Marsupial Egg and Embryo Coats

Abstract

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.

Evolution of the shell coat and yolk in amniotes: a marsupial perspective

“…All marsupial conceptuses are enclosed in the shell coat as the zygote passes through the uterotubal junction and into the uterus (Selwood, 2000), where coat deposition continues until late in pregnancy, until at least the primitive streak stage (Casey et al, 2002). The shell coat is thin, 1.2-14 mm wide (Selwood, 2000), highly transparent, durable, and capable of being stretched to cover initially, a zygote plus zona pellucida and mucoid coat of around 250 mm and eventually a blastocyst many mm in diameter.…”

“…Mature oocytes of viviparous marsupials are between 150-250 mm in diameter (Selwood, 2000) and exhibit numerous clear vesicles that contain a variety of granules, droplets and smaller vesicles, which have been identified as originating from mitochondria (Ullmann and Butcher,'96) or from Golgi bodies including endocytotic contributions (Kress,'96). Falconnier and Kress ('92) concluded that some of the material within the vesicles was glycoprotein.…”

Mammalian diversity: gametes, embryos and reproduction

“…These proteins are being considered as targets for marsupial immunocontraception for non-indigenous pest species (Roberts et al 1997). A unique type of oocyte develops in dasyurids (Selwood 2000). Like the wallaby and other marsupials, there is a zona pellucida, thick mucoid coat and shell.…”

Texture of the zona pellucida of the mature pig oocyte. The mammalian egg envelope revisited

“…These glycoproteins all belong to the ZP family and generally contain a common domain or "ZP module" [9]. There are three single-copy ZP genes in mammals ZPA, ZPB, ZPC [3] including marsupials [11]. Depending on the species, ZPA in the mouse [40] or ZPB in the pig [3,41] would constitute sperm receptors.…”

“…The site of ZP synthesis is the oocyte during its growth phase, at least in the mouse [6][7][8] and/or the follicular cells in other species [3,9]. A unifying hypothesis, at least on the composition of these envelopes, has been offered by molecular biology: the constitutive proteins are homologous and all belong to the ZP protein family [3,[10][11][12]. Does a common composition induce a similar structure of egg envelopes?…”