EARLY CLEAVAGE TO FORMATION OF THE UNILAMINAR BLASTOCYST IN THE MARSUPIAL ANTECHINUS-STUARTII - ULTRASTRUCTURE

The development of Antechinus stuartii from the 2-cell stage to the bl astocyst stage in vivo was examined by routine transmission electron m icroscopy. The 2-8-cell stages had a similar organization of organelle s, whereas the 16- to 32-cell stages had pluriblast cells and trophobl ast cells forming an epithelium closely apposed to the zona pellucida. Specialized cell-zona plugs were formed at the 8-cell stage, and prim itive cell junctions appeared in later conceptuses. The cytoplasmic or ganelles included mitochondria, lysosomes, aggregates of smooth endopl asmic reticulum, lipid and protein yolk bodies and fibrillar arrays, p ossibly contractile in function. Nuclei had uniformly-dispersed dense chromatin. Nucleoli of 2-4-cell conceptuses were dense, compact and fi brillar, and those of 8-cell conceptuses and later conceptuses were fi nely granular and became progressively reticulated. The embryonic geno me is probably not switched on before the 8-cell stage. Sperm tails we re detected in cells in several early conceptuses. The yolk mass had t he same organelles as cells. Centrioles were discovered for the first time in marsupial conceptuses. These were prominently situated at a sp indle pole in a 32-cell blastomere and were associated with a nucleus and sperm tail at the 4-cell stage. It is very likely that the paterna l centrosome is inherited at fertilization and perpetuated in Antechin us embryos during cleavage.