Ultrastructural Features of Blastocyst Attachment and Trophoblastic Invasion in the Rat

Abstract

Morphological features of the interaction between the im¬ planting 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 tropho¬ blastic erosion of the uterine epithelium and early decidual transforma¬ tion 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.

Apoptosis at the time of embryo implantation in mouse and rat

“…94 Decidual cells adjacent to rat implantation chambers are also strongly positive for tissue transglutaminase, an enzyme catalyzing cross-linking reactions and whose activation is associated with apoptosis. 95 Investigations in the mouse 96 and in the rat 97 confirmed the digestion of displaced epithelial cells by nearby trophoblasts. When histological sections through rat implantation sites are analyzed for acid phosphatase activity, however, highest levels of this marker of phagocytosis are found in decidual cells, 95 suggesting that both trophoblast and decidual cell types are likely to participate in the elimination of dying cells from the regressing area.…”

Uterine receptivity and the plasma membrane transformation

“…Under the influence of either hormone alone, change in the apical plasma membrane is mostly limited to alterations in the height and frequency of microvilli [1,2,3 for review]. However, during early pregnancy when progesterone and oestrogen act together in the peri-implantation period leading to uterine receptivity for implantation of the blastocyst, the apical plasma membrane of uterine epithelial cells undergoes a more marked form of structural change during which over the several days of early pregnancy, and beginning as early as d 2 of pregnancy, it gradually loses regular microvilli and becomes very flat [2,[4][5][6][7]. Indeed, the membranes of uterine epithelial and trophoblast cells in the implantation chamber come to run more or less parallel all along the area of contact as more generally do the membranes of opposing uterine epithelial cells outside the implantation chamber especially in species with uterine closure [8,9].…”

The Usefulness of Alkaline Solutions for Clearing the Uterus and Staining Implantation Sites in Rats

“…Rat blastocyst implantation begins when the blastocyst attaches to the surface of the uterine luminal epithelium, on the antimesometrial side [2,9,14). After implantation, the placenta and metrial glands are formed on the mesometrial side of the uterus [26,27]. The spots after delivery are cicatrices resulting from uterine wall injury following placental decollement.…”