from the B6.YDOMSex-Reversed Female Mouse for Maturation, Fertilization, and Embryonic Developmentin Vitro

Abstract

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.

Onset and Progress of Meiotic Prophase in the Oocytes in the B6.YTIR Sex-Reversed Mouse Ovary1

“…However, the deficit in the oocyte population at the Z/P stage is too early to result from a P checkpoint mechanism of homologous pairing. Furthermore, the X-Y pairing failure alone cannot explain the observation that many oocytes survive and contribute to folliculogenesis in the XY ovary [17,19]. We speculate that the germ cells that enter meiosis at early developmental stages already have less chance to complete the meiotic prophase and the additional defect due to the XY genotype further reduces the number of oocytes that survive in the XY ovary.…”

“…It remains to be determined if the excessive oocyte death is associated with X-Y pairing failure. Although oocytes in the cortical region survive and continue to develop, they cannot develop properly after fertilization [15,[17][18][19].…”

The behavior of the X- and Y-chromosomes in the oocyte during meiotic prophase in the B6.YTIR sex-reversed mouse ovary

“…The few ovulated eggs can be fertilized either in situ or in vitro but they cease development at the 1-or 2-cell stage (Merchant-Larios et al 1994). A larger number of eggs can be obtained from B6.Y TIR females by collecting fully grown oocytes and maturing them in culture (Amleh et al 1996, Villemure et al 2007). These oocytes, although they mature at a normal rate, again fail to develop after fertilization.…”

Transmission of Y chromosomes from XY female mice was made possible by the replacement of cytoplasm during oocyte maturation

“…The resultant XY sex-reversed females are anatomically normal at young ages but fail to produce offspring (13,18). Our previous studies have demonstrated that the primary cause of infertility lies in the incompetence of the oocytes from these females to initiate embryonic development (19)(20)(21). The meiotic cell cycle proceeds normally up to the second metaphase (MII) in these oocytes in culture despite sex chromosome aneuploidy; however, the second meiotic division goes awry after activation or fertilization and very few oocytes reach the 2-cell stage (22).…”