Breed W G, Leigh C M
Department of Anatomy and Histology, University of Adelaide, South Australia.
J Morphol. 1990 May;204(2):177-96. doi: 10.1002/jmor.1052040207.
This light and transmission electron microscopical study shows that the first polar body is given off before ovulation and that part of its cell membrane and that of the surrounding oocyte have long microvilli at the time of its ejection. Several layers of cumulus cells initially surround the secondary oocyte and first polar body, but the ovulated oocytes in the oviducts in the process of being fertilized do not have cumulus cells around them. Partly expelled second polar bodies occur in the oviduct; they are elongated structures that lack organelles and have electron-dense nuclei. A small fertilization cone appears to form around the sperm tail at the time of sperm entry into the egg and an incorporation cone develops around the sperm head in the egg cytoplasm. In three fertilized eggs a small hole was seen in the zona, which was presumably formed by the spermatozoon during penetration. Cortical granules, present in ovarian oocytes, are not seen in fertilized tubal or uterine eggs; release of their contents probably reduces the chances of polyspermy, although at least one polyspermic fertilized egg was seen and several other fertilized eggs had spermatozoa within the zona pellucida. In the zygote the pronuclei come to lie close together, but there was no evidence of fusion. A "yolk mass," which becomes eccentric before ovulation, is extruded by the time the two-cell embryos are formed, but many vacuoles remain in the non-yolky pole of the egg. A shell membrane of variable thickness is present around all uterine eggs but its origin remains undetermined.
这项光学显微镜和透射电子显微镜研究表明，第一极体在排卵前排出，其细胞膜及其周围卵母细胞的细胞膜在排出时具有长微绒毛。最初有几层卵丘细胞围绕着次级卵母细胞和第一极体，但在输卵管中处于受精过程的排卵卵母细胞周围没有卵丘细胞。部分排出的第二极体出现在输卵管中；它们是细长结构，缺乏细胞器，有电子致密的细胞核。精子进入卵子时，似乎在精子尾部周围形成一个小受精锥，在卵子细胞质中围绕精子头部形成一个包入锥。在三个受精卵中，在透明带上看到一个小孔，推测是精子穿透时形成的。卵巢卵母细胞中存在的皮质颗粒，在输卵管或子宫内的受精卵中未见；其内容物的释放可能会降低多精受精的几率，尽管至少看到一个多精受精卵，并且其他几个受精卵在透明带内有精子。在合子中，原核彼此靠近，但没有融合的证据。一个“卵黄块”，在排卵前变得偏心，在形成二细胞胚胎时被挤出，但在卵子的非卵黄极仍有许多液泡。所有子宫内的卵子周围都有一层厚度可变的壳膜，但其来源尚不确定。
