Implantation and extravillous trophoblast invasion: From rare archival specimens to modern biobanking

Abstract

Extravillous trophoblast invasion serves to attach the placenta to the uterus and to enable access to nutrients for the embryo throughout pregnancy - secretions of the uterine glands in the first trimester, maternal blood in the second and third trimester. For assessing extravillous trophoblast invasion, histology (in combination with immunohistochemistry) still plays a major role in placental research. This is especially true for the re-assessment of rare archival specimens from early human implantation sites or placenta in utero with the background of recent knowledge which may help to strengthen current hypotheses. This review summarizes the recently expanded picture of extravillous trophoblast invasion, gives an overview about fundamental archival specimens in placental research, presents new images of archival specimens, gives insights into the latest developments in the field of biobanking and provides insight into the current situation on sample usage in the absence of biobanks. Modern techniques allow expanding our hitherto believed concept of extravillous trophoblast invasion, which is not restricted to spiral arteries: Extravillous trophoblasts also invade into uterine glands and uterine veins and thereby connect all these luminal structures with the intervillous space. All biomedical research dramatically depends on the quality of the assessed biological samples. Hence, researchers should be aware that the time between collection of a sample from a body and the beginning of analysis (pre-analytical phase) may have more impact on the outcome of a study than previously assumed.

Keywords: Biobanking; Endoglandular trophoblast; Extravillous trophoblast; Human implantation; Placenta; Uterine gland; Uterine vein.

Fig. 1. Connection between uterine glands and the early conceptus during implantation.

(1–22) Hematoxylin and eosin stained serial sections of a human implantation site at day 10 (archival specimen) in putative serial order. Images in (13) and (14) demonstrate a clear connection (arrows) between the early conceptus (EC) and the uterine gland beneath. Uterine glands are marked by asterisks. Lumina of uterine glands beneath the EC seem expanded and filled with fluid (probably a mixture of glandular secretion products and maternal blood).

Fig. 2

Nomenclature of various subtypes of trophoblast cells depending on their location.

Fig. 3. Extravillous trophoblasts (EVTs) invade uterine veins (endovenous invasion).

Serial sections of invaded first trimester decidua with immunohistochemical double staining (gestational age 7 weeks). Image in (a) is immuno-double stained for von Willebrand factor (VWF) (blue, serves as marker for vascular endothelial cells) and major histocompatibility complex, class I, G (HLA-G) (brown, serves as marker for EVTs). Image in (b) is immuno-double stained for EphB4 (blue, serves as marker for venous endothelial cells) and HLA-G (brown). (a) Interstitial trophoblasts (examples marked by asterisks) are situated within the decidua, the uterine vessel (circle) is nearly completely surrounded by EVTs. (b) The neighboring section shows that the endothelium of the vessel (circle) is Eph4 positive (blue) and thus identifies the vessel as uterine vein. Arrows point to EVTs within the venous lumen and within the endothelium. AB, air bubble; no nuclear counterstain.

Fig. 4. Eroded glands especially at the edge of the developing placenta.

Serial sections of an archival placenta in utero from hysterectomy, image in (a) is immunostained for HLA-G (brown), images in (b, c) are immuno-double-stained for HLA-G (brown, serves as marker for extravillous trophoblasts) and Ck7 (blue, serves as marker for uterine glands). (a) Overview: shows the embryo in the center of the uterus, the uterus with a thick layer of myometrium underneath the decidua, the non-invaded decidua parietalis, the junctional zone between mother and fetus (decidua basalis, strongly invaded by extravillous trophoblasts), the villous part of the placenta with the intervillous space inbetween, and cell islands. (b, c) Details from inset in (a). Inset marks the zone of the placental edge and thus the transitional zone between invaded decidua basalis, non-invaded decidua parietalis and the uterine cavity. (b) Uterine glands (UG) are invaded by endoglandular trophoblasts particularly at the edge of the expanding placenta. The most upper gland appears nearly completely eroded, endoglandular trophoblast (green arrow) replace the glandular epithelium. (c) Higher magnification of uterine glands invaded by endoglandular trophoblasts (green arrows) at the edge of the placenta. Nuclei were counterstained with Hemalaun (a) or no nuclear counterstain (b, c).