Fig. 3. High power SEM of a stage 4 embryo in vitro that is attached to an endometrial cell culture. Measuring bar = 60 µm. Several cells contact the endometrial surface (arrows) and possibly illustrate early penetration. The asterisk indicates a naked area without microvilli. The trophoblast cells exhibit many microvilli. (From: Bentin-Ley et al. 1999. Reproduced with permission of the publisher, Oxford University Press.)