pronucleus /-no̅o̅″klē·əs/ pl. pronuclei [Gk, pro + L, nucleus, nut kernel] , the nucleus of an ovum or a spermatozoon after fertilization but before fusion of the chromosomes to form the nucleus of the zygote. Each pronucleus contains the haploid number of chromosomes, is larger than the normal nucleus, and is diffuse in appearance. The pronucleus of the ovum is formed only after it has completed its second meiotic division and the second polar body has formed, which occur after the spermatozoon has penetrated. It then loses its nuclear envelope, releasing the chromosomes so that synapsis with the chromosomes of the male pronucleus, which is contained in the head of the spermatozoon, can occur. Also called germinal nucleus, germ nucleus. See also oogenesis, spermatogenesis.
