Ultrastructural study of the intercellular junction and lanthanum penetration in the mouse blastocyst

: Cell junctions in mouse blastocyst were ultrastructurally investigated with or without lanthanum tracer. Tight junctions, gap junctions and desmosomes were observed in the trophectoderm. The tight junction was located near the zona pellucida in all trophoblast interspaces, whereas the gap junction and the desmosome, which were infrequently observed, were localized far from the zona pellucida. However, the desmosomes in the trophectoderm of the expanded blastocyst after culture increased in number and came to be located near the zona pellucida. The trophoblast layer excluded lanthanum whose invasion was interrupted by the tight junction. There were a few intermediate junctions in the interspace between the trophoblast and the inner cell mass cell as well as between the inner cell mass cells. These findings indicate that a substances whose molecular weight exceeds that of lanthanum (138.9) may not flow into the blastocyst through the intercellular space. Moreover, it is speculated that the intercellular connection is strengthened during the expansion of the blastocyst.