Early embryonic development and evaluation of equine embryo viability

Tremendous progress has been made in the development of assisted reproductive techniques that may enhance the reproductive efficiency of the horse. However, techniques that involve the manipulation of oocytes and/or embryos may themselves be detrimental to embryo viability and subsequent development. Therefore, an objective method of assessing viability of embryos before and/or after oocyte/embryo manipulation is desirable. At this time, morphologic evaluation is the most widely used method of determining the viability of equine embryos. Although morphologic assessment of embryo quality will not always accurately predict the survival of individual embryos, it is very useful for predicting the survival of groups of embryos. Other tests that have been used to evaluate equine embryo viability include (1) development during culture in vitro; (2) quantitating metabolism of the fluorescent substrate fluorescein diacetate; (3) quantitating uptake of the fluorescent stain DAPI; and (4) quantitating embryo metabolism. Although these tests offer potential advantages over morphologic assessment alone, their current limitations have prevented their routine use for embryo evaluation. It is likely that as improvements are made in these evaluation methods, they may offer advantages for use alone or in combination to more accurately assess the viability of equine embryos.