Marsupial egg and embryo coats.

Egg and embryo coats of marsupials are reviewed. Marsupial eggs are enclosed by a zona pellucida, mucoid coat and an outer shell coat. An extra-cellular matrix coat that lines the zona pellucida also occurs in some species. The zona pellucida consists of three zona proteins (ZPA, ZPB, ZPC) with considerable similarity to those of eutherian mammals. The zona is thought to be the sperm receptor site and, in some species, but not others, is a barrier to polyspermy. Immunostaining, in situ hybridisation, histochemistry and electron microscopy indicate that the zona originates in the oocyte at the onset of oocyte growth. It changes greatly in shape during ovulation and early development and plays an essential role in blastocyst formation. The mucoid coat is an acid glycoprotein that is produced by non-ciliated cells at all levels of the oviduct and has been implicated in providing a barrier to polyspermy, nourishment of the embryo and an osmotically stable environment for the embryo. Its width varies widely in marsupials. The shell coat is secreted by the epithelia of the utero-tubal junction and upper uterus during oestrus and early cleavage. Evidence is provided for a second wave of secretion at bilaminar and trilaminar blastocyst stages. Shell-free embryos do not survive in vitro during bilaminar and early trilaminar blastocyst stages. Hatching from the shell coat occurs between 65 and 85% of the gestation period usually when the embryo is undergoing early somitogenesis. The extra-cellular matrix is secreted by the oocyte or by the cells of early cleavage stages and initially separates cells from the zona pellucida and from each other. It plays an essential role in blastocyst formation. The zona, mucoid and shell coats provide a framework for blastocyst construction and normal embryos do not form in their absence in some species.