Amphimixis (Fusion of Pronuclei)
Amphimixis (also called karyogamy) is the final step of fertilization, where the genetic material from the male gamete (sperm) and female gamete (egg) combine to form a diploid zygote nucleus. This process ensures the formation of a new individual with a complete set of chromosomes (46 in humans). The sperm enters the secondary oocyte at the fertilization cone or cone of reception. This triggers the completion of the second meiotic division, producing a haploid mature ovum and the second polar body, which degenerates. The head of the sperm, which contains the nucleus, detaches from the middle piece and tail. The sperm nucleus swells and transforms into the male pronucleus. The nucleus of the ovum also swells and is now called the female pronucleus. The male and female pronuclei move towards each other inside the cytoplasm of the ovum. As they approach, their nuclear membranes disintegrate. The chromosomes from both the male and female gametes align on a spindle apparatus formed between centrioles. The paternal and maternal chromosomes mix, leading to the formation of a diploid zygote nucleus. A diploid zygote (2n) with 46 chromosomes is formed. The zygote now begins its mitotic divisions (cleavage) to develop into an embryo. The mother is now considered pregnant, as the fertilized egg moves towards implantation in the uterus.
Zygote Nucleus → The diploid nucleus formed by the fusion of male and female pronuclei.
Penetration Path → The initial movement of the male pronucleus towards the female pronucleus.
Copulation Path → The final movement when both pronuclei fuse to form the zygote nucleus.
Spindle Formation → The structure formed between centrioles where chromosome alignment occurs.
