Trophoblast invasion during implantation of the mouse embryo.

The interaction between trophoblastic and maternal cells was analysed by electron microscopy on days 6, 7 and 8 of pregnancy, in the mouse. Special emphasis was given to phagocytic activity and invasiveness by the trophoblast. On the sixth day of pregnancy, the trophoblast cells are in contact with the epithelial cells of the implantation crypt, with the basal lamina of the crypt, and with cells of the antimesometrial decidua. On the seventh and eighth days of pregnancy, the trophoblastic cells interact with those of the antimesometrial decidua. The giant trophoblastic cells engulf epithelial cells, maternal blood cells and decidual cells although the pattern of phagocytosis of these structures differs. Both whole epithelial cells and blood cells were ingested. The epithelial cells were deteriorated whereas the blood cells had a normal morphology; the decidual cells were ingested following a severe process of deterioration. Processes of trophoblastic cells interposed between the epithelium of the implantation crypt and its basal lamina seem to contribute to the displacement of the epithelial cells. The invasion of the endometrium by the trophoblast begins with the penetration by processes of trophoblastic cells between the decidual cells. The contact between the surface of both cell types may be very close: adherens type junctions and focal contacts are formed between trophoblastic cells and healthy decidual cells. Trophoblastic cells ingest deteriorated or fragmented cells and gradually occupy the spaces left by the latter.