Fig. 8 . TEM of a stage 4 embryo in vitro showing details of the contact zone between the endometrial cells (EC) and the cytotrophoblast (CTr). Measuring bar = 100 µm. The cultured endometrial cells are stacked. The cytotrophoblasts have long protrusions (arrows) that extend into the spaces between the endometrial cells. (From: Lindenberg et al., 1986. Reproduced with permission of the publisher, Oxford University Press.)