The ultrastructure of delayed implanting mouse blastocysts during the period of Days 10 through 30 showed no differences throughout the period, but was different in the following points, as compared with that of intact blastocysts on Day 4.
I. Trophoblastic cells.(1) Elongation of trophoblastic cells that had long cytoplasmic projections at the intercellular portion near the blastocoele, (2) Appearance of the basement membrane-like material along the inner surface of the trophoblast, (3) Irregularity in the shape of the nucleus with invaginations of the nuclear membrane, and the thickening in the density of the nuclear substrate, (4) Disappearance of vacuolated cristae in mitochondria, (5) Decrease in the number of rough-surfaced endopl asmi c reticula, of polysomes and of Golgi apparatus, (6) Increase in the number of lipid droplets, (7) Disappearance of fibrous inclusions.
II. Inner cells.(1) Irregularity in the shape of inner cells with fine cytoplasmic projections, (2) Phenomena concerning nuclear and cytoplasmic organelles and fibrous inclusions similar to those in trophoblastic cells.