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

Abstract

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.

Fig. 1

Location of samples (I – III) collected from the maternal surface of the placenta. The mean weight of the sample: 49.76 ± 4.54 g

Fig. 2

Immunohistochemical visualization of OXTR. a, b – placental samples (the image captured through optical microscope was subjected to series of digital transformations for morphometric purposes; initial magnification 400×); c – trophoblast cells in culture (phase –contrast image); d – trophoblast culture immunostained for OXTR (the image digitally transformed for morphometric purposes). The OXTR is represented by the category of blue hues ranged from dark blue to violet blue. Scale bar = 100 μm. CT – cytotrophoblast, ST – syncytiotrophoblast, fv – fetal vein, fa – fetal artery

Fig. 3

Oxytocin (OXT) concentration in placental tissue obtained from central a and peripheral b regions of the maternal surface. The median values for the groups were marked

Fig. 4

Group I vs. group II. Mean fold increase in OXTR expression in placental samples a and the trophoblast cultures b. The data are expressed as mean ± SEM (* indicates p < 0.05; indicates p < 0.02)