Transmission electron microscopy of mouse blastocysts activated and growth-arrested in vivo and in vitro.

Abstract

Blastocysts from mice in a state of delayed implantation were examined by electron microscopy, either directly or after a culture period, to find out if activation and growth arrest in vitro were similar to the corresponding stages in vivo. It was found that activation in vitro, obtained by culturing the blastocysts in an outgrowth medium, was accompained by morphological signs similar to those during oestrogen activation in vivo, namely an increase in polyribosomes and glycogen granules. Analogously, growth arrest in vitro, obtained by culturing the blastocysts in a medium deprived of glucose, arginine and leucine, was accompanied by a morphology similar to that of blastocysts obtained during delay of implantation, namely scattered ribosomes of the monosome type, few membranes of endoplasmic reticulum and a lack of glycogen granules, all signs of a low metabolism. However, the morphological parallelism between the in vitro and the in vivo systems does not necessarily mean that the growth-controlling factors are the same in both cases. Activated blastocysts were transplanted to uteri of mice in delay of implantation to characterize the trophoblast ultrastructure four days after the transplantation. The experiments demonstrated that both in blastocysts activated for 24h in vivo and in those activated for 6 h in vitro the morphology reverted to that of inactive blastocysts, indicating that the functional inactivity is related to reversion into a morphologically inactive state rather than to uterine blocking of a morphologically active state.