Trophoblast development gradient and its relationship to placental hemodynamics

Optimising sample collection for placental research

Biobanks provide an important repository of samples for research purposes. However, for those samples to reflect the in vivo state, and for experimental reliability and reproducibility, careful attention to collection, processing and storage is essential. This is particularly true for the placenta, which is potentially subjected to stressful conditions during delivery, and sample collection may be delayed owing to routine postpartum inspection by clinical staff. In addition, standardisation of the collection procedure enables samples to be shared among research groups, allowing larger datasets to be established. Here, we provide an evidence-based and experts' review of the factors surrounding collection that may influence data obtained from the human placenta. We outline particular requirements for specific techniques, and propose a protocol for optimal sample collection. We recognise that the relevance of these factors, and of the sample types collected to a particular study will depend on the research questions being addressed. We therefore anticipate that researchers will select from the protocol to meet their needs and resources available. Wherever possible, we encourage researchers to extend their collection to include additional samples that can be shared on an international collaborative basis, with appropriate informed consent, to raise the quality, as well as quantity, of placental research.

Syncytial knots, sprouts, apoptosis, and trophoblast deportation from the human placenta

The syncytiotrophoblast (STB) that forms the epithelial covering of the placental villous tree has a unique cell biology on account of its syncytial nature. The tissue is in a terminally-differentiated, postmitotic state, and expands through the recruitment by fusion of underlying progenitor cytotrophoblast cells. This process occurs from the time of implantation until term, and so its nuclei will be of various ages, producing a spectrum of contrasting appearances; whilst some are euchromatic, others display dense condensations of heterochromatin, the latter often aggregating to form clusters referred to as syncytial knots. These appearances have led to the suggestion that knots are apoptotic, and a hypothesis has developed that the nuclei are transcriptionally inactive and transit through the STB before being shed into the maternal circulation. Here, we review the evidence for this hypothesis, looking at the morphology of the nuclei, their number throughout gestation, evidence of transcriptional activity, and trophoblast deportation. We conclude that there is little evidence to support the concept that turnover of syncytial nuclei takes place in the normal placenta, or that this occurs through an apoptotic-related process. Instead, we suggest that a proportion of syncytial nuclei are transcriptionally active, that epigenetic modifications underlie the changes in chromatin appearance, and that syncytial nuclei continue to accumulate until term. We recognize that apoptotic changes can occur in pathologic pregnancies, but consider the deportation of trophoblast that has been linked to preeclampsia to be most likely of necrotic origin following ischemic injury.

Pre-eclampsia and Fetal Growth Restriction: How Morphometrically Different is the Placenta?

Both pre-eclampsia (PET) and fetal growth restriction (FGR) pose a heavy burden on fetal and maternal health and may disrupt pregnancy outcome. Using design based stereological techniques, placental vascular and villous morphology were assessed to determine the individual role played by both PET and FGR on placental growth during the third trimester. The following placentas delivered between 25 and 41 weeks of gestation were included into the study; controls (n=16), PET (n=20), FGR (n=17) and PET-FGR (n=16). Each placenta was uniformly randomly sampled and the sampled tissue processed to paraffin. Sections were stained with a CD34 antibody and the following morphometric parameters estimated: volumes, surface areas, length, diameters and the shape factor of the villous (terminal and intermediate) and vascular placental features. For stereologically estimated parameters pure PET had an effect on IVS and terminal villi volume only. FGR alone or when coexisting with PET contributed towards significant reductions in volumetric and surface area terminal villous and vascular features. FGR factors also contributed towards a significant reduction in the lengths of all parameters estimated and in the terminal villi diameter. Additionally, FGR was associated with a significant difference in shape factor indices for both intermediate and terminal villi. This study has shown that PET on its own has limited influence on the placental morphology studied, since the vascular features estimated do not differ stereologically from age matched normal controls. However, placental morphology is different between PET and PET-FGR and between PET-FGR and FGR. PET and FGR may have a cumulative effect on placental villous and vascular morphology as seen in the PET-FGR but there is no synergistic effect. These morphological abnormalities may have major physiological implications in terms of placental function and fetal growth.

The correlation between sampling site and gene expression in the term human placenta

Using oligonucleotide microarrays we recently identified a set of transcripts that were up-regulated in hypoxic human trophoblasts. To test the hypothesis that expression of hypoxia-related placental transcripts depends on sampling site we analyzed nine different sites from term human placentas (n=6), obtained after uncomplicated pregnancies. These sites spanned the placental center to the lateral border and the basal to the chorionic plate. Relative gene expression at each site, determined using quantitative PCR, was correlated with villous histology. The expression of vascular endothelial growth factor (VEGF) and connective tissue growth factor (CTGF), the cytoskeleton proteins lamininA3 and alpha-tubulin, and the signal transduction protein Rad was enhanced in the subchorionic lateral border compared to medial basal site (1.6-2.9 fold, p<0.05). In contrast, the expression of NDRG1, adipophilin and human placental lactogen was unchanged. Enhanced villous maturation, syncytial knots and fibrin deposits were more frequent in the subchorionic placental lateral border, and correlated with up-regulation of hypoxia-related transcripts (p<0.05). The association between sample site and expression level was not observed in placentas with marginal cord insertion. The expression of hypoxia-related genes in the term human placenta is dependent on sampling site within the placental disk, likely reflecting local differences in villous perfusion.

Elevation of serum macrophage colony-stimulating factor before the clinical manifestations of preeclampsia

OBJECTIVE

The purpose of this study was to determine whether the increase in serum macrophage colony-stimulating factor (M-CSF) levels preceded the onset of preeclampsia.

STUDY DESIGN

We selected 146 women, of whom 36 were nonpregnant women participating in the preliminary study and 110 were normotensive pregnant women at risk for preeclampsia who were carrying single fetuses at about 18 weeks of gestation. The blood was collected and serum was stored at -20 degrees C until assay. Sixteen women had preeclampsia develop at a later stage of pregnancy (preeclamptics), whereas 89 women continued to have normotensive pregnancies until delivery. Thirty-five of the 89 women with normotensive pregnancy who were matched for age and parity were selected to form a control group (controls). Serum M-CSF levels were determined by the sandwich enzyme-linking immunosorbent assay method with use of three antibodies.

RESULTS

Serum level of M-CSF was 1295 U/mL (median) in preeclamptics and 957 U/mL in controls. Serum M-CSF levels were significantly higher (P<.0001) in preeclamptics than in controls.

CONCLUSION

The increase in serum M-CSF levels markedly precedes the development of clinical manifestations of preeclampsia. Elevation of serum M-CSF supports M-CSF elevation in the placenta. This elevation at 18 weeks of gestation may be related to placental hypoxia, which is considered the cause of preeclampsia.

Intralobular differences in antioxidant enzyme expression and activity reflect the pattern of maternal arterial bloodflow within the human placenta

The aim was to determine whether the activities and mRNA concentrations of antioxidant enzymes in human placental tissues reflect the prevailing oxygen tension or developmental maturity of the villi. Advantage was taken of contrasting gradients within lobules of the mature placenta. The central region is well-oxygenated compared to the periphery, owing to the direction of maternal blood flow. However, central villi are morphologically and enzymatically immature compared to peripheral villi. Activity of catalase (t=8.72, P< 0.001) and glutathione peroxidase (t=2.17,P< 0.05) was higher in central than peripheral villi, but no difference was detected for total superoxide dismutase (t=1.08, P> 0.05). The degree of change in catalase activity across the lobule correlated closely with the radius (r=-0.70, P< 0.01). The mRNA concentration was higher in the centre for catalase (t=2.81, P< 0.05) and for glutathione peroxidase (t=3.33, P< 0.05), but no differences were found for copper/zinc or manganese superoxide dismutase. In separate experiments, first trimester villi cultured under 10 per cent oxygen contained higher concentrations of catalase mRNA than controls maintained under 2.5 per cent oxygen. We conclude that the activities of catalase and glutathione peroxidase reflect gradients established by the pattern of maternal intralobular bloodflow, and that oxygen tension is one regulatory factor in vitro.

Increased levels of macrophage colony-stimulating factor in the placenta and blood in preeclampsia

PROBLEM

Macrophage colony-stimulating factor (M-CSF) is considered an essential cytokine for placental growth and maintenance. We evaluated whether M-CSF levels in the placenta and blood in preeclampsia differed from those in normal pregnancies.

METHOD OF STUDY

The subjects were 37 pregnant women carrying single fetuses, of whom 19 were women with normal pregnancies and 18 were women with preeclampsia. Their average gestational age at entry was 38 weeks of gestation. Blood was collected before the onset of labor, and separated serum was obtained after centrifugation. A tissue segment of the placenta was cut immediately after delivery. The frozen placental tissue was placed in a plastic tube containing phosphate-buffered saline. The tissue was fully homogenized and then centrifuged. Separated supernatant was used for subsequent determination. M-CSF levels in separated serum were measured, and M-CSF and total protein (TP) levels in separated supernatant were also measured.

RESULTS

Both M-CSF/TP levels in the placenta and M-CSF levels in blood were significantly higher (P < 0.05-0.01) in preeclampsia than in normal pregnancies.

CONCLUSIONS

This is the first report concerning high placenta levels of M-CSF/ TP in preeclampsia. Increased M-CSF in the placenta supports the hypothesis that immunological abnormalities contribute to the etiology of preeclampsia.

Regulation of placental vascular endothelial growth factor (VEGF) and placenta growth factor (PIGF) and soluble Flt-1 by oxygen--a review

Morphological studies show poor placental vascular development and an increase in the mitotic index of cytotrophoblast cells in intrauterine growth restriction (IUGR). We hypothesized that the reported relatively high oxygen level in the intervillous space in contact with IUGR placental villi will limit angiogenesis by changes in vascular endothelial growth factor (VEGF) and placenta growth factor (PIGF) expression and function. Western immunoblot analysis demonstrates a diametric expression of PIGF and VEGF proteins throughout pregnancy, with P1GF levels increasing and VEGF levels decreasing, consistent with placental oxygenation. PIGF mRNA and protein is increased in IUGR as compared to gestationally matched normal placentae. Increasing oxygen tension upregulates P1GF protein in term placental villous explants, whereas hypoxia downregulates P1GF and VEGFR-1 (Flt-1) autophosphorylation in term trophoblast choriocarcinoma cell line (BeWo). Levels of soluble Flt-1 (sFlt-1) protein in supernatant of term villous explants were upregulated by 1 per cent hypoxia, whereas hyperoxia (40 per cent) decreased sFlt-1 levels, indicating that under conditions of increasing oxygen tension, PlGF function may remain unopposed. The addition of PlGF-1 to a spontaneously transformed first trimester cytotrophoblast cell line (ED27) stimulated cell proliferation while PlGF-2 had little effect. In contrast, the addition of PlGF-1 had little effect on endothelial cell proliferation while this was inhibited by PIGF-2. Taken together these changes provide a molecular explanation for the observed poor angiogenesis in the pathogenesis of IUGR.

Stereological study of the effect of altitude on the trophoblast cell populations of human term placental villi

Paraffin-embedded histological material was examined from 10 placentae from uncomplicated pregnancies at high altitude (3000 m). This was compared with material from 10 placentae delivered at low altitude (500 m). The sample groups were matched for maternal age, gestational age and parity. Within terminal and intermediate villi the volume-weighted mean cytotrophoblast cell volume did not significantly change at high altitude (754.1 microm3 at low altitude versus 796 microm3 at high altitude). The fractional volume of the villi occupied by cytotrophoblastic cells and their nuclei number per 10000 microm3 of villous tissue were significantly greater in placenta from high altitude (3.17 and 1.86 per cent, respectively) than those from low altitude (1.05 and 0.79 per cent, respectively) (P<0.0004 and P<0.0058, respectively). No significant differences in either fractional volume of the syncytiotrophoblast or its nuclei number per 10000 microm3 of villous tissue were observed between placentae from high (26.01 and 11.6 per cent, respectively) and low altitude (26.33 and 11.89 per cent, respectively). These results suggest an increase in the number of cytotrophoblastic cells at high altitudes without any changes in their volume. Exposure to hypobaric hypoxia is thought to be the principal aetiological factor.

Methodological problems in placental morphometry: apologia for the use of stereology based on sound sampling practice

Several problem areas in morphometry of human and animal placentae are reviewed. Attention is given to methods of tissue processing (handling, mode of fixation, embedding, shrinkage) and sampling (of organs, tissue blocks, sections, micrographs). Principal sources of bias and sampling variability are identified and the crucial importance of randomized sampling is emphasized. Methods for obtaining structural quantities from sections are compared. The case is made for estimating absolute values (volumes, surface areas, lengths, numbers, thicknesses) using stereological principles rather than relying on planar data (profile areas, perimeter lengths, numbers, apparent thicknesses). Absolute values may be obtained simply and efficiently without resort to expensive measuring devices. Finally, morphological descriptors suitable for correlating with functional data or for comparing normal and diseased organs are surveyed.

Interpretation of syncytial sprouts and bridges in the human placenta

Human placental villi from normal and pathological material from the eighth to the 40th week of gestation were examined by light, scanning and transmission electron microscopy. Trophoblastic specializations--such as syncytial sprouts of early and late pregnancy, syncytial knots and syncytial bridges--were classified into proliferative and degenerative processes or artefacts caused by tangential sectioning. In early pregnancy and in diabetes mellitus most syncytial sprouts represent the initial phases in the development of villi. In late pregnancy, in particular in pre-eclampsia, most syncytial knots, sprouts and bridges are histological artefacts, caused by tangential sectioning of the trophoblastic surface. The chance of producing such artefacts is increased with increasing section thickness and with increasing branching and tortuosity of the villi. However, a small proportion of the syncytial knots, sprouts and bridges in the last-trimester placenta, those containing coarse pyknotic nuclei, are trophoblastic specializations of a probably degenerative character.

Regional anatomy of the term human placenta

We have studied regional anatomical variability in four term placentae, comparing both whole placental regions and intralobar zones and plates. In addition, we have emphasized the need for careful selection of the area to be sampled, rather than strictly randomized sampling of the whole placenta. A unique contribution is our quantitative data for a number of structures of the several intralobar zones and plates. The data confirm the hypothesis that the area best suited to physiological exchange is the central region parabasal plate. The relative homogeneity of the intralobar zones in this area make it a representative area for sampling for various placental studies.

Morphometric studies of fetal placental stem arteries in hypertensive disorders ('toxaemia') of pregnancy

The mural thickness of fetal stem arteries of 3rd order was assessed morphometrically in 50 placentae from each of the 'toxaemia', normal pregnancy and acute fetal distress groups. Several clinical maternal and fetal variables and the syncytial sprout proliferation of the placentae were correlated with the morphometric findings. The results show that: (1) there was a significant reduction in the ratio of lumen-to-whole-diameter of the fetal arteries in 'toxaemia' as compared with the two other groups; (2) the mean lumen-to-whole-diameter ratio also differed between regions of the placenta in all groups, the most marked reduction being in the parachorial region and the least prominent in the parabasal zone; (3) no significant differences in the mean diameter ratio were found among the three sub-groups of the toxaemic pregnancies, i.e., the preeclampsia, essential hypertension and renal disease group; and (4) there was an inverse relationship between the lumen-to-whole-diameter ratios and the syncytial sprout counts in the toxaemic group.

Functional significance of the zonal morphologic differences in the normal human placenta. A morphometric study

Previous investigations have shown that the human placenta is a complex organ with different zones, each with a unique morphology. These zonal morphologic difference have been related to a probable blood PO2 gradient operating in the intervillous space. This study describes zonal variations in the vertical plane of the placenta by the application of different morphometric parameters. It demonstrates morphologic changes, which suggest that zonal differences may be the result of a physiologic adaptation of the placenta, which is dependent upon its structural relationships with the maternal tissues. This adaptation found in various areas of the placenta increases its functional efficiency.

A reflection on the oxygen diffusion of the terminal villus of the human placenta (author's transl)

By using a simple model, transformed to cartesian coordinates, we examined the behavior of the fetal and maternal oxygen partial pressure on the terminal villus of the human placenta during a constant arteriovenous oxygen difference. For this purpose we divided the flow length of the cotyledon in central and a peripheral part. We found that the oxygen exchange takes place at a lower utero-umbilical oxygen pressure gradient it the reduction of the maternal blood flow velocity amounts to 30% of the normal value and if the diffusion distance is doubled. However under these circumstances a sufficient the oxygen supply of the fetal organism. This is the case with the total flow length of a single cotyledon, but some of the terminal villi in the periphery of the cotyledon, could receive at the same time a diminished oxygen supply leading to degenerative cell changes. At a progression of these morphological changes or during labor the oxygen transport can get insufficient.