Marsupials were popular models for early development in the early 1900s , with classic morphological studies performed by notable embryologists such as J.P. Hill, C.G. Hartman, T.T. Flynn (Errol Flynn’s father) and E. McCrady. These workers were fascinated by the marsupial mode of blastocyst formation, which contrasts starkly with that of eutherians. One key difference is that marsupial zygotes contain large amounts of deutoplasm (often called yolk, but probably with a minimal nutritive role), which is expelled into the extracellular space during cleavage. Consequently, the marsupial conceptus never forms a morula and early blastomeres instead adhere to the inner surface of the zona pellucida where they form a single-cell-layered (unilaminar) epithelium. The resulting blastocyst thus lacks an inner cell mass and the embryonic disc develops much later from cells within a restricted region of the epithelium. Marsupial early development thus retains features that are reminiscent of their much yolkier ancestors but on a smaller scale. They can help us to understand how eutherian early development evolved by elucidating homologies in morphological mechanisms and cell lineages among vertebrates.
Now that three marsupial genomes (wallaby, opossum and Tasmanian devil) have been sequenced, new doors are opening for exploring these homologies. In our first detailed molecular analysis of marsupial early cell lineage specification, one of the more interesting findings is that key regulatory factors known from mouse development appear to be uniformly expressed and localised in all cells of the early unilaminar blastocyst, although underlying biases in cell fate could still exist. This raises the possibly that totipotent stem cells could be derived for the first time in any mammal. We plan to explore this and other avenues and hope that our study is just the beginning of a renaissance in marsupial early embryology!