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Genetic defects of the zygote, such as chromosome aberrations, are the most frequent causes of abnormal embryonic development and spontaneous abortion. However, the underlying mechanisms remain unknown. Chromosome aberrations likely cause changes in placental morphology and function (such as size, shape, vascularity, and the presence of trophoblastic inclusion). We postulated that chromosome aberrations may affect rates of cell proliferation or programmed cell death (apoptosis) during the differentiation of chorionic villi. To address these questions, we evaluated cell proliferation using a monoclonal antibody to Ki-67 (a cell-cycle marker) and apoptosis using the in situ end-labeling method (TUNEL) on paraffin-embedded placental tissues. Tissues were obtained from spontaneous abortions in early gestational periods with normal (11 cases) and abnormal karyotypes (15 cases), as well as eight normal control placentas from elective abortions. Apoptotic cells were found in the stroma of all cases, but were significantly higher in number in the stroma of chromosomally abnormal versus chromosomally normal spontaneous abortions. The apoptotic index of the trophoblasts was not significantly different between groups. Cell proliferation was higher in muscularized blood vessels in chromosomally normal placentas (both elective and spontaneous abortions) versus chromosomally abnormal spontaneous abortions. Cell proliferation was different in the trophoblast and stroma between the groups but to a lesser degree than in blood vessels. The morphological and biological data presented here suggest that: (1) chromosomally abnormal spontaneous abortions may occur because of different mechanisms than chromosomally normal spontaneous abortions, (2) apoptosis of the stromal cells and cell proliferation in blood vessels and stroma play an important role in the differentiation and functioning of villi, and (3) these changes could explain the etiology of spontaneous abortion and growth retardation of chromosomally abnormal embryos. Copyright © 2000 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]