Cytotrophoblastic cells (CTBs) from first trimester placenta follow one of two existing differentiation pathways: villous CTBs (vCTBs) form a monolayer of polarized epithelial stem cells which proliferate and eventually differentiate by fusion to form a syncytiotrophoblast (STB) covering the entire surface of the villus, or they can break through the STB at selected sites (in anchoring villi) to form multilayered columns of non-polarized but invasive CTBs. In vitro, CTBs invade a reconstituted basement membrane, they thus behave like metastatic cells. This invasive behaviour is due to the ability of CTBs to secrete matrix metalloproteinases (MMPs) since tissue inhibitor of MMP (TIMP) inhibits their invasiveness. MMPs are a family of at least 17 human zinc-dependent endopeptidases collectively capable of degrading essentially all components of the extracellular matrix (ECM). Although CTBs behave like metastatic cells, in vivo they are only transiently invasive (first trimester) and their invasion is normally limited only to the endometrium and to the proximal third of the myometrium. This temporal and spatial regulation of trophoblast invasion is believed to be mediated in an autocrine way by trophoblastic factors and in a paracrine way by uterine factors. Several types of regulators have been investigated: hormones, cytokines, growth factors and ECM glycoproteins. This review is not intended to be an exhaustive catalogue of all the potential regulators but is aimed at emphasizing those factors relevant in trophoblast-endometrial interactions. (C) 2000 IFPA and Harcourt Publishers Ltd.
The earliest cell fate decision in the mammalian embryo separates the extra-embryonic trophoblast lineage, which forms the fetal portion of the placenta, from the embryonic cell lineages. The body plan of the embryo proper is established only later at gastrulation, when the pluripotent epiblast gives rise to the germ layers ectoderm, mesoderm and endoderm. Here we show that the T-box gene Eomesodermin performs essential functions in both trophoblast development and gastrulation. Mouse embryos lacking Eomesodermin arrest at the blastocyst stage. Mutant trophoectoderm does not differentiate into trophoblast, indicating that Eomesodermin may be required for the development of trophoblast stem cells. In the embryo proper, Eomesodermin is essential for mesoderm formation. Although the specification of the anterior-posterior axis and the initial response to mesoderm-inducing signals is intact in mutant epiblasts, the prospective mesodermal cells are not recruited into the primitive streak. Our results indicate that Eomesodermin defines a conserved molecular pathway controlling the morphogenetic movements of germ layer formation and has acquired a new function in mammals in the differentiation of trophoblast.
Non-classical MHC class I molecule HLA-E is the ligand for CD94/NKG2 NK cell receptors. Surface expression of HLA-E requires binding of specific HLA class I leader sequences. The uterine mucosa in early pregnancy (decidua) is infiltrated by large numbers of NK cells, which are closely associated with placental trophoblast cells. In this study we demonstrate that trophoblast cells express HLA-E on their cell surface in addition to the previously reported expression of HLA-G and HLA-C. Furthermore, we show that the vast majority of decidual NK cells bind to HLA-E tetrameric complexes and this binding is inhibited by mAb to CD94. Thus, recognition of fetal HLA-E by decidual NK cells may play a key role in regulation of placentation. The functional consequences of decidual NK cell interaction were investigated in cytotoxicity assays using polyclonal decidual NK cells. The overall effect of CD94/NKG2 interaction with HLA-E is inhibition of cytotoxicity by decidual NK cells. However, since decidual NK cells are unable to kill trophoblast even in the presence of mAb to MHC class I molecules and NK cell receptors, HLA-E interaction with CD94/NKG2 receptors may regulate other functions besides cytolysis during implantation.
During placentation the extravillous trophoblast (EVT) cells migrate through the decidua towards the maternal spiral arteries. The walls of the arteries are then destroyed by trophoblast resulting in an increased blood flow to the fetus. These EVT express HLA-G, HLA-E and HLA-C, an unusual combination of two non-classical and one classical MHC class I molecules. The decidua is infiltrated by distinctive uterine natural killer (NK) cells during the time of trophoblast invasion. These cells express a variety of receptors (CD94/NKG2, KIR and ILT) which are known to recognize HLA class I molecules. There is, therefore, a mechanism for molecular recognition of the placental trophoblast cells. The possible functional consequences of this uterine NK cell-trophoblast interactions are uncertain. One possible result is in an altered NK cell cytokine profile which modulates the invasive proclivity of the EVT. In this way placentation could be controlled. (C) 2000 IFPA and Harcourt Publishers Ltd.
The placenta is comprised of an inner vascular network covered by an outer epithelium, called trophoblast, all designed to promote the delivery of nutrients to the fetus. Several specialized trophoblast cell subtypes arise during development to promote this function, including cells that invade the uterus to promote maternal blood flow to the implantation site, and other cells that fuse into a syncytium, expand and fold to increase the surface area for efficient transport. Mutation of many genes in mice results in embryonic mortality or fetal growth restriction due to defects in placental development. Several important principles about placental development have emerged from these studies. First, distinct molecular pathways regulate the differentiation of the various trophoblast cell subtypes. Second, trophoblast proliferation, differentiation and morphogenesis are highly regulated by interactions with adjacent cell types. Finally, the specific classes of mutant phenotypes observed in the placenta of knockout mice resemble those seen in humans that are associated with preeclampsia and intrauterine growth restriction. Copyright 2000 Academic Press
In this paper definitive evidence that the classical class I product, HLA-C, is expressed on the surface of normal trophoblast cells is provided. HLA-C transcripts were sequenced from cDNA isolated from first trimester trophoblast cells obtained by flow cytometric sorting. Both paternal and maternal alleles were transcribed. HLA-C proteins were demonstrated by biochemical analysis and found on the cell surface in association with beta(2)-microglobulin. Upregulation of cell surface HLA-C but. not HLA-G expression after interferon (IFN)-gamma treatment was demonstrated by flow cytometric analysis. Immunohistology has confirmed HLA-C is expressed by all extravillous subpopulations in vivo. The question of whether trophoblast HLA-C molecules interact with decidual NK cells expressing killer Ig-like receptors (KIR) has also been addressed. Our results demonstrate that extravillous trophoblast expresses at least two HLA class I molecules, HLA-G and HLA-C on the cell surface. (C) 2000 Harcourt Publishers.
Leptin is a circulating hormone which plays an important role in the regulation of energy balance, haemopoiesis and reproduction. Leptin and its receptor (leptin-R) are localized in human placental tissue but their function is not known. In this study we have investigated the expression of leptin and leptin-R in the human placenta with particular attention to extravillous cytotrophoblastic cell islands and cell columns which play a pivotal role in trophoblast invasion and placental growth. We demonstrate that leptin-R immunoreactivity shows a strong expression in the distal extravillous cytotrophoblastic cells of cell columns invading the basal plate, whereas leptin expression is homogeneously expressed in all the cellular components of cell columns. Since the invasive ability of the distally located extravillous cytotrophoblast of cell columns is known to be regulated by a variety of proteases and some extracellular matrix molecules, we tested the influence of leptin on the in-vitro production of matrix metalloproteinase (MMP)-2, MMP-9 and fetal fibronectin (fFN) by cytotrophoblastic cells. We demonstrate that leptin increases, in a dose-dependent manner, the secretion of immunoreactive MMP-2 and fFN and enhances the activity of MMP-9 in cultured cytotrophoblastic cells. Our results suggest that leptin and leptin-R could have a role in the invasive processes of the extravillous cytotrophoblastic cells by modulating the expression of MMPs. In addition, these results provide a foundation for studying pathological conditions characterized by insufficient or excessive trophoblast.