Anti-adhesive glycosylation of fibronectin-like molecules in human placental matrix-type fibrinoid

A variety of opportunities for immune interactions during trophoblast development and invasion

During human implantation and placentation, the direct cell to cell contact of fetal and maternal tissues gives room for a variety of immune interactions. Especially, the invasion of a subset of fetal trophoblast cells, called extravillous trophoblast, generate a very close interplay between the two individuals, enabling the attachment of the placenta to the uterine wall and the transformation of maternal spiral arteries to facilitate adequate nutrition of the fetus. During pregnancy, maternal and fetal factors closely interact to maintain pregnancy and smooth the process of delivery. At each and every stage and site, immunological interactions take place, including attachment of the blastocyst, development and invasion of trophoblast, and flow of maternal plasma and blood through the intervillous space of the placenta. Control mechanisms tightly regulate these interactions helping to evade fetal rejection by the mother. In this review, we highlight the morphological sites of development and feto-maternal interaction to help immunological interested scientists and clinicians to develop hypotheses on the feto-maternal immunological network during pregnancy.

The feto-maternal interface: setting the stage for potential immune interactions

Human implantation and placentation comprise the direct contact of fetal with maternal tissues culminating in the erosion of maternal tissues by fetal cells. A complex interplay of maternal and fetal factors is key to maintain pregnancy until delivery. Immunological interactions can be found at different stages, such as blastocyst attachment, trophoblast invasion into maternal tissues, and flow of maternal blood through the placenta. These interactions need tightly controlled mechanisms to avoid rejection of the conceptus. In this study, these sites of interaction are introduced on a morphological level to help immunologists create their hypotheses on how the immunological interactions may work.

Human placental explants in culture: approaches and assessments

Placental explant cultures in vitro are useful for studying tissue functions including cellular uptake, production and release of secretory components, cell interactions, proliferation, growth and differentiation, gene delivery, pharmacology, toxicology, and disease processes. A variety of culture conditions are required to mimic in utero environments at different times of gestation including differing oxygen partial pressures, extracellular matrices and culture medium. Optimization of explant methods is examined for first and third trimester human placental tissue and the biological processes under investigation.

Extracellular pH modulates the secretion of fibronectin isoforms by human trophoblast

Differentiation of human trophoblast from the proliferative to the invasive phenotype takes place in a hypoxic and thus likely an acidic microenvironment. During differentiation, the secretion pattern of fibronectin isoforms changes. Therefore, we analysed the relation between extracellular pH, secretion of fibronectin splice variants and invasiveness. By means of immunohistochemistry and biochemistry, cellular non-oncofetal fibronectins were found in placental stroma and around extravillous trophoblast, whereas oncofetal isoforms only marked the extracellular matrix of extravillous trophoblast. In vitro, mesenchymal cells produced non-oncofetal fibronectins only, whereas choriocarcinoma cell lines, extravillous trophoblast and choriocarcinoma/trophoblast hybrid cells secreted both non-oncofetal and oncofetal isoforms. When the pH of the culture medium was either lowered or increased (between 6.0 and 8.0), the trophoblast hybrids, but not choriocarcinoma and mesenchymal cells, responded with increased secretion of fibronectins and a shift towards oncofetal isoforms. These changes were preserved after pH normalisation. Histochemical determination of local tissue acidity revealed that the site of the lowest detectable tissue pK coincided with the starting point of invasion, the proximal part of trophoblastic cell columns. Therefore, it is concluded that the local pH plays an important role as regulator of differentiation of human trophoblast as reflected by the synthesis of oncofetal fibronectins by the invasive phenotype of extravillous trophoblast.

The placenta in pseudoxanthoma elasticum: clinical, structural and immunochemical study

Pseudoxanthoma elasticum (PXE) is a rare genetic disorder clinically characterized by skin, cardiovascular and eye manifestations, mainly due to calcification and fragmentation of elastic fibres. Although infrequent, complications during pregnancy in women affected by PXE have been reported. The aim of the present study was to compare structural features of placentae at term from 14 control and 15 PXE-affected women, in order to better understand if and how abnormal mineral and/or matrix accumulation might affect placental function in PXE. In all cases, pregnancy, fetus growth and delivery were normal. Both gross and light microscopy examination did not reveal dramatic differences between placentae of PXE patients and controls, with regard to weight, dimensions, infarcts, thrombi, inflammatory lesions or vessels. However, necrotic changes and mineralization appeared statistically more pronounced in PXE. By electron microscopy the most remarkable differences between PXE and control placentae were observed in the localization and morphology of mineral precipitates; a significant higher deposition of mineral precipitates was observed associated with the "matrix"-type fibrinoid and among collagen fibrils, especially on the maternal side. Immunocytochemistry revealed the presence of vitronectin and fibronectin associated with the PXE-specific mineralizations and the absence of mineralization on the small and scarce elastic fibres in either controls or in PXE.

A stereological method for testing whether or not there is random deposition of perivillous fibrin-type fibrinoid at the villous surface: description and pilot applications to term placentae

We present a stereological method for testing whether or not there is random deposition of fibrin-type fibrinoid (FTF) at the villous surface of human placenta. The method requires random sampling of tissue with test lattice lines superimposed on microscopic fields at random positions and orientations. Test lines are used to generate chance intersections with specified sub-domains of the villous surface. At least three sub-domains are distinguishable: non-syncytial knots (nonSK), syncytial knots (SK) and areas of trophoblast de-epithelialization (DEP). Other sub-domains may be included to suit individual circumstances and project aims. The relative numbers of intersections with sub-domains provide the basis for an 'expected' distribution. Subsequently, this is compared with an 'observed' distribution which can be calculated from empirical estimates of the numbers of intersections with sub-domains associated with perivillous FTF (e.g. nonSK+FTF, SK+FTF and DEP+FTF). Expected and observed distributions can be compared by a chi-squared analysis. If the null hypothesis (no difference) is rejected, chi-squared values for individual sub-domains can be analysed in order to localize and interpret sites of preferential deposition. Comparisons may be drawn for individual placentae as well as a group of placentae, thereby permitting assessment of inter-placental variability. Finally, between-group comparisons may be drawn in order to test whether or not FTF deposition patterns differ in control and other pregnancies. Worked examples of the statistical procedures are provided. Preliminary results of applications to placentae from normal and complicated (hypobaric hypoxia) pregnancies are presented. They show that FTF deposition is non-random and preferentially located at sites of de-epithelialization. De-epithelialization may be a consequence of syncytial degeneration but also, at least in part, of continuous trophoblast turnover in which syncytial fragments rich in (pre-) apoptotic nuclei detach from the epithelium and are deported from the maternal intervillous space. The nascent detachment site is immediately covered by FTF prior to repair by re-epithelialization.

Cytogenetic and DNA-fingerprint characterization of choriocarcinoma cell lines and a trophoblast/choriocarcinoma cell hybrid

We report the successful fusion of human choriocarcinoma cells with normal human trophoblast cells to a choriocarcinoma/trophoblast hybrid. The hybrid cells ACH1P were derived from fusion of primary male trophoblast cells with the HGPRT-defective choriocarcinoma cell line AC1-1. The karyotypes of the parental choriocarcinoma cell line JEG-3, its HGPRT-defective mutant clones AC1-1, AC1-5, and AC1-9, and the choriocarcinoma/trophoblast hybrid ACH1P are presented, together with a detailed characterization of the AC1-specific chromosomal marker add(X)(q26) using conventional cytogenetic banding techniques and multiplex-fluorescence in situ hybridization (M-FISH). To our knowledge, this is the first report of a stably proliferating human cell hybrid of trophoblastic origin, providing a unique cell culture model to study trophoblast-related invasion and its underlying genetic mechanisms.

Immunohistochemical identification of the receptor for urokinase plasminogen activator associated with fibrin deposition in normal and ectopic human placenta

The receptor for urokinase plasminogen activator (uPAR) is a key molecule in cell surface-directed plasminogen activation. uPAR binds urokinase plasminogen activator (uPA) and thereby focuses plasminogen activation on the cell surface. Plasmin dissolves fibrin deposits and facilitates cell migration during tissue repair processes by degrading the extracellular matrix. During human implantation and placental development, plasmin is considered important for both trophoblast migration/invasion and for fibrin surveillance. This study examined the expression of uPAR in normal and ectopic human placentae by immunohistochemistry. In first and third trimester normal placentae as well as in tubal ectopic placental tissues, a high uPAR expression was seen in the trophoblast associated with deposits of fibrin-type fibrinoid. Extravillous trophoblast of the basal plate, of the cell islands, and of the cell columns was also positive for uPAR in the first trimester whereas at term the expression of the protein was decreased. Moreover, uPAR immunostaining was observed in decidual cells throughout normal gestation and in endometrial tissues of patients with ectopic pregnancies. These findings suggest that uPAR participates in placental development and in trophoblast invasion particularly in the first trimester of pregnancy and that uPAR is involved in repair mechanisms of the trophoblast and fibrin surveillance.

The fibrinoids of the human placenta: origin, composition and functional relevance

Placental fibrinoids are extracellularly deposited materials which are histologically glossy and acid staining, and can be found in every normal and pathological placenta at all stages of pregnancy. The amount of fibrinoid is, in general, independent of pregnancy outcome and fetal wellbeing. According to new findings, the classical histological term "fibrinoid" covers two distinctive extracellular matrices which differ as regards structure, composition and function. Fibrin-type fibrinoid is mostly composed of fibrin together with other molecules derived from blood clotting or degenerative processes. It is mainly a maternal blood-clot product which is used (a) to adapt the intervillous space to optimized flow conditions and (b) to control growth of the villous trees by encasing new villous branches which caused intervillous stasis or turbulence of maternal blood. Moreover, fibrin-type fibrinoid replaces degenerative syncytiotrophoblast at the maternofetal exchange surfaces, thus acting as a kind of substitute barrier. Matrix-type fibrinoid is a secretory product of invasive extravillous trophoblast cells. It shares some similarities with basement membranes, however, it is secreted in an apolar fashion, embedding the secreting cells. Like basement membranes, it contains laminins, collagen IV, and heparan sulfate. In addition, oncofetal fibronectins, vitronectin, and i-glycosylated molecules but no collagens I, III, and VII can be found. Matrix-type fibrinoid is thought to regulate trophoblast invasion by specific interactions with cell surface integrins. As a kind of "glue", it anchors the placenta to the uterine wall and seems to play an important role in materno-fetal immune interactions at this particular site. Both types of fibrinoid are usually co-localized, thus indicating close morphogenetic and functional interrelations.

Extracellular matrix components of the placental extravillous trophoblast: immunocytochemistry and ultrastructural distribution

Invasive extravillous trophoblast cells of the human placenta are embedded in a self-secreted extracellular matrix, the matrix-type fibrinoid. The ultrastructure and molecular composition of the matrix-type fibrinoid of the term human placenta were studied by transmission electron microscopy and immunogold labelling. We used antibodies directed against different matrix proteins such as collagen type IV, laminin, vitronectin, heparan sulfate, various fibronectin isoforms, and against the oncofetal blood group antigen, "i". Immunogold labelling patterns of matrix proteins are the basis for the subdivision of the trophoblast-derived matrix-type fibrinoid into mosaic-like patches of structurally and immunocytochemically different compartments. Firstly, fine granular patches with structural similarities to basal lamina material are composed solely of collagen type IV and laminin. Secondly, an ultrastructurally amorphous glossy substance shows reactivity with antibodies against heparan sulfate and vitronectin. A third type of patches, fine fibrillar networks embedded in the above-mentioned glossy matrix, are reactive with antibodies against normal fibronectin isoforms (IST-4, IST-6, IST-9) and oncofetal isoforms (BC-1, FDC-6). The blood group precursor antigen "i" was not only expressed on the surfaces of the extravillous trophoblast cells but was associated with the fibronectin-positive fibrils. In conclusion, within this extracellular matrix, clear compartments of different composition can be distinguished from each other. Glycosylation with "i" in this matrix may be involved in immunological masking, thus preventing rejection of placenta and fetus.