The blastocyst epithelium is not a protoderm in dasyurid marsupials: a review of the evidence.

Evidence from studies of cleavage and blastocysts in dasyurids in reviewed to show that the unilaminar blastocyst is not a protoderm but consists of two cell types. Cleavage and blastocyst formation in marsupials has been most comprehensively studied in dasyurids, in which the secondary oocyte and zygote are polarized with respect to the position of the nucleus, cytoskeletal elements and cytoplasmic vesicles. Polarity is reinforced by fertilization. Early cleavage divisions are associated with the polarized elimination of a yolk mass and many vesicular structures into the perivitelline space. Because secretion of the vesicular structures, of which several types are found, facilitate blastomere-zona then blastomere-blastomere associations during cleavage, a unilaminar blastocyst forms without an intervening morula stage. Polarization of the cleavage cavity is related to the appearance of two cell lineages, pluriblast and trophoblast, at the 16-cell stage. In species in which the yolk mass persists, the tier of eight cells lying nearest the yolk mass forms the pluriblast (future embryonic and extra-embryonic lineages), and the other tier forms the trophoblast (future extra-embryonic ectoderm of the yolk sac). Thus, the unilaminar blastocyst epithelium is not a protoderm. Blastocyst expansion is associated with increased cell numbers mainly in the trophoblast. Pluriblast cells begin to increase just before the appearance of the hypoblast. The two cell populations can be distinguished by ultrastructural and histological features at the end of cleavage. During blastocyst expansion they differ in appearance and behaviour in vitro and in cell-doubling time. Two populations of cells have also been found in cleavage and unilaminar blastocyst stages in other marsupials, such as opossums, brushtail possums and bandicoots.