Development of early cell lineages in marsupial embryos: an overview

All major embryonic and extra-embryonic cell lineages are established before implantation in marsupials. In reptiles, birds, monotremes and most marsupials, the zygote is polarized, sometimes markedly so, and the cleavage pattern in relation to the polarized state provides the mechanism for the generation of positional signals. These ensure that the embryonic cell lineages develop in the centre of the developing blastoderm or blastocyst epithelium and the extra-embryonic lineages at the periphery. The evolution of the vertebrate yolky egg was accompanied by a decreasing dependence on maternal determinants and increasing dependence on positional signals to determine cell fate. It is proposed that when a less yolky egg evolved, the mechanisms for determination of cell fate in a developing epithelium were retained. It is proposed that in marsupials, positional signals are involved in the determination of cell fate of embryonic and trophectoderm cells but do so in a two-dimensional epithelium not a three-dimensional morula. The next lineage formed is the primary endoderm which separates off from the primitive ectoderm in the embryoblast and eventually lines the blastocyst cavity. Positional signals are responsible for the determination of primary endoderm in eutherian mammals, birds and probably also marsupials. Order of cell division during cleavage provides a mechanism whereby some cells in the embryoblast of marsupials have earlier and greater contact with their neighbouring cells. The mechanism for determination of primary endoderm cells in the blastocyst epithelium is examined in the Virginia opossum and the stripe-faced dunnart.