Villous Trophoblast: Morphometric Perspectives on Growth, Differentiation, Turnover and Deposition of Fibrin-type Fibrinoid During Gestation

Abstract

Villous trophoblast growth and deposition of perivillous fibrin-type fibrinoid were examined in human placentas from 10–41 weeks of gestation. The main aims were: (1) to study growth of different trophoblast domains implicated in epithelial turnover (proliferation, differentiation, extrusion, denudation); (2) to test predictions about relationships between fibrinoid deposits and intervillous volume or villous surface area; and (3) to derive baseline data for future studies on complicated pregnancies. Microscopical fields on trichrome-stained paraffin sections were selected by systematic random sampling. Volumes were estimated stereologically by point counting and surface areas by intersection counting. Apparent differences were tested by analyses of variance and relationships by regression and contingency table analyses. All compartments increased in absolute volume and/or surface area although not at the same rates. Relative volumes of cytotrophoblast were greater at earlier stages (10–20 weeks) but, due to differential growth, syncytiotrophoblast nuclear aggregation sites (syncytial knots and ‘bridges’) occupied greater proportions of trophoblast volume and surface near term (37–41 weeks). Fibrinoid volume correlated positively with intervillous volume and villous surface area but, relative to intervillous volume, seemed to increase near term. Findings confirm that the incidence of syncytial knots increases during gestation and contributes to trophoblast thickness variability. Greater relative volumes and surfaces of syncytial ‘bridges’ are consistent with increased incidences of true intervillous bridges and/or villous branching points. These findings support the notion that fibrinoid deposition during normal gestation is influenced by the quality of vascular perfusion but also emphasize that the villous surface is another important factor. Haemostatic events operate at the maternal surface of trophoblastic epithelium and influence the steady state between coagulation and fibrinolysis. Fibrinoid is deposited at sites of trophoblast de-epithelialization and these arise following trauma (e.g. abruption of intervillous bridges) or during the extrusion phase of normal epithelial turnover. Like knots and bridges, sites of de-epithelialization also expand at a faster rate than overall villous surface area. These and other events in villous development can be interpreted in terms of a coherent concept of epithelial turnover in which proliferation early in gestation is mainly for growth whilst that at later stages is mainly for renewal and repair.