Research on Equine Yolk Sac Remnant

Researchers state that mares are the only animals that have large and/or ossified yolk sac remnants.

“The yolk sac placenta plays a vital role in early embryonic development,” noted researchers. iStock/Rocter

Researchers from the Sharjah Equine Hospital in the United Arab Emirates published an article about the equine yolk sac and its remnant. The paper published in the Journal of Equine Veterinary Science from Elsevier was titled, “The yolk sac of the equine placenta. Its remnant and potential problems,” and was authored by Laura Vilaregut, Marco Lores and Sandra Wilsher. 

Summary: “This review details the current state of knowledge about the equine yolk sac and its remnant (YSR) in the pregnant mare, which, incidentally, is the only animal species known to exhibit large and/or ossified YSR. It also describes the clinical significance of the YSR and details a case of a strangulating YSR that caused fetal death and abortion.”


  • “The yolk sac placenta plays a vital role in early embryonic development.
  • “The yolk sac regresses and the allantochorion facilitates fetomaternal exchange.
  • “A yolk sac remnant is present on all term placentas.
  • “Yolk sac remnants may become enlarged and ossified.
  • “Enlargement and anomalous positioning can, on rare occasions, cause abortion.”

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