Bidirectional Extracellular Vesicle-Mediated Maternal-Embryonic Exchange in the Lecithotrophic Teleost Guppy

The placenta is defined as an organ that mediates the exchanges of nutrients, hormones, and other substances between the mother and embryo in viviparous animals. Its structure is diverse due to interspecific differences in fetal tissues and variation in the forms of maternal-fetal interfaces. Matrotrophic poeciliid teleosts, in which the embryo develops within the maternal ovarian follicle, possess functional placentas formed from maternal follicular and embryonic tissues. However, the physiological mechanisms underlying substrate exchange between mother and embryo remain unclear. Furthermore, although similar embryonic-maternal interfaces are observed in lecithotrophic poeciliids, it is unknown whether nutrient exchange occurs in these species. Therefore, this study investigated whether substance exchange occurs between the mother and embryo in the lecithotrophic teleost guppy (Poecilia reticulata) and identified the underlying physiological mechanisms. Histological analysis revealed that guppies have embryonic-maternal interfaces consisting of the maternal follicle and the embryonic yolk sac and pericardial sac. Additionally, tracking of 2,000 kDa fluorescein isothiocyanate-dextran injected into pregnant guppies confirmed its transport from mother to embryo. Immunofluorescence staining and electron microscopy revealed that substance transport from mother to embryo occurs via extracellular vesicles. Moreover, immunofluorescence staining and pharmacological experiments revealed exosome transport from embryo to mother. This study demonstrates that lecithotrophic guppies possess a functional placenta that mediates maternal-embryonic substrate transfer via extracellular vesicles. These findings provide fundamental insight into the evolution of placental strategies within the Poeciliidae family. Significance StatementViviparity, in which embryos develop within the maternal body, has evolved independently across diverse animal lineages. In lecithotrophic viviparity, embryos are thought to rely primarily on yolk-derived nutrients, with maternal-fetal exchange limited to small molecules such as gases. Here, using macromolecular tracer experiments and ultrastructural analyses in guppies, we show that large macromolecules (2000 kDa) are exchanged bidirectionally between mother and fetus via extracellular vesicles, despite the absence of direct tissue attachment. These findings challenge the conventional view of lecithotrophic viviparity and reveal a previously unrecognized mechanism of maternal-fetal communication. Our results suggest that extracellular vesicle-mediated exchange may represent a widespread and evolutionarily conserved strategy for maternal-fetal interaction across viviparous animals.