Ultrastructural studies of the mouse blastocyst substages.

The mouse blastocyst stage covers approximately 2 days. During this period, embryonic development advances through four ultrastructurally distinct substages. Specific ultrastructural characteristics, such as changes in cell shape and/or the distribution of intracellular organelles, may be used to characterize the various cell types from each other at each substage (e.g. trophoblast from inner cell mass cells), as well as to distinguish the same cells from each other at different substages (e.g. substage 1 abembryonic trophoblast cells from substage 2 abembryonic trophoblast cells). A distinguishing characteristic of these blastocyst substages are the membrane contacts (tight junctions, desmosomes, focal tight junctions) between cells. The apical surfaces of adjacent trophoblast cells, from the time of cavity formation until the initial stage of implantation, are connected by a tight junction and a desmosome. The basal region of the apposed trophoblast membranes, however, vary both in their spatial interrelationship with each other and in their defined cellular contacts. Likewise, the cell contacts between the inner cell mass cells, and between these cells and the embryonic trophoblast cells, are distinctive for each blastocyst substage.