The ultrastructure of preimplantation hamster blastocysts developed in vivo and in vitro.

Scanning electron microscopy of zona-free blastocysts showed bulging trophoblast cells possessing slender microvilli and many small projections of various lengths. The trophoblast cells contained mitochondria in two shapes. One type consisted of spheroids with a few mitochondrial inner membranes. The other type had a more conventional appearance. Fibrous material varied in amount. The trophoblast cells from blastocysts cultured for 1 day had an appearance similar to that of trophoblast tissue fixed in vivo. However, the large, dense mitochondria with few cristae had diminished in number, and globular mitochondria occurred more frequently. Furthermore, the fibrous material was more sparsely distributed after culture. The experiments demonstrated that cultured hamster blastocysts have ultrastructural characteristics similar to those of in vivo blastocysts. Experiments are in progress to test whether the in vitro technique can determine the growth requirements of the hamster blastocyst.