The in vitro transport of 14C-alpha-aminoisobuturic acid into blastocysts from mice in delay and after activation for implantation.

The uptake of 14C-AIB (14C-alpha-aminoisobuturic acid) into mouse blastocysts was studied in vitro. The flushed blastocysts fell into two size categories, namely large or "expanded" and small or "contracted". The arithmetic mean volume for the large blastocysts grew from 0.58 nl for delayed blastocysts to 0.76 nl for 8 h activated ones, while both delayed and activated small blastocysts had a mean volume of 0.22 nl. Delayed and 8 h activated large blastocysts showed an enhanced uptake of 14C-AIB with time of incubation, the uptake increasing 3-fold from 15 min to 60 min. The small blastocysts generally showed a higher uptake than the large ones, but the activated small blastocysts were atypical in that they unexpectedly displayed a 3-fold decrease in uptake at 60 min incubation as compared with 15 min. The early peaks of 14C-AIB uptake in vivo by uterine tissue and blastocysts were shown to be well correlated to the uptake in vitro. The presence of a working System A carrier in the activated blastocysts was demonstrated by showing that the uptake of AIB was sodium dependent and could be inhibited by N-methyl-AIB. Ultrastructural studies by scanning and transmission electron microscopy revealed that the trophoblast of large blastocysts was elongated and covered by numerous microvilli on a slightly bulging surface. The small blastocysts on the other hand displayed cuboidal trophoblast cells, sometimes in a pseudostratified arrangement, with the outer surface covered by many ridges.