Structural and functional properties of trophoblast cells of mouse egg-cylinders in vitro.

Embryos at the egg-cylinder stage were obtained by culturing blastocysts in vitro on an agarose surface for 4 days. The adhesiveness and outgrowth activity of the trophoblast of the egg-cylinder were compared with those properties of the flushed adhesive blastocyst. Trophoblast cells of egg-cylinders were found to be more adhesive and their outgrowth activity greater than in the case of trophoblast cells of adhesive blastocysts. The cultured egg-cylinders presented two subpopulations of giant trophoblast cells. They represent probably two stages of differentiation of the same trophoblast population. The most distinctive morphological differences of these subpopulations were that the surface of one was covered with small blebs and the cytoplasm had relatively few ribosomes, while the surface of the other subpopulation was covered with large blebs and microvilli and the cytoplasm was rich in ribosomes and large, dense granules. The two types of giant trophoblast cells of the day 7 implantation site consist of one subpopulation whose cytoplasm has few organelles, and the other subpopulation, whose cytoplasm is rich in ribosomes, lysosome-like bodies, and heterophagosomes. Hence, we conclude that the in vitro grown trophoblast cells have counterparts in the in vivo implanting embryos. The lectin binding pattern of the agarose cultured egg-cylinder trophoblast cells was similar to that of the adhesive and/or invasive trophoblast cells grown in vivo. Thus, the in vitro grown egg-cylinders are appropriate material for the analysis of trophoblast cells at the invasive stage of implantation.