Deep trophoblast invasion and spiral artery remodelling in the placental bed of the lowland gorilla

In contrast to baboon or rhesus macaque, trophoblast invasion in the human placental bed occurs by the interstitial as well as the endovascular route and reaches as deep as the inner myometrium. We here describe two rare specimens of gorilla placenta. In the light of recent findings in the chimpanzee, we postulated the occurrence of deep invasion in gorilla pregnancy. Tissues were processed for histology (PAS, orcein), lectin staining (Ulex europaeus agglutinin 1) and immunohistochemistry (cytokeratin 7/17, α-actin). A specimen of young but undetermined gestational age included deep placental bed tissue, showing interstitial and spiral artery invasion of the inner myometrium as well as the decidua. The cell density and depth of trophoblast invasion was equivalent to a human placental bed of 10-14 weeks. Intraluminal trophoblasts were not seen in any of the invaded vessels, allowing no definite conclusions about the origin of the intramural trophoblast and the time-course of spiral artery invasion. A different late second trimester placenta specimen showed scattered extravillous trophoblast in the basal plate and underlying decidua, as well as a remodelled spiral artery containing intramural trophoblast. Absence of inner myometrial tissue precluded assessment of invasion depth in this later specimen. Despite the limited material we can conclude that key aspects of trophoblast invasion are shared by the three hominid species: gorilla, chimpanzee and human.