If a haploid sperm meets a mature haploid egg cell (ovum, see 1), it releases specific proteolytic enzymes from the top part of its head (acrosome), so that it can enter the egg cell completely (acrosome reaction). As soon as it has reaches the zona pellucida (see light blue layer), it is modified biochemically, so that further sperms cannot enter (poly sperm block, see 1). The tail seperates from the head which releases the paternal pro-nucleus (sperm nucleus) into the egg cell (see 2). The paternal pro-nucleus grows on its way to the maternal one (see 2). The paternal and maternal chromosomes fuse and the fertilized egg cell (zygote) starts mitotic cell division (see 3 and 4). Hereby the spindle apparatus consisting of microtubules and spindle poles (see blue structures in 3 and 4) forms. The blue balls between the zona pellucida and the cell membrane of an oocyte (perivitelline space) are the polar bodies and signal the completion of the second meiosis that was in a resting state before.
