Regional ultrastructural differences in placental villi in cotyledons of a mature human placenta

Myometrial contractility influences oxytocin receptor (OXTR) expression in term trophoblast cells obtained from the maternal surface of the human placenta

Background

Oxytocin (OXT) acts through its specific receptor (OXTR) and increased density of OXTR and/or augmented sensitivity to OXT were postulated as prerequisites of normal onset of labor. Expression of OXTR in the placental term trophoblast cells has not yet been analyzed in the context of contractile activity of the uterus. Here we examine comparatively OXT contents in the placental tissue adjacent to the uterine wall and expressions of OXTR in this tissue and corresponding isolated placental trophoblast cells.

Methods

Twenty eight placentae after normal labors at term (group I, N = 14) and after cesarean sections performed without uterine contractile activity (group II, N = 14) have been collected. Tissue excised from the maternal surface of examined placenta was used for OXT concentration measurement, cytotrophoblast cell cultures preparation and immunohistochemistry of OXTR. Concentration of OXT was estimated in the tissue homogenates by an enzyme immunoassay with colorimetric detection. Cytotrophoblast cells were isolated using Kliman’s method based on trypsin, DNase, and a 5–70 % Percoll gradient centrifugation. The cultures were incubated for 5 days in normoxia. Both placental specimens and terminated cytotrophoblast cultures were fixed and embedded in paraffin before being immunostained for OXTR. Using light microscopy with computed morphometry for quantitative analysis, OXTR expressions were estimated in calibrated areas of the paraffin sections.

Results

There were not significant differences between the groups in respect to the mean OXT concentration. However, in both groups the median value of OXT concentration was significantly (p < 0.05) higher in the tissue obtained from the peripheral regions of the maternal surface of the placenta, compared to the samples from the central region of this surface. In placental tissue the mean expression of OXTR in group I was significantly (p < 0.05) increased by approximately 3.2-fold and 3.45-fold (the samples collected from central and peripheral regions, respectively) compared to the values obtained in group II.

In the isolated primary trophoblast cultures the differences were even more evident (p < 0.02) and the mean change in OXTR expression in group I comprised approximately 6.9-fold increase and 6.5-fold increase (the samples collected from central and peripheral regions, respectively) compared to the values obtained in group II.

Conclusions

Upregulation of OXTR within placental trophoblast cells localized close or adherent to uterine wall may play a crucial role in labor with efficient contractile activity (vaginal delivery). Further studies may disclose if this local OXT/OXTR signaling is utilized in the third stage of labor to elicit placental detachment or contribute in a more versatile way throughout the labor period.

Taking tissue samples from the placenta: an illustration of principles and strategies

Tissue samples are removed from placentas for a variety of reasons associated with a host of investigative techniques, including chorionic villus sampling, villus explant culture, cell culture, proteomic analysis, gene expression profiling, microscopy and morphometry. Apart from the latter, especially stereological analysis, many studies provide extremely limited information on how the samples were selected. At worst, we learn little more than the placenta was sampled. Sometimes, studies provide sufficient detail to reveal flaws in sampling, e.g. the selection of placentomes based on size rather than mere presence. Occasionally, the reader is informed, without further explanation, that representative samples were taken or that samples from placentas in different study groups were taken from standard or similar sites. Such statements raise doubts about the unbiasedness of the sampling process, leave the reader in ignorance of the quality of the final sample, thwart attempts at achieving study repeatability and compromise interpretations of the validity of study outcomes. And yet study outcomes depend critically on the selection process because sampling influences study errors, notably precision (random error) and bias (systematic error). This article aims to review the basic principles and virtues of random sampling in general and the practical utilities of variants of it. For many functional and structural studies, it suffices to randomise the positions of tissue samples but, in certain structural studies, orientation must also be randomised. Therefore, sampling tools for stereological estimation of membrane surface areas, tubule lengths and layer thicknesses are mentioned. Although emphasis is accorded to the placenta, the principles apply equally well to other organs and to lower levels of organisation including the subcellular. It is hoped that this review will inform future study designs, encourage greater transparency and facilitate sampling improvements.

Observer reliability in assessing placental maturity by histology

AIMS

To evaluate the ability of five experienced perinatal pathologists to assess placental maturity reliably by histology.

METHODS

Twenty four haematoxylin and eosin slides, six each from placentas of 27, 31, 35, and 39 weeks' gestation, were circulated to five pathologists on three separate occasions. The slides were labelled with the correct or incorrect gestational ages.

RESULTS

The mean absolute error over all 360 readings was 2.72 weeks. Only 54% of the slides were assessed within two weeks of the correct gestation. Pathologist tended to overestimate younger gestations and underestimate older gestations. Two, and possibly three, pathologist were influenced by the gestational age state on the label. One pathologist, who did not appear to be influenced by the label, was more accurate in diagnosing gestation of the placentas than other colleagues.

CONCLUSIONS

Experienced pathologists can have difficulty in assessing the villous maturity of placentas by histology. They can also be influenced by clinical information provided, such as gestational age. Other observer reliability studies must address the issue of the influence of labelled information on observer variation. A difference in maturation would have to be of a six week magnitude to have a chance of being detected by current methods. This may limit the value of the histological diagnosis of placental dysmaturity as a surrogate marker for uteroplacental ischaemia.

Cellular junctions on the free surface of human placental syncytium

Chorionic villi of human placenta at full term have been studied in thin section. On the free surface of syncytium, cellular junctions such as desmosomes, desmosome-like junctions and tight junctions, but not gap junctions, are present. Theories regarding the formation and functions of the junctions on the syncytial free surface are discussed.

The influence of mode of fixation on morphometrical data derived from terminal villi in the human placenta at term: a comparison of immersion and perfusion fixation

The placenta is a highly vascular organ which is subjected to a variable degree of compression during delivery. Partial collapse of the fetal capillary network inevitably results, and this has significant effect upon the maternofetal diffusion distance. Fixation via a dual perfusion system operating at physiological pressures can redistend the vascular tree to its antenatal state. Morphometrical analysis provides a volume fraction for the fetal capillaries within the terminal villi of 38.4 per cent, a figure which approximates closely to values obtained from in situ biopsy of the placenta. The harmonic mean diffusion distance was found to be 3.6 micron. Perfusion fixation resulted in a lower coefficient of variation for many parameters compared with traditional immersion fixation. By negating the unpredictable influence of the placental transfusion, it is considered that perfusion fixation will make true interplacental variations easier to detect.

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.

Morphological study of the fetal vasculature of the human term placenta: scanning electron microscopy of corrosion casts

Microvascular corrosion casts of the fetal vessels of the normal human placenta were studied using the scanning electron microscope. All available evidence suggests that when prepared under standard conditions the resultant casts accurately replicate the original luminal morphology of the vessels, and are comparable between placentae. In conjunction with stereo-pair photography this technique allows for the rapid and unequivocal interpretation of the three-dimensional configuration of villous vasculature. Previously described structures such as dilated capillary loops and perivascular capillary networks can be viewed in new and dramatic perspective. It is hoped the technique may thus be of assistance to physiologists in the interpretation of experimental data concerning placental transport. Further work is in progress to apply the technique to the investigation of possible changes in fetal vasculature occurring in abnormal pregnancies.