Morphological features of the interaction between the implanting blastocyst and the uterine endometrium were studied by electron microscopy, using material fixed in situ. The observations extended from the pre-attachment stage to the completion of trophoblastic erosion of the uterine epithelium and early decidual transformation of the stromal cells.
After shedding of the zona pellucida, the blastocyst establishes contact with the tips of microvilli and with bleb-like cytoplasmic protrusions of the epithelial cells. Attachment occurred during the afternoon of the 5th day post coitum (p.c.). Patches of`bristle-coated' membrane appeared on the trophoblast membrane at that time. A lattice-like substructure was discernible in the cytoplasmic plaques, which are abundant in the early blastocyst. Sperm-tail inclusions were observed in both trophoblast and inner cell mass as late as Day 5 p.c.
By the 6th day p.c., the epithelial microvilli were usually lost and the trophoblast membrane interlocked with that of the epithelial cells, with frequent formation of tight junctions. Leucocytes were occasionally seen in the epithelial layer at the attachment site. Unusually electron-dense cells were often found wedged between the trophoblast and epithelial cells; their possible relation to Wilson bodies is discussed.
The earliest morphological response of the stromal cells to blastocyst attachment was the appearance of changes in nuclear shape, chromatin distribution and nucleolar structure in the subepithelial layer. The deeper subepithelial stroma became oedematous, and diapedesis, erythro-phagocytosis and later the formation of sinusoids could be observed.
During Day 7 p.c., the basement membrane generally disappeared and the trophoblast cells were then in close contact with stromal cells : the distance between the two unit membranes was in the range 100 to 200 Å. A `periplacental fibrinoid barrier' could not be recognized at this stage, but the existence, or later development, of such an immunological barrier could not be excluded.
Stromal cells undergoing decidual transformation showed an abundance of membrane-bound ribosomes and a striking accumulation of fibrils, 80 to 90 Å in diameter, displacing the cytoplasmic organelles.
The development of Reichert's membrane was traced from its first appearance as an amorphous deposit along the inner wall of the trophoblast on Day 6 p.c., to the formation of a stratified, membranous structure on the 8th day.