054. TROPHOBLAST, PLACENTA AND EARLY EMBRYO: HOW THE MARSUPIAL DEVELOPS

Abstract

In marsupials, the blastocyst forms as a single cell layer of cells. The marsupial blastocyst has no inner cell mass, so the 80–100 cell tammar embryo remains in diapause as a unilaminar blastocyst. All marsupials have a unilaminar stage, but what is unusual is that in the tammar the total cessation of cell division and cell metabolism lasts for 11 months each year. Marsupials are placental mammals. The yolk sac forms the definitive placenta up to birth. Only very few marsupials, such as the bandicoot, have a chorio-allantoic placenta, which supplements the placental functions of the yolk sac. However, the understanding how the unilaminar layer of trophoblast cells of the diapausing blastocyst become specified into placental and embryonic tissues has been an ongoing puzzle. To identify genes that do become differentially expressed in tammar development, we targeted the stage of the earliest appearance of the embryonic disc, at which the remainder of the blastocyst is then defined as trophoblast, as well as early cleavage stages. Intriguingly, we found no evidence for early differential expression of the canonical pluripotency genes POU5F1, SOX2 and NANOG, or of CDX2. By contrast, we found overt differential expression of GATA3, the closely related gene GATA2, and FGF4. This expression profile suggests that in the tammar, mechanisms regulating trophoblast- and pluriblast-specific expression of POU5F1, SOX2, NANOG and CDX2 are temporally secondary to those regulating GATA2 & -3 and FGF4 expression. Together, our results may signify the evolution of alternative mechanisms of early lineage specification in marsupials, or alternatively reveal a general hierarchy of signalling mechanisms that are masked in the relatively rapid and ‘compressed’ development of mice. The results of our ongoing study have important implications for understanding not only marsupial stem cells but the early development of all therian mammals.