ABSTRACT
After fertilization, a series of cleavage divisions transforms the one-cell zygote into 2-cell, 4-cell and 8-cell stage embryos. Each individual cell of the embryo, called blastomeres, is maintained within the outer acellular zona pellucida. The developing embryo is at the morula stage when 16 or more blastomeres are present. A gap, termed the perivitelline space, may be present between the blastomeres and the zona pellucida of a compact morula. As embryonic development continues, the morula transitions into a blastocyst as cell divisions proceed and a small fluid filled cavity or blastocoele begins to form within the center of the embryo. The blastocyst consists of an outer rim of trophoblast cells and a distinct inner cell mass embedded within the wall of trophoblast cells. The trophoblast layer will eventually form the placenta and the inner cell mass will form the embryo proper. An equine embryo is usually at the morula or early blastocyst stage of development when it first enters the uterus. Trophoblast cells of in vivo maturing embryos produce an acellular glycoprotein coat called the capsule in between the trophoblast layer and the outer zona pellucida. The capsule is unique to equine embryos and can be observed microscopically as a pale-yellow refractile layer underneath the zona pellucida. As the blastocyst expands, the zona pellucida becomes thinner and eventually is shed. The equine embryo does not “hatch” from its zona pellucida like ruminant embryos. After the zona pellucida has been lost, the capsule is the only membrane surrounding the embryo.
