Conceptus Polarity and Cell-Zona Adhesion during Early Cleavage (Fertilized Tubal Egg to 8-Cell Stage) in the Marsupial SMINTHOPSIS MACROURA.

摘要

This study outlines the ultrastructural changes that occur in SMINTHOPSIS MACROURA tubal zygotes to the 8-cell stage in relation to observations of development in vitro, oocyte polarity and cell-zona adhesion. The extremely polarized mature oocytes and zygotes have nuclear material at one pole and accumulated vesicular bodies at the other. The first division is associated with extrusion of vesicular bodies and some cytoplasm as a membrane-bound yolk mass into the perivitelline space. Early cleavage is accompanied by the appearance of an extensive, highly structured extracellular matrix (ECM) comprised of amorphous substance, granules and filaments. At the 2- and 4-cell stage the decrease in density of the ECM in the vicinity of the blastomeres may facilitate cell-zona contact. At the 8-cell stage, discharge of vesicular bodies, which mostly appear to be empty, may contribute to the ECM by increasing the area of plasma membrane for synthesis of a hyaluronan-like ECM. As in other marsupials, the precedence of cell-zona adhesion over cell-cell contacts prevents morula formation. The earliest cell-zona contacts appear when microvilli contact the zona in the uterine zygote 12-16 h after uterine entry and continue at later stages. This early contact is possible because of the absence of a dense subzonal ECM in this species. Between late zygote and late 4-cell stage the cytoplasm also contains, beside a large amount of vesicular bodies, demarcated areas where smooth endoplasmic reticulum encloses mitochondria, vesicles, granular material and fibrillar arrays. The latter develop in the late zygote stage and are found outside demarcated areas as well, often closely surrounding large vesicles, probably helping vesicle extrusion. A putative germ plasm was identified at the 4-cell stage.