The trophoblast plug during early pregnancy: a deeper insight

Abstract

During the first trimester of pregnancy, foetal endovascular trophoblasts invade into maternal spiral arteries, accumulate and form plugs in the lumen of the vessels. These plugs only allow blood plasma to seep through. Hence, during the first trimester of pregnancy, a first flow of fluids through the placental intervillous space is established, resulting in a physiological oxygen gradient between mother and foetus. The trophoblast plugs block spiral arteries until the beginning of the second trimester (11-14 weeks). In parallel, uterine glands are invaded and opened by endoglandular trophoblasts towards the intervillous space of the placenta, without showing the formation of plugs (Moser et al. in Hum Reprod 25:1127-1136, 2010, Hum Reprod Oxf Engl 30:2747-2757, 2015). This enables histiotrophic nutrition of the embryo prior to onset of maternal blood flow into the placenta. Failure of these endovascular and endoglandular invasion processes may lead to miscarriage or pregnancy disorders such as intrauterine growth restriction (IUGR). After dissolution of the plugs, the onset of maternal blood flow allows maternal blood cells to enter the intervillous space and oxygen concentrations rise up. In this study, we demonstrate for the first time serial cross sections through a trophoblast plug in a first trimester placental bed specimen. Invaded and plugged arteries as well as invaded uterine glands in week 11 of gestation are visualized with specific immunohistochemical double staining techniques. We show that spiral artery plugs appear throughout the placental invasion zone and illustrate erythrocytes stowed due to trophoblast plugs. In addition, we give evidence of the presence of MMP-1 in plugs of invaded spiral arteries. The results reveal a better understanding and a closer insight into the morphological appearance of trophoblast plugs and the consequences for placental and uterine blood flow.

Keywords: Endoglandular trophoblast; Endovascular trophoblast; Placenta; Spiral artery; Trophoblast plug; Uterine gland.

Fig. 1

Trophoblast invasion and arterial trophoblast plugs—overview. Immunohistochemical double staining of serial cross sections of invaded decidua basalis (gestational age 11 weeks) for major histocompatibility complex, class I, G (HLA-G) (brown, serves as marker for extravillous trophoblasts) and von Willebrand factor (vWF) (blue, serves as marker for vascular endothelial cells). No nuclear counterstain. a Schematic outline of structures of interest within the cross section in (b). Decidual stroma is dotted, plugged arteries are marked red, invaded arteries are marked with red stripes. Uterine glands are marked in blue and uterine vessels in grey. b Corresponding histological cross section to the scheme in image a. Various invaded and plugged sectors of spiral arteries can be identified within the section. c Progression of the section in (b) with a distance of 55 µm. Alterations of plugs and invaded vessels and glands (specifically identified in respective stainings of serial sections, not shown) are present. d Further changes of invaded structure with a distance of another 60 µm. Scale bar represents 50 µm

Fig. 2

Arterial trophoblast plugs and invasion of uterine glands—details. Immunohistochemical double staining of serial cross sections of invaded decidua basalis (gestational age 11 weeks) for major histocompatibility complex, class I, G (HLA-G) (brown, serves as marker for extravillous trophoblasts) and von Willebrand factor (vWF) (blue, serves as marker for vascular endothelial cells) (a–d) or cytokeratin 7 (KRT7) (blue, serves as marker for glandular epithelium here) (e). No nuclear counterstain. (a–c) Progression of a trophoblast plug (asterisk) in consecutive cross sections: In (a) and (b) the trophoblast plug fills the complete lumen of the spiral artery, whereas in (c) there are only few endovascular trophoblasts left. (b) is in 55 µm distance to (a) and in 60 µm distance to (c). (d) highlights the most prominent plugged artery in the current placental specimen. Erythrocytes (circle) appearing yellow-greenish-blue are stowed by the trophoblast plug (asterisk). (e) demonstrates invasion of endoglandular trophoblasts (arrows) into a uterine gland (triangle). The gland is opened towards the intervillous space. Scale bar 50 µm

Fig. 3

MMP-1 expression in arterial trophoblast plugs. Immunohistochemical single (a, c, e, g) and double (b, d, f, h) staining of serial sections (a–b, c–d, e–f, g–h) of invaded decidua basalis (gestational age 11 weeks a–d and gestational week 7 e–h) for matrix metalloproteinase 1 (MMP-1) and for major histocompatibility complex, class I, G (HLA-G) (brown, serves as marker for extravillous trophoblasts) together with von Willebrand factor (vWF) (blue, serves as marker for vascular endothelial cells). Nuclei are counterstained with Haemalum (a, c, e, g) or no nuclear counterstain (b, d, f, h). a, c, e, g Endovascular trophoblasts within trophoblast plugs (asterisk) are clearly positive for MMP-1. Additionally interstitial trophoblasts as well as decidual stroma cells are positive for MMP-1. b, d, f, h Immunohistochemical double staining in serial sections enables a clear classification of the assessed structures and cells and clearly confirms the extravillous origin of the trophoblasts composing the trophoblast plug. Scale bar 50 µm

Fig. 4

Different perspectives of trophoblast plugs within spiral arteries. Trophoblast plugs (A) are composed of endovascular trophoblast cells. Depending on the stage of invasion and on the localization of sectioning the lumen of the spiral artery can be either completely filled with trophoblasts (1, 2) or partly infiltrated (3, 4). In a plugged artery, the approaching erythrocytes are stowed by the trophoblast plug (1). Uterine glands are invaded and opened towards the intervillous space (C) by endoglandular trophoblasts. Glandular epithelial cells are replaced by endoglandular trophoblasts (5)