TEM of a stage 4 embryo in vitro showing the most peripheral contact area between two trophoblast cells (T) and cultured endometrial cells (E). Measuring bar = 2 µm. One trophoblast cell adheres to the endometrial epithelial cell (arrow). Another trophoblast cell has started epithelial penetration (arrowhead).
From: Bentin-Ley et al., 2000. Reproduced with permission of the Society for Reproduction and Fertility.
Source: The Virtual Human Embryo.