Implantation in the marmoset monkey: Expansion of the early implantation site

This study was initiated to examine the early stages of trophoblast adhesio n and invasion during implantation in the marmoset. Seven implantation site s were found in the uteri of four marmosets taken between days 13 and 15 of gestation. Three implantation sites in two uteri were examined in detail b y electron microscopy. Between days 13 and 15, the marmoset implantation si te expanded peripherally by adding areas where syncytial trophoblast penetr ated between uterine luminal epithelial cells. Such penetrating masses ofte n bridged openings of endometrial glands, shared junctional complexes with the uterine epithelial cells between which they are infiltrating, and subse quently reached the residual basal lamina of the uterine luminal epithelium . Centripetal to the peripheral region was an intermediate region in which syncytial trophoblast overlay individual clusters of epithelial cells and r ested along the basal lamina. In this region there was some evidence of fus ion of syncytial trophoblast with uterine epithelial cells. In the central region of the implantation site near the inner cell mass and amnion the tro phoblast formed elaborate lamellipodia in relation to the basal lamina. In one of the three specimens examined with electron microscopy there were two foci where trophoblast penetrated through the basal lamina. It was also in the central region that trophoblast penetrated farthest into the uterine g lands. The gland cells closest to trophoblast were less closely associated and lost their columnar shape, forming large round cells similar to the epi thelial plaque cells of other primates. Where two blastocysts implanted on the same side of the uterus a conjoint membrane was formed which in regions consisted solely of syncytial trophoblast with two basal surfaces and two basal laminas. The prolonged period of time when the implantation site expa nds within the plane of the uterine epithelium (trophoblastic plate stage) and the peripheral to central sequence in extent of development make this p rimate a particularly useful animal for studies of trophoblast adhesion to and penetration of the uterine luminal epithelium. Anat Rec 256:279-299, 19 99. (C) 1999 Wiley-Liss, Inc.