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Notes: During gestation, the uterus of eutherian mammals is subject to extensive modifications, which are most prominent in species with a haemochorial placenta. Pregnancy-associated changes in haemochorial placental types includes a rearrangement of endometrial blood vessels together with a rapid invasion of the endometrium by trophoblast cells, inducing the alteration of the phenotype of stroma cells, which is called decidualization. The aim of this study is to re-evaluate these modifications with special reference to the development of the placental vascularization during gestation of the rat. 15 Wistar rats were sacrificed on days 4, 6, 9, 12, 16, 19 and 20 of gestation (2 per day) and during estrous cycle (1) and perfusion-fixed. Implantation chambers and/or placental tissues were excised and paraffin-embedded. Additionally vascular corrosion casts were produced on the same days of gestation from another series of rats, and examined scanning electron microscopically. Paraffin-embedded sections were used for immunohistochemistry with antibodies against cytokeratin, α-smooth muscle (sm) actin, and histochemistry with lectin (Lycopersicum esculentum). The histological findings were compared to the scanning electron microscopical results. Cytokeratin labelled epithelial cells, namely uterine luminal and glandular epithelium as well as most types of trophoblast cells. α-sm actin was observed in smooth muscle cells, maternal vascular endothelium and decidua cells. Lycopersicum esculentum lectin was detected in endothelial cells and in yolk sac epithelium. The uterine microvasculature in the course of gestation showed specific alterations, like the development of spiral arteries and a central artery running straight to the fetal placental surface where it branches into arterioles. These arterioles feed the placental labyrinth through blood channels, which are lined by syncytiotrophoblast instead of endothelial cells. In parallel, the vascular diameter increases distinctly, and comparison to histology confirms the presence of protruding endovascular trophoblast cells as indentations on vascular casts. The present findings allow to establishing a reference defining the principal stages of the constitution and development of the chorioallantoic placenta, particularly with regard to the modifications concerning the morphology of the placental and uterine vessel tree, and the development of the cellular composition in different parts of the placenta.