Uterine asynchrony: a cause of embryonic loss.

During early gestation, hormonal events associated with corpora lutea formation and embryonic synthesis of proteins, prostaglandins, and steroids result in synthesis and release of endometrial secretory products into the uterine lumen. The embryo, inherently and in response to secretory products of the uterus, develops and grows. However, considerable embryonic mortality occurs when uterine secretions become altered in such a manner that they are asynchronous to the developing embryo. Factors that advance or retard development of the uterus and embryo have been utilized to document utero-embryonic asynchrony, and it has been observed that the uterus will not "wait" for embryos to become synchronous. However, the reverse is possible: embryonic development can be accelerated or decelerated. Furthermore within the uterus, localized areas might also exist that favor development of some embryos at the expense of others. This review will consider causes of utero-embryonic asynchrony and offer models of embryonic loss associated with an asynchronous environment in cattle, sheep, and swine.