Origin, Lineage Specification, and Molecular Regulatory Mechanisms of the Primate Extraembryonic Mesoderm
ZHAO Yijuan1,2,3, CHU Chu1,2*, NIU Yuyu1,2,3*
The EXM (extraembryonic mesoderm) is a key lineage in mammalian early embryonic development that gives rise to extraembryonic structures such as the yolk sac, allantois, umbilical cord, and placenta, and is involved in primitive hematopoiesis, vasculogenesis, and the establishment of the maternal-fetal interface. In mice, EXM originates from the primitive streak, whereas in primate, EXM cells appear prior to primitive streak formation. Hypotheses regarding the origin of EXM in primate involve multiple sources, including the trophectoderm, epiblast and hypoblast, but direct evidence remains lacking. This review focuses on the origin and specification of EXM, systematically summarizing the controversies surrounding EXM origin in primate, lineage functional specification, and molecular regulation. Current studies indicate that EXM functions are conserved between rodent and primate, but the timing of its emergence and the pathways of origin exhibit species differences. Future efforts are needed to strengthen in vivo lineage tracing, heterogeneity analysis, and in vitro model validation of EXM in primate.
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