The maternal-fetal interface is an early hematopoietic niche enriched in distinct erythro-myeloid progenitors.

MIXER (Maternally Interfaced eXtraEmbRyonic) tissue at the maternal-fetal interface in mice is a robust early hematopoietic niche enriched in distinct erythro-myeloid progenitors, including eEMPROR cells.

• Why it matters: Embryonic hematopoiesis is thought to occur exclusively in the extraembryonic yolk sac before HSCs arise, leaving the maternal-fetal interface’s potential role unclear.
• What they did: The study identifies MIXER in mice, characterizes abundant CSF1R+ erythro-myeloid progenitors and a unique CD4+ CSF1R− eEMPROR population, and uses lineage tracing to track eEMPROR contributions to fetal macrophages.
• The result: eEMPROR cells contribute to select macrophage subpopulations, primarily in the perivascular space, and early human placental villi before HSC emergence also contain CD4+ nucleated erythrocytes, supporting a conserved maternal-fetal hematopoietic role.