The ultrastructure of the oocyte and zygote reveals in great details the processes of oocyte growth,
maturation and fertilization. In this Chapter these details are addressed in cattle in comparison with pig, horse, fox,
mouse and man. In the growing oocyte a variety of common and oocyte-specific organelles and inclusions are build
up resulting in the complex ultrastructure of the oocyte; one of the largest cells of the mammalian body. During
development, the oocyte is surrounded by cumulus cells of which the innermost establish gap junctions with the
oolemma. However, when oocyte maturation is initiated and meiosis resumed, this intimate contact is broken and
the cytoplasm of the oocyte is restructured towards a more independent fate allowing for cytoplasmic oocyte
maturation. As one important aspect, cortical granules migrate to solitary positions along the oolemma immediately
prior to ovulation. The fertilizing spermatozoon completes acrosome reaction on the surface of the zona pellucida,
penetrates the zona, and fuses with the oolemma at the equatorial segment. Consequently, the oocyte is activated
resulting in exocytosis of the cortical granules, establishing the block against polyspermic fertilization, and in
resumption of meiosis from metaphase II. The maternal and paternal chromatin is gradually surrounded by nuclear
envelope developed from the smooth endoplasmic reticulum to form pronuclei that later swell to their typical
spherical shape. Later, the pronuclei appose each other, the nuclear envelopes are dissolved, and the maternal and
paternal chromosomes are arranged in the center of the zygote forming the metaphase of the first mitosis.
