Electron microscopic studies on the mammalian blastocyst in the pre-implantation stages.

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This work was designed to investigate the fine structures of the rat and the rabbit blastocysts with both transmission and scanning electron microscopes.The cells of the trophoblast form a single layer beneath the intact zona pellucida and the inner cell mass and blastocystic cavity are formed. In the blastocyst on day 5 to 6 of pregnancy, the trophoblast cells were differentiated from the inner cell mass cells. However, the difference in density was not distinct and the distribution and structures of the organelles were almost similar in both cells.The tight junctional complex between the trophoblast cells and the loose association among the inner cell mass cells were observed. The organelles of the blastocysts were quite specific. In the rabbit blastocyst, as inclusions restricted to the blastocyst stage, crystal-like inclusions were present.From the standpoint of functional aspects the incorporation of 3H-uridine and 3H-thymidine into rat blastocysts was studied using electron microscopical radioautography. The result showed that incorporated 3H-uridine was found in all regions and 3H-thymidine mostly in nucleus. Moreover, with scanning electron microscope the surface of blastocyst was observed after the zona pellucida was removed with pronase. The surface of zona-free rat blastocyst was wholly covered with dense, fine microvilli and abembryonic proliferation was seen. Each bulging trophoblast cell was bounded by grooves and distinguishable to one another.