Ultrastructure of trophoblast giant cell transformation during the invasive stage of implantation of the mouse embryo.

The fine structure of abembryonic and mural trophoblast cells of mouse embryos was analyzed during the initial stages of invasion of the endometrial stroma by the embryo (days 6-8 of pregnancy). On day 6 of pregnancy, most trophoblastic cells are flat and have spindle-shaped nuclei. A few large, round trophoblastic cells (giant cells) are present at the abembryonic pole. As pregnancy proceeds through days 7 and 8, the area occupied by the trophoblast becomes larger because of an increase in the trophoblastic cell population, growth of giant cells, and rearrangement of the latter cells into a network containing maternal blood. As flat cells transform into giant cells, their content of ribosomes, granular endoplasmic reticulum, Golgi complexes, lysosomelike bodies, and heterophagosomes increases. Reichert's membrane is always lined by cell bodies or by laminar processes of trophoblastic cells that are provided with small pores. Transformation of flat cells into giant cells is associated with an activation of the giant cells and their acquisition of invasive behavior.