Equine Embryology: What's in the Flush?

Unlike the embryos of other farm animal species, that of the equine has a relatively long oviducal transport time and it cannot be flushed from the mare’s uterus any earlier than 6–6.5 days after ovulation. Before the embryo reaches the uterus it has undergone a series of developmental changes. It begins its existence in the oviduct as an oocyte encased in a glycoprotein membrane, the zona pellucida. Once the oocyte is fertilised it is termed a zygote and mitotic cell divisions are initiated to produce cells known as blastomeres. These adhere to one another in the form of a spherical clump within the confines of the zona pellucida. After 3 or 4 divisions a clump of 8–16 blastomeres exists within the zona pellucida; this embryonic stage is known as amorula (Latin) due to its mulberry-like appearance and the stage is reached 4–5 days post ovulation. Cell division continues and those cells that come into contact with the zona pellucida effectively eliminate any space within the developing embryo. Once this occurs the equine embryo expands rapidly to form ablastocyst and the zona pellucida thins markedly. Formation of a fluid filled cavity called the blastocoele is facilitated by rearrangement of the blastomeres into 2 populations – a thintrophectoderm layer which surrounds the blastocoelic cavity and a small, more compact mass, known as the inner cell mass (ICM) which projects into the blastocoele and will eventually form the embryo proper. In addition, before the embryo ‘hatches’ from its zona pellucida at the early blastocyst stage, it becomes surrounded by a tough, elastic, glycoprotein layer known as theblastocyst capsule which is secreted by the trophectoderm cells. It is around this stage of embryonic development at Day 7–8 after ovulation, that the mare’s uterus is flushed to recover the embryo in traditional embryo transfer procedures. [...]