An unusual blastocyst stage of the marsupial Isoodon macrourus

Abstract

Four blastocysts, identical in size (diameter 1.1 mm) and appearance, were obtained from one uterus of a short-nosed bandicoot I. macrourus and examined by light and electron microscopy. These blastocysts were unusual in that each one had a conspicuous region of protoderm about 0.3-0.5 mm in width and mostly more than one cell thick. The remainder of the protoderm was thin and unilaminar. Cell divisions were more numerous in the thickened region than in the remainder of the protoderm. Some entire cells and blebs of cytoplasm had been shed or were being shed from the thickened region into the blastocoele. The egg membranes were similzr to those of bilaminar blastocysts, the shell membranes being intact and about 1.0 um thick, and the mucoid coat and zona pellucida being discontinuous.