Early embryology af the marsupials Isoodon macrourus and Perameles nasuta

Abstract

Twelve embryos, ranging from a four-celled stage to late unilaminar blastocysts, were obtained from the bandicoots I. macrourus and P. nasuta and examined by light and electron microscopy. These early stages covered at least one-quarter of the 12.5-day gestation period. The three non-cellular egg membranes characteristic of marsupials (zona pellucida, mucoid coat and shell membrane) were present, although the zona was sometimes absent or discontinuous in the intermediate and late unilaminar blastocysts examined. At the four-celled stage the blastomeres were close to the zona, but they had lost contact with each other, probably due to the extrusion of yolk, a phenomenon which has been described in other marsupials. The embryo did not increase in diameter until it was composed of at least 75 contiguous cells, which were in contact with the zona. In several of the larger blastocysts the protoderm cells had lost contact with the zona. Subsequently, the number of cells increased considerably and they were flattened against the egg membranes to form the late unilaminar blastocyst stage. Electron microscopy of the protoderm cells revealed the presence of numerous microvilli, particularly on the outer surface, and a range of other structures as great as those found in eutherian mammals. Remnants of spindle bridges were common in one 75-celled embryo. The yolk material in the blastocoele was also composed of a variety of structures, including small crystalloid inclusions composed of hexagonal units about 8-10 nm in diameter. Similar crystalloids have been described in the cells of early mouse and rabbit embryos and in egg and embryonic cells of various amphibians.