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.
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.
- 1The 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:
- 2The 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.
- 3The second cleavage (44 hours) and third cleavage (approx. 72 hours) produced small blastomeres with no cell contact. Yolky vesicles surrounded the blastomeres.
- 4The 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.
- 5Unilaminar blastocysts were formed about 5 days after fertilization.
- 6Development in vitro followed a similar time course to that in vivo, however, most embryos were arrested at the 8-cell stage.