Of note, placental-like structures can also be found in non-placental mammals, as well as in other taxa such as reptiles, fishes or even invertebrates ( Carter, 2012 Roberts et al., 2016 Griffith and Wagner, 2017). Strikingly, it is one of the most variable organs in terms of shape, structure and cell types, as illustrated in Figure 1, where the cellular organization of the human and mouse placental tissues is compared. The placenta provides nutrient and oxygen supply and it also ensures waste elimination, protection against infections and hormone synthesis ( Burton and Fowden, 2015). The placenta is a transient organ that develops at the interface between maternal and fetal tissues and orchestrates diverse physiological functions during pregnancy. Here, we summarize our knowledge on the evolutionary, molecular, and physiological relevance of these epigenetically-regulated, recently-evolved miRNAs, by focusing on their roles in placentation and possibly also in pregnancy diseases (e.g., preeclampsia, intrauterine growth restriction, preterm birth). Interestingly, these imprinted miRNA genes form large clusters composed of many related gene copies that are co-expressed with a marked, or even exclusive, localization in the placenta. Three evolutionarily distinct genomic intervals contain the vast majority of imprinted miRNA genes: the rodent-specific, paternally expressed C2MC located in intron 10 of the Sfmbt2 gene, the primate-specific, paternally expressed C19MC positioned at human Chr.19q13.4 and the eutherian-specific, maternally expressed miRNAs embedded within the imprinted Dlk1-Dio3 domains at human 14q32 (also named C14MC in humans). In mammals, the expression of a subset of microRNA (miRNA) genes is governed by genomic imprinting, an epigenetic mechanism that confers monoallelic expression in a parent-of-origin manner. 2Laboratoire de Biologie MolГ©culaire Eucaryote, Centre de Biologie IntГ©grative, CNRS, UPS, UniversitГ© de Toulouse, Toulouse, France.1Centre de Physiopathologie de Toulouse Purpan, UniversitГ© de Toulouse, CNRS, INSERM, UPS, Toulouse, France.CГ©cile Malnou 1, David Umlauf 2, MaГЇlys Mouysset 1 and JГ©rГґme CavaillГ© 2 *

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