Fertilization and Early Development of Cow Ova

Abstract

Cow ova and embryos were recovered in 1–16-cell stages and studied by light and electron microscopy. Events associated with normal fertilization and early development in other species were documented by observations of acrosome-reacted sperm cells embedded in the matrix of the zona pellucida, presence of sperm remnants within ooplasm and disappearance of cortical granules, appearance of centrioles at the 8-cell stage, and changes in mitochondria with development. Few sperm cells, apparently limited in penetration to one-third the thickness of the zona, along with complete ovum penetration by only one sperm cell suggested a strong zona block to polyspermy. Sperm remnants were occasionally found in blastomeric cytoplasm of 2-cell stage ova. Prominent granules were seen in mitochondria of bovine ova before and after fertilization. Efforts to achieve fertilization in vitro by combining ova, recovered near the expected time of ovulation from follicles or oviducts, with bull sperm treated with high ionic strength medium resulted in sperm penetration and development to 2- and 4-cell stage embryos judged normal by light and electron microscopy. Use of a different bull was associated with aberrant ovum activation with retention of cortical granules, thus emphasizing a need for further definition of conditions compatible with bovine fertilization.