Histological and immunohistochemical events during placentation in pigs

The early morphological events in pig placental development are summarized and related to the known data on differences in placental vascular efficiency between Meishan and US breeds. The activation and localization of a number of factors, the ligands and their receptors, such as insulin-like growth factor (IGF), transforming growth factor beta (TGF beta), platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF), as well as retinoids and calcium, is described. The comparison between these factors gives a strong impression of their complex interactions and hormonal relationships during placentation and vascular development in pigs. This review also emphasizes that retinoids are of great importance for placental function and that the transport of vitamin A appears to take place in the areolar gland complex only, whereas based on histochemistry and electron energy dispersive analysis, the calcium transport may be confined to the interareolar route across the interhaemal barrier.