Significant pregnancy losses due to fertilization failure do not appear to occur in mature cows, but may account for up to 50 percent of the conception failures in virgin heifers. Cleavage Cleavage of the newly fertilized embryo is simply mitotic divisions of one cell into two cells, two cells into four cells, four cells into eight cells, and so on. In each mitotic division, the genetic information is duplicated such that each daughter cell arising from the original cell contains exactly the same chromosomes. At this point, the embryo is free-floating within the uterus, and depends upon uterine secretions for nourishment. The embryo is now known as a morulla.
Fluid begins to collect between the cells, and an inner cavity or blastocyst appears.
Once the cavity begins to expand, the embryo is known as a blastocyst. Some embryonic loss occurs during the time of blastocyst formation and may be due to embryonic chromosome defects. In cattle, the blastocyst stage occurs between day seven to eight after ovulation and is the time when embryos are flushed from a donor cow and implanted into recipient cows during embryo transfer. A single layer of large flattened cells, the trophoblast layer, surrounds a knob of smaller cells which lie to one side of the central cavity. In the cow, the zona pellucid shed at about day eight and blastocyst elongation begins a few days later.