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Human endometrial and decidual stromal cells are the same cells in different environments (nonpregnancy and pregnancy, respectively). Although some authors consider decidual stromal cells to arise solely from the differentiation of endometrial stromal cells, this is a debatable issue given that decidualization processes do not end with the formation of the decidua, as shown by the presence of stromal cells from both the endometrium and decidua in both undifferentiated (nondecidualized) and decidualized states. Furthermore, recent functional and transcriptomic results have shown that there are differences in the decidualization process of endometrial and decidual stromal cells, with the latter having a greater decidualization capacity than the former. These differences suggest that in the terminology and study of their characteristics, endometrial and decidual stromal cells should be clearly distinguished, as should their undifferentiated or decidualized status. There is, however, considerable confusion in the designation and identification of uterine stromal cells. This confusion may impede a judicious understanding of the functional processes in normal and pathological situations. In this article, we analyze the different terms used in the literature for different types of uterine stromal cells, and propose that a combination of differentiation status (undifferentiated, decidualized) and localization (endometrium, decidua) criteria should be used to arrive at a set of accurate, unambiguous terms. The cell identity of uterine stromal cells is also a debatable issue: phenotypic, functional, and transcriptomic studies in recent decades have related these cells to different established cells. We discuss the relevance of these associations in normal and pathological situations., (© The Author(s) 2022. Published by Oxford University Press on behalf of Society for the Study of Reproduction. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The human decidua contains an unusually high proportion of lymphocytes, mainly NK and T cells, which are potentially cytotoxic to the trophoblast when they are stimulated with certain cytokines. Given the high incidence of spontaneous abortion in humans and other species, our working hypothesis is that decidual lymphocytes are involved in immunological mechanisms that attack the trophoblast and induce abortion when any gestational problem arises. To test this hypothesis, flow cytometry was used to compare decidual lymphocyte populations in first-trimester spontaneous abortions and elective terminations of first-trimester pregnancy. We found significantly higher proportions of decidual lymphocytes that expressed activation markers, and of T cells (mainly T helper cells) in spontaneous abortions than in elective terminations of pregnancy. Decidual lymphocytes from spontaneous abortion, like decidual lymphocytes from elective termination of pregnancy and peripheral blood lymphocytes, were however, unable to lyse the JEG-3 extravillous cytotrophoblast cell line in a (51)Cr-release assay. Nevertheless, decidual lymphocytes from spontaneous abortion, unlike decidual lymphocytes from elective termination of pregnancy and peripheral blood lymphocytes, induced apoptosis in JEG-3 cells as determined by DNA fragment-release assay. Hematoxylin and eosin staining showed a significantly higher proportion of apoptotic JEG-3 cells when these cells were treated with decidual lymphocytes from spontaneous abortion than when JEG-3 cells were cultured with decidual lymphocytes from elective termination of pregnancy. The ultrastructural signs of apoptosis were confirmed by electron microscopy. These data support the hypothesis that activated decidual lymphocytes participate in human spontaneous abortion by inducing apoptosis but not necrosis of the trophoblast.

The origin and function of decidual stromal cells (DSC), the main cellular component of the decidua, are uncertain. Although the general consensus is that they are fibroblastic cells involved in fetal trophoblast nutrition, several authors have demonstrated that these cells can carry out immunological functions and that at least a subpopulation of them may be of hematopoietic origin. Human DSC precursors or predecidual cells (preDSC) purified by expansion in culture express a surface phenotype recalling that of dendritic cells. In the present study, we show by flow cytometry that these cells also express B7-1 (CD80) and B7-2 (CD86), two antigens involved in the costimulation of T cells by antigen-presenting cells. Cultured DSC were also able to stimulate allogenic T cells in vitro. Using an immunohistochemical technique, we found that in cryostatic sections of early human decidua, CD80 and CD86 were expressed mainly by DSC located around the decidual vessels, a location compatible with preDSC rather than fully differentiated DSC. Our results suggest that preDSC are involved in antigen presentation.