TIMETABLE OF IN-VIVO EMBRYONIC-DEVELOPMENT IN THE GREY SHORT-TAILED OPOSSUM (MONODELPHIS-DOMESTICA)

The timing of development was examined in 496 embryos from female Mono delphis domestica, collected at known time intervals after video recor ded mating. Ovulation occurred approximately 20 hr (day 1) after matin g, and fertilization was observed by 24 hr. Transport through the ovid ucts was rapid, and pronuclear stage embryos were recovered from the u terus as early as 24 hr after mating. Second cleavage had occurred by 55 hr after mating. Three-celled embryos were among those collected on day 3 after mating, indicating that asynchronous cleavage of blastome res can occur from the two-cell stage. The four-cell stage persisted f or approximately 24 hr, and embryos that had undergone third cleavage were first recovered 74 hr after mating. Embryos that had undergone fo urth to fifth cleavage were found 96-100 hr (4 days) after mating and complete unilaminar blastocysts by 5.5 days after mating. Primary endo derm formed from an already distinct embryonic area of the unilaminar blastocyst early on day 7 after mating. Formation of the bilaminar bla stocyst was completed rapidly, on day 7 after mating. The primitive st reak appeared on day 10 after mating, and organogenesis rapidly ensued on a timetable similar to that reported for Didelphis virginiana (McC rady, 1938). Close contact with the maternal circulation was establish ed on day 11 and by day 12 maternal and embryonic tissues could not be separated without damage. The length of the gestation period from fer tilization to birth was approximately 13.5 days. These observations pr ovide the basis for further embryological cellular and molecular studi es of this species as a laboratory model for marsupial development. (C ) 1994 Wiley-Liss, Inc.