Abstract At the tips of anchoring villi, cytotrophoblast (CTB) proliferation leads to a process of multilayering in which cells lose their attachment to the villous basement membrane and develop into columns, within which they adhere to one another using desmosomes, with associated intermediate filament bundles. Non-desmosomal cadherins, tight junction proteins and other adhesion molecules are also present, suggesting that actin-associated adhesions contribute to placental anchorage. In the distal columns, cell–cell interactions diminish, cells upregulate β1 integrins and bind to a provisional fibrinoid extracellular matrix, eventually detaching to migrate into the decidual stroma and myometrium, where interstitial and endovascular extravillous trophoblast (EVT) populations show distinct repertoires of adhesion molecules.
Abstract In the placental villus, cells attach to basement membrane via integrin α6β4 and adhere both laterally and apically to their neighbours. The most prominent adhesive specialisation seen using the electron microscope is the desmosome, which connects cytotrophoblast cells (CTB) laterally and also contributes to the attachment of CTB to the overlying syncytium. However, numerous cadherins and other junctional proteins are also present in the corresponding plasma membrane domains, indicating a multiplicity of adhesive interactions. Integrins, tight junction components and cadherins are all found in the syncytial microvillous membrane, perhaps reflecting its ability to form intersyncytial bridges. There is a wide gulf to be filled between molecular anatomy and functional studies, with much to be learned about the role of adhesion molecules in regulating villous epithelial integrity, homeostasis and growth.
Abstract During human pregnancy the semi-allogeneic/allogeneic fetal graft is normally accepted by the mother's immune system. Initially the contact between maternal and fetal cells is restricted to the decidua but during the 2nd trimester it is extended to the entire body. Two contrary requirements influence the extent of invasion of extravillous fetal trophoblast cells (EVT) in the maternal decidua: anchorage of the placenta to ensure fetal nutrition and protection of the uterine wall against over-invasion. To establish the crucial balance between tolerance of the EVT and its limitation, recognition of the semi-allogeneic/allogeneic fetal cell by maternal leukocytes is prerequisite. A key mechanism to limit EVT invasion is induction of EVT apoptosis. Apoptotic bodies are phagocytosed by antigen-presenting cells (APC). Peptides from apoptotic cells are presented by APC cells and induce an antigen-specific tolerance against the foreign antigens on EVT cells. These pathways, including up-regulation of the expression of IDO, IFNγ and CTLA-4 as well as the induction of Tregulatory cells, are general immunological mechanisms which have developed to maintain peripheral tolerance to self-antigens. Together these data suggest that the mother extends her “definition of self” for 9 months on the foreign antigens of the fetus.
Oxygen is a critical regulator of placentation. Early placental development occurs in a predominantly low oxygen environment and is, at least partially, under the control of hypoxia signaling pathways. In the present study, in vivo hypobaric hypoxia was used as an experimental tool to delineate hypoxia-sensitive events during placentation. Pregnant rats were exposed to the equivalent of 11% oxygen between days 6.5 and 13.5 of gestation. Pair-fed pregnant animals exposed to ambient conditions were included as a control group. Uterine mesometrial blood vessels in the hypoxia-exposed animals were greatly expanded and some contained large cuboidal cells that were positive for cytokeratin and other markers characteristic of invasive trophoblast cells. Unlike later in gestation, the route of trophoblast cell invasion in the hypoxia-exposed animals was restricted to endovascular, with no interstitial invasion observed. Hypoxia-activated endovascular trophoblast invasion required exposure to hypoxia from gestation day 8.5 to day 9.5. Activation of the invasive trophoblast lineage was also associated with an enlargement of the junctional zone of the chorioallantoic placenta, a source of invasive trophoblast cell progenitors. In summary, maternal hypoxia during early stages of placentation activates the invasive endovascular trophoblast cell lineage and promotes uterine vascular remodeling.
Abstract Since 1995 the number of publications investigating apoptosis in villous trophoblast has increased exponentially. This scientific interest is in part due to observations that this specialised form of cell death is increased in pregnancy complications such as pre-eclampsia and intra-uterine growth restriction. In addition, apoptosis is described in normal villous trophoblast and elements of the apoptotic machinery are involved in the fusion between cytotrophoblast and the overlying multinucleate syncytiotrophoblast. The increase in descriptions of apoptotic cell death in villous trophoblast has been accompanied by investigations of regulators of apoptosis. It is anticipated that understanding the regulation of apoptosis in villous trophoblast may provide new insights into placental pathologies. This review describes current knowledge regarding the expression and function of these regulators in villous trophoblast, both in normal and complicated pregnancies.
Abstract Placenta creta is associated with massive postpartum hemorrhage and commonly leads to emergency hysterectomy. While the exact pathogenesis of placenta creta is unknown, proposed hypotheses include a primary deficiency of decidua, abnormal maternal vascular remodeling, excessive trophoblastic invasion, or a combination thereof. To assess these changes in placenta creta, we retrospectively reviewed 49 cases of gravid hysterectomy, 38 with and 11 without the diagnosis of creta, gathered clinical data, and evaluated histopathology of extravillous trophoblast. Specifically, we evaluated maternal vessels for remodeling by endovascular trophoblast, as well as the morphology and depth of invasion of interstitial trophoblast at the implantation site. Compared to controls, cases with creta had decreased proportion of remodeled vessels, with many vessels displaying partial physiologic change. Cases with creta also demonstrated vascular remodeling deeper in the myometrium; however, vascular remodeling of large outer myometrial vessels was only demonstrated in increta and percreta cases, and was absent in both non-creta and accreta. As previously reported, interstitial trophoblast invaded the uterine wall to a significantly greater depth in placenta creta; however, there was no significant difference between creta subtypes. Finally, Ki-67 staining was rarely observed in extravillous trophoblast, except in the trophoblast columns of first trimester creta cases. We, therefore, conclude that the pathogenesis of placenta creta is multi-dimensional, involving increased, but incomplete trophoblast invasion in a background of absent decidua. We further propose that placenta increta and percreta are not due to a further invasion of extravillous trophoblast in the uterine wall, rather they likely arise secondary to dehiscence of a scar, leading to the presence of chorionic villi deep within the uterine wall, and thus give extravillous trophoblast greater access to the deep myometrium.
ATRX is an X- encoded member of the SNF2 family of ATPase/ helicase proteins thought to regulate gene expression by modifying chromatin at target loci. Mutations in ATRX provided the first example of a human genetic disease associated with defects in such proteins. To better understand the role of ATRX in development and the associated abnormalities in the ATR- X ( alpha thalassemia mental retardation, X- linked) syndrome, we conditionally inactivated the homolog in mice, Atrx, at the 8- to 16- cell stage of development. The protein, Atrx, was ubiquitously expressed, and male embryos null for Atrx implanted and gastrulated normally but did not survive beyond 9.5 days postcoitus due to a defect in formation of the extraembryonic trophoblast, one of the first terminally differentiated lineages in the developing embryo. Carrier female mice that inherit a maternal null allele should be affected, since the paternal X chromosome is normally inactivated in extraembryonic tissues. Surprisingly, however, some carrier females established a normal placenta and appeared to escape the usual pattern of imprinted X- inactivation in these tissues. Together these findings demonstrate an unexpected, specific, and essential role for Atrx in the development of the murine trophoblast and present an example of escape from imprinted X chromosome inactivation.
Abstract Trophoblast invasion during placental development helps to establish efficient physiological exchange between maternal and fetal circulatory systems. Trophoblast stem cells differentiate into multiple subtypes, including some that are highly invasive. Signalling to the trophoblast from decidua, uterine natural killer cells and vascular smooth muscle can regulate extravillous trophoblast differentiation. Important questions remain about how these cellular interactions promote trophoblast invasion and the signalling pathways that are involved. New and established biological models are being used to experimentally examine these interactions and the underlying molecular mechanisms.
Genome endoreduplication during mammalian development is a rare event for which the mechanism is unknown. It first appears when fibroblast growth factor 4 (FGF4) deprivation induces differentiation of trophoblast stem (TS) cells into the nonproliferating trophoblast giant (TG) cells required for embryo implantation. Here we show that RO3306 inhibition of cyclin-dependent protein kinase 1 (CDK1), the enzyme required to enter mitosis, induced differentiation of TS cells into TG cells. In contrast, RO3306 induced abortive endoreduplication and apoptosis in embryonic stem cells, revealing that inactivation of CDK1 triggers endoreduplication only in cells programmed to differentiate into polyploid cells. Similarly, FGF4 deprivation resulted in CDK1 inhibition by overexpressing two CDK-specific inhibitors, p57/KIP2 and p21/CIP1. TS cell mutants revealed that p57 was required to trigger endoreduplication by inhibiting CDK1, while p21 suppressed expression of the checkpoint protein kinase CHK1, thereby preventing induction of apoptosis. Furthermore, Cdk2(-/-) TS cells revealed that CDK2 is required for endoreduplication when CDK1 is inhibited. Expression of p57 in TG cells was restricted to G-phase nuclei to allow CDK activation of S phase. Thus, endoreduplication in TS cells is triggered by p57 inhibition of CDK1 with concomitant suppression of the DNA damage response by p21.