Oocyte-stored Mif promotes preimplantation development as an embryonic extracellular signal

Oocyte-stored macrophage migration inhibitory factor (Mif) persists after fertilization in mice and peaks 3–4 days post fertilization, matching the strongest Mif knockout phenotype.

• Why it matters: Embryos grow more efficiently in groups, implying embryonic extracellular factors, but the endogenous signals driving this density-dependent effect are not well defined.
• What they did: An unbiased proteomic screen identified Mif as a stable maternal oocyte protein; spent medium quantification showed embryo release with a maternal-derived fraction, and recombinant Mif rescued blastocyst growth defects.
• The result: Maternal Mif loss, but not zygotic loss, impaired blastocyst growth in group and individual culture, and extracellular Mif promoted growth by activating PI3K-Akt-mTORC1 via Cd74.

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