Endocytic and Transcytotic Processes in Villous Syncytiotrophoblast: Role in Nutrient Transport to the Human Fetus

Abstract

The supply of nutrients to the developing fetus is a major function of the human hemochorial placenta, a placenta type in which the fetal chorion is in direct contact with the maternal blood. At term, nutrients have to be transported across two cell layers in chorionic villi, the syncytiotrophoblast (STB) and fetal endothelial cells. The STB is a continuous syncytium covering the entire surface of chorionic villi. This polarized epithelium is specialized in exchange processes and membrane trafficking between the apical membrane facing the maternal blood and the basal membrane facing the fetal endothelium. To meet placental and fetal requirements, the STB selectively takes up and transports a variety of nutrients, hormones, growth factors and cytokines and also transfers passive immunity to the fetus by receptor-mediated transcytosis. In this review in vivo and in vitro systems currently used to study STB functions are discussed and the potential mechanisms of transplacental IgG, iron, lipoprotein and glucose transport are presented. As revealed in this article, the placenta is a tissue where intensive cell biological research is required to unravel endocytic trafficking pathways in a highly specialized cell such as the STB.

A cohort study of haemoglobin and zinc protoporphyrin levels in term Zambian infants: effects of iron stores at birth, complementary food and placental malaria

“…A further explanation is that transplacental transport of iron is disturbed by pathological changes of the syncytiotrophoblast with placental malaria. Although there is general agreement that iron is bound to transferrin at the maternal side of the placenta, very little is known about how iron is subsequently transported across the syncytiotrophoblast (Srai et al, 2002;Fuchs and Ellinger, 2004;Brabin et al, 2004a). Seasonal factors could also alter anaemia risk, but in view of the short study duration it was not possible to assess these.…”

Trophoblast phagocytic program: roles in different placental systems

“…The localization of IgG and human neonatal Fcγ-receptor (hFcRn) in the syncytiotrophoblast suggests that hFcRn is a relevant mediator of the IgG transcytosis, although part of the internalized IgG is not transferred to the fetal organism, but directed to a degradative pathway, into acidic early endosomes (Fuchs and Ellinger, 2004). The exact mechanisms of the selective and active transfer of IgG across the placental barrier are not fully understood, but there is no doubt that receptors for Fc region of IgGs are important.…”

The placenta: the forgotten essential organ of iron transport

“…Immunohistochemical studies have localized TFR1 and transferrin to the apical membrane of syncytiotrophoblasts. 25,26 Concordantly, immunoelectron microscopy demonstrated the presence of transferrin [26][27][28] and TFR1 29 on the membrane of cellular invaginations and intracellular vesicles, likely representing clathrin-coated endosomes that contain transferrin, 30,31 TFR1, 32 and TFR1-transferrin complexes. 30,32 Kinetic studies in BeWo cells 33 and placental microvillous membrane preparations 17,34,35 showed that diferric transferrin uptake is a specific and saturable process, with binding affinities similar to values reported in K562 erythroleukemia cells 36 and reticulocytes.…”