The origin of intra-arterial cells in the pregnant uterus of the Macaque (Macaca mulatta)

Modification of uterine vasculature during pregnancy in macaques

Embryonic development in macaques includes extensive modification of the uterine vasculature by fetal trophoblast cells. Soon after the onset of blastocyst attachment to the endometrium, syncytial trophoblast cells intrude between endometrial epithelial cells, resulting in focal epithelium loss. Trophoblast cells continue to move into the endometrial stroma and encounter superficial uterine capillaries. These capillaries are penetrated by trophoblast, which permits maternal blood to leave the maternal circulation and enter lacunae formed within the mass of trophoblast cells. Cytotrophoblast cells enter the uterine vessels and attach to the endothelium via cell adhesion molecules prior to migration into confluent spiral arterioles, against the flow of blood. As intra-arterial cytotrophoblast cells migrate, they displace adjacent endothelium, produce matrix metalloproteinases, traverse the tunica intima, and reside in the tunica media as intramural trophoblast. Intramural trophoblast cells disrupt the tunica media and become surrounded by an extensive extracellular matrix. In areas proximal to the placenta, the entire circumferences of spiral arteries are modified in this way. In the same arteries, distal to the placenta and farther "upstream," trophoblast-mediated changes to the arterial wall are less extensive. Uterine veins are modified by trophoblast only in the area immediately next to the trophoblast shell, with no trophoblast migration. The functional consequence of this trophoblast activity may be to ensure an adequate flow of maternal blood to the placenta, thus enhancing the survival of the fetus.

Early stages of trophoblastic invasion of the maternal vascular system during implantation in the macaque and baboon

Trophoblastic invasion and remodeling of the uteroplacental (spiral) arteries in primates are well-documented, but virally nothing is known of the early stages of these phenomena. Therefore, we examined invasion of the maternal vasculature in macaques and baboons at, and immediately following, implantation. Following penetration of the uterine epithelium (day 9), trophoblast spreads along the residual epithelial basal lamina. By day 10, cytoplasmic processes penetrate the epithelial and endothelial basal laminae, and syncytial trophoblast insinuates itself between maternal endothelial cells. As lacunae develop, both syncytial and cytotrophoblast are exposed to maternal blood. Endovascular cytotrophoblast was first observed in subepithelial dilated capillaries and venules. These vessels are lined by increasingly hypertrophied endothelial cells. The spiral arterioles are unmodified at this time. Particularly interesting was the observation that there is rapid extensive endovascular trophoblast invasion of the spiral arterioles immediately beneath the implantation site. By day 14-16 nearly all of the small arterioles directly beneath the site are completely occluded. There is no invasion of the veins in this region. Somewhat later, the deeper arterioles in the principal zone are invaded. Rather than a continuous stream of cells invading the deeper arterioles, these endovascular cells occur in clusters ranging from a few cells to groups of cells that completely plug the lumen. Our results indicate that trophoblastic invasion of maternal vessels occurs very early; and, at least initially, trophoblast can migrate between and along endothelial cells without causing their lysis. The endovascular cells eventually interrupt the endothelial lining of the arterioles and penetrate the walls of the vessels. The occlusion of arterioles underneath the site suggests that circulation through the lacunae at this stage is indirect. Corresponding stages of human development were examined, and no invasion of arterioles could be observed prior to formation of an extensive cytotrophoblastic shell.

Trophoblast invasion and alteration of mesometrial arteries in the pregnant hamster: light and electron microscopic observations

During gestation in the Syrian golden hamster, an extensive migration of trophoblast into the maternal uteroplacental arterial system takes place. The present light and electron microscopic study has focused on one facet of this phenomenon, namely the trophoblastic invasion and alteration of the mesometrial 'vascular knot' (a complex of anastamosing, small arterial channels carrying blood to the chorio-allantoic placenta). Beginning at about the 8th day of gestation, motile trophoblastic giant cells (TGC), originating from the primitive trophospongium (Träger), invade and migrate as a wave up the central maternal arteries of the decidua basalis. During the 12th and 13th days of gestation, they reach the mesometrium and successively settle in the lumina of the vascular knot arterial channels. Between this time and parturition (day 16) the arrested TGC sequentially remove and replace the arterial endothelium, breach the elastic lamina, and apparently destroy (by lytic activity) nearly all of the mural smooth muscle. Near term, the vascular knot arterial channels are composed almost entirely of TGC. The functional significance of these events, and, more generally, the phenomenon of trophoblastic invasion of maternal uterine arteries in mammalian species having haemochorial-type placentae, are still uncertain. These invasions may be requisite to adapting, structurally and physiologically, the maternal uterine arteries to support of the fetoplacental unit and/or to maintaining the immunologically privileged status of the latter.

The pregnant Syrian hamster as a model to study intravascular trophoblasts and associated maternal blood vessel changes

In pregnant Syrian hamsters (Mesocricetus auratus) used as an animal model for studying the migration of fetal trophoblasts and the associated changes in maternal blood vessels, intravascular trophoblasts migrated well beyond the blood vessels of the uterus and into the vessels of the mesometrium. They migrated beyond the decidua of the uterus, into the lumina of maternal uterine and mesometrial arteries, but not into veins. The arterial changes, which were often segmental, resembled those seen in the decidua and consisted of a replacement of normal smooth muscle cells by poorly differentiated stromal cells. Ultrastructurally, the trophoblasts were either above or below maternal endothelial cells. They occurred also as single or multiple layers within the lumina of arteries that lacked an endothelial lining. Apparent penetration of the elastic membrane by the fetal trophoblasts brought them into close apposition to maternal cells in the arterial wall. Histochemical studies showed heightened metabolic activity of the intravascular trophoblasts as suggested by strong histochemical reactions to nonspecific esterase, succinic dehydrogenase and the glycerophosphate dehydrogenase reactions. Thus, these metabolically active fetal trophoblasts actively migrate into the maternal arterial system, resulting in loss of endothelial cells and changes in the wall of the maternal arteries similar to those in the decidua at the uteroplacental junction.