Trophectodermal processes regulate the expression of totipotency within the inner cell mass of the mouse expanding blastocyst.

Mouse blastocysts, aged 0, 2, 6 and 12 h from the onset of cavitation, were examined by transmission (TEM) and scanning (SEM) electron microscopy. In TEM sections, trophectoderm cells (TE) differed morphologically from those of the inner cell mass (ICM) by their flattened shape, paler cytosol staining and polarized disposition of both junctional complexes (apicolateral) and intracellular secondary lysosomes (SL; basal). Throughout this period of development, cytoplasmic processes, characterized by abundant SLs, cover approximately 80% of the juxtacoelic face of the ICM. These processes are shown to be derived from the basal surface of TE cells intermediately placed between polar and mural regions. In SEM preparations of the juxtacoelic ICM surface, revealed by 'cracking open' blastocysts, the processes appear as tongue-shaped, centripetally oriented structures which terminate collectively at a central area on the ICM surface. The potential of cultured ICMs to generate TE was demonstrated following their immunosurgical isolation from blastocysts aged up to 12 h post cavitation and by examining the sequence of ultrastructural changes associated with TE generation by ICMs from 2 h blastocysts. In contrast, the juxtacoelic cells of similarly aged ICMs observed in situ in ultrasections of intact embryos showed little or no evidence of totipotency expression as judged by the absence of TE characteristics. Since TE expression within presumptive ICM cells is thought to be generated by an asymmetry of cell contacts (Johnson & Ziomek, 1983), we propose that the juxtacoelic TE processes, by providing a cellular cover to the ICM, function in suppressing the expression in situ of ICM totipotency.