Investigation of the determinative state of the mouse inner cell mass. I. Aggregation of isolated inner cell masses with morulae.

Inner cell masses (ICMs) were dissected from 3 1/2- and 4 1/2-day blastocysts and cultured in contact with 2 1/2-day morulae. Blastocysts and morulae were homozygous for different electrophoretic variants of the enzyme glucose phosphate isomerase (GPI). Aggregation of ICMs and morulae was observed, and such aggregates were able to form blastocysts in vitro and morphologically normal foetuses in utero. GPI analysis of these conceptuses revealed that most were chimaeric. However, donor ICM-type isozyme was only detected in the embryonic and extra-embryonic fractions of the chimaeras and never in the trophoblastic fraction. Thus, ICM cells appear unable to form trophoblast derivatives even when exposed to 'outside' conditions as experienced by developing trophoblast cells. This is evidence that ICM cells, although not overtly differentiated, are determined by 3 1/2 days.