PARENT SESSION

(75) PAIRING FAILURE AND OOCYTE LOSS IN XY AND XO MOUSE OVARIES.

Alton, Michelle¹, Taketo, Teruko¹, ¹ McGill University, Montreal, QC, Canada

ABSTRACT- Female reproductive life is limited by oocyte reserve, which is determined by the number of germ cells that enter meiosis and subsequent oocyte loss. The cause or biological role of the massive oocyte loss in normal development remains poorly understood. Our previous studies have shown a continuous loss of oocytes in the normal ovary throughout fetal and neonatal life, suggesting the involvement of multiple causes. To test the hypothesis that failure in homologous chromosome pairing is a cause of oocyte death, we examined the number of germ cells and meiotic progression in XY sex-reversed and XO female mice, two mouse models lacking pairing between the sex-chromosomes. The distribution of germ cells in ovarian sections was examined by immunohistochemical staining of GCNA1. The total number of GCNA1-positive cells was counted in chromosome spreading preparations from each ovary. The substage of meiotic prophase was assessed by immunostaining of synaptonemal complex components. The results indicate that the distribution and density of germ cells in the XY ovary was comparable to the control XX ovary until 15.5 dpc. Between 17.5 and 19.5 dpc, germ cells in the central region disappeared in the XY ovary. The progression of meiotic prophase was comparable up to 18.5 dpc, at which stage some oocytes reached the pachytene stage. There was a temporal accumulation of pachytene oocytes in the XY ovary at 19.5 dpc, but no further difference was recognized at later developmental stages. The total number of germ cells decreased similarly in XY and XX ovaries from 14.5 through 20.5 dpc. In the XO ovary, distribution of germ cells, the total number of germ cells, and progression of meiotic prophase were comparable to the control XX ovary at all developmental stages examined. These observations suggest that failure in sex-chromosome pairing is not sufficient to cause oocyte death. The excessive loss of oocytes in the XY ovary can be attributed to the activity of the Y-chromosome and not to the absence of the second X-chromosome. Supported by CIHR.

KEY WORDS: oocyte, meiosis, sex chromosome pairing, mouse ovary