Abstract
The B6.YTIR (XY) mouse develops bilateral ovaries despite the expression of the testis-determining gene Sry during gonadal differentiation. We reported that the oocytes of the XY female are defective in their cytoplasm, resulting in a failure in the second meiotic division after activation or fertilization in vitro. However, the mechanism of meiotic failure or the cause of infertility remained to be clarified. In the present study, we obtained mature oocytes from XY females by superovulation and confirmed that these oocytes also fail in zygotic development. By using confocal microscopy 3D-analysis, we demonstrated that meiotic spindles were properly positioned and oriented in the MII-oocytes from XY females. After parthenogenic activation, fewer oocytes from XY females extruded the second polar body, and in those oocytes, sister-chromatids were often separated but neither set entered the second polar body. ARP2, F-actin, and ORC4, known to play roles in asymmetric meiotic division, were initially localized along the ooplasmic membrane and concentrated over the MII-spindle but lost their cortical polarity after activation while the sister-chromatids moved away from the oolemma in the oocytes from XY females. Our results indicate that the second polar body extrusion is uncoupled from the sister-chromatids separation in the oocytes from XY female mouse.
Highlights
In mammalian development, the germ cells undergo sexual differentiation according to their gonadal environment, testis or ovary, which is determined by the presence or absence of the Y-linked Sry gene
We have previously reported that the germ cells in the B6.YTIR fetal ovary enter meiosis and go through the Meiotic Prophase I (MPI), but the X and Y chromosomes fail to pair at the pachytene stage, unlike the XY spermatocyte[12,13,14]
in vitro matured (IVM) oocytes are known to be inferior to ovulated oocytes[18, 19], and we cannot exclude the possibility that the cytoplasmic defects that we identified in the XY oocyte were exacerbated by the IVM conditions
Summary
The germ cells undergo sexual differentiation according to their gonadal environment, testis or ovary, which is determined by the presence or absence of the Y-linked Sry gene. When most copies of Rbmy repeats on the Y chromosome are deleted, the Sry gene is repressed during gonadal differentiation and sex reversal ensues These XYd1 females are nearly as fertile as XO females on the MF1 background[6]. In the XY female on the B6 genetic background, named B6.YTIR or B6.YPOS, the Sry gene is intact and expressed during gonadal differentiation and yet fails to initiate testicular differentiation due to inefficient coordination with its target Sox[98–11]. These XY females are sterile except for one litter at an early backcross generation[8, 12]. Our results indicate that the second polar body extrusion is uncoupled from the sister-chromatids separation in the oocyte from XY female mouse
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