Morphological changes in the oocyte and its surrounding vestments during in vivo fertilization in the dasyurid marsupial sminthopsis crassicaudata

Abstract

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.

Marsupial Egg and Embryo Coats

“…This mechanism has been demonstrated in A. agilus (= A. stuartii) [Selwood, 1994] and in T. vulpecula ] and is probably widespread in marsupials because similar vesicular arrays are a common feature in peripheral cytoplasm of other marsupials at the time of cell-cell or cell-zona adhesion, e.g. in Macropus eugenii [Renfree and Lewis, 1996], S. crassicaudata [Breed and Leigh, 1990]. Time-lapse studies of cleavage in A. agilus and S. macroura [Selwood and Smith, 1990] release of the vesicular arrays occurs at specific times in the cell cycle, immediately before and after M phase.…”

“…It appears to be the initial site of sperm egg binding. In the species examined, the zona pellucida provides a barrier to polyspermy in didelphid marsupials but not in dasyurid marsupials [for review, see Breed, 1996] in which polyspermy in vivo has been observed in Antechinus stuartii (= A. agilus) [Selwood, 1982] and S. crassicaudata [Breed and Leigh, 1990]. …”

Comparison of the production, quality, and in vitro maturation capacity of oocytes from untreated cycling and intermediate phase equine serum gonadotropin-treated fat-tailed dunnarts (Sminthopsis crassicaudata)

“…Ovarian stimulation in the fat-tailed dunnart maintaining cellular processes from granulosa cells that form gap and tight junctions through the zona pellucida for endocrine and nutritional support of the oocyte (Breed & Leigh 1990). This was not observed in more mature GOC in preparation of the complete removal of granulosa cells prior to ovulation (Breed & Leigh 1988).…”

Structural and Ultrastructural Aspects of Folliculogenesis in Didelphis albiventris, the South-American Opossum

“…Oocytes maturation and in vitro fertilization studies in marsupials, especially Dasyuridae, were described by Selwood, (1982), Breed & Leigh, (1990), Breed, (1996 and Hickford et al (2001). Kress et al described the oogenesis pattern in Sminthopsis macroura and found that the timetable of oogenesis is accelerated as in other marsupials showing relatively early female maturation.…”