When a haploid sperm encounters a mature, also haploid ovum (1), it releases specific proteolytic enzymes out of the upper part of its head (acrosome) that enable it to completely enter the ovum (acrosome reaction). As soon as it reaches the pellucid zone (light blue layer), biochemical modifications prohibit further sperms from entering (block of polyspermy, 1). The tail separates from the head, which discharges the so-called paternal pronucleus (sperm nucleus) into the ovum (2). The paternal pronucleus increases in size on its way to the maternal pronucleus (2). Paternal and maternal chromosomes fuse and the therefore fertilized ovum (zygote) begins with the mitotic cell division (3 and 4). Meanwhile, the spindle apparatus is formed, that consists of spindle fibers (microtubules) and spindle poles (blue structures in 3 and 4). The brown globules between the plasma membrane and envelope of the ovum (perivitelline space) are the so-called polar bodies and signalise the completion of the second meiosis, that had been in a resting state beforehand.
