Early embryonic development of the grey short‐tailed opossum, Monodelphis domestica, in vivo and in vitro

Abstract

Eggs and embryos (up to the unilaminar blastocyst stage) were collected from grey short‐tailed opossums, Monodelphis domestica, at known time intervals after mating. Some were fixed immediately for examination by light and electron microscopy while others were maintained in culture at 37 °C in 5% CO2 in Ham's F10 medium containing foetal calf serum. Cleavage to the early blastocyst stage was obtained in vitro although most embryos displayed arrested development at the eight‐cell stage. From gross morphological and ultrastructural observations a pattern of embryogenesis to the unilaminar blastocyst was constructed. Comparisons were made with the events of early development in other marsupials. Summary Oocytes were fertilized soon after ovulation and their zonae pellucidae surrounded by a mucoid layer originating from mucopolysaccharide secretion of oviductal epithelial cells. Within 24 hours zygotes passed into the uterus and shell membrane formation commenced. The early embryonic development in vivo and in culture of the grey short‐tailed opossum, Monodelphis domestica, was investigated by light and electron microscopy. The following timetable of events was constructed: The first cleavage at about 28 hours after ovulation (54 hours after mating) was at right angles to the equator of the zygote and was accompanied by the vesiculation of yolk mass. The resulting two blastomeres were composed of peripheral polar cytoplasm. The second cleavage (44 hours) and third cleavage (approx. 72 hours) produced small blastomeres with no cell contact. Yolky vesicles surrounded the blastomeres. The fourth cleavage division (approx. 96 hours) occurred in the same equatorial plane as the second resulting in two tiers of cells which subsequently flattened against the inner surface of the zona pellucida. Tight junctions formed between adjacent blastomeres. Unilaminar blastocysts were formed about 5 days after fertilization. Development in vitro followed a similar time course to that in vivo, however, most embryos were arrested at the 8‐cell stage.

Unexpected oocyte growth after follicular antrum formation in four marsupial species

“…During first cleavage a large amount of cytoplasm (about one third) is separated from the embryo by formation of either an anuclear yolk mass (e.g. dasyurids, Selwood & Smith, 1990) or a mass of cytoplasmic yolk bodies (Tyndale-Biscoe & Renfree, 1987;Baggott & Moore, 1990). Despite the many general references to the yolky character of marsupial eggs, the manner of yolk formation and its role in development has not been examined directly.…”

Mechanisms for pattern formation leading to axis formation and lineage allocation in mammals: a marsupial perspective

“…All early conceptuses show polarity but do not all share the same polarized features. However, all early conceptuses that have been analysed ultrastructurally show polarity related to the distribution of vesicles at one pole and nucleus or mitochondrial-rich cytoplasm at the other (Selwood and Sathananthan, 1988;Baggott and Moore, 1990;Renfree and Lewis, 1996;Frankenberg and Selwood, 1998). A variety of anchoring devices characterize marsupial zygotes so that the mitochondrial-rich cytoplasm is retained when vesicles or the yolk mass are discharged (Breed et al, 1994(Breed et al, , 1995Merry et al, 1995;Frankenberg and Selwood, 1998).…”

Conceptus Polarity and Cell-Zona Adhesion during Early Cleavage (Fertilized Tubal Egg to 8-Cell Stage) in the Marsupial <i>Sminthopsis macroura</i>

“…The mature oocytes of S. macroura exhibit the most extreme form of polarity, with the nuclear material at one pole and an accumulation of vesicular bodies at the opposite pole. Many other marsupials, such as in Monodelphis domestica [Baggott and Moore, 1990;Falconnier and Kress, 1992;Breed et al, 1994], Sminthopsis crassicaudata [Breed, 1996;Breed andLeigh, 1988, 1990;1992] or Trichosurus vulpecula [Frankenberg and Selwood, 2001], however, show only a polarised nucleus, with the vesicular bodies located centrally and a more pronounced polarity appearing after fertilization.…”