the sperm cell penetrates an egg by cortical reaction.
the sperm cell penetrates an egg by cortical reaction.
how does the sperm cell penetrate an egg?
cortical reaction
in mammals, the binding of the spermatozoon to the galt initiates the acrosome reaction. this process releases the hyaluronidase that digests the matrix of hyaluronic acid in the vestments around the oocyte. additionally, heparin-like glycosaminoglycans (gags) are released near the oocyte that promote the acrosome reaction. fusion between the oocyte plasma membranes and sperm follows and allows the sperm nucleus, the typical centriole, and atypical centriole that is attached to the flagellum, but not the mitochondria, to enter the oocyte. the protein cd9 likely mediates this fusion in mice (the binding homolog). the egg "activates" itself upon fusing with a single sperm cell and thereby changes its cell membrane to prevent fusion with other sperm. zinc atoms are released during this activation.
fertilisation
the female pronucleus is the female egg cell once it has become a haploid cell, and the male pronucleus forms when the sperm enters into the female egg. while the sperm develops inside of the male testes, the sperm does not become a pronucleus until it decondenses quickly inside of the female egg. when the sperm reaches the female egg, the sperm loses its outside membrane as well as its tail. the sperm does this because the membrane and the tail are no longer needed by the female ovum. the purpose of the cell membrane was to protect the dna from the acidic vaginal fluid, and the purpose of the tail of the sperm was to help move the sperm cell to the egg cell. the formation of the female egg is asymmetrical, while the formation of the male sperm is symmetrical. typically in a female, meiosis starts with one diploid cell and becomes one ovum and three polar bodies. in a male, meiosis starts with one diploid cell and ends with four sperm. in mammals, the female pronucleus starts in the center of the egg before fertilization. when the male pronucleus is formed, after the sperm cell reaches the egg, the two pronuclei migrate towards each other. however, in brown alga pelvetia, the egg pronucleus starts in the center of the egg before fertilization and remain in the center after fertilization. this is because the egg cells of brown alga pelvetia, the egg pronucleus is anchored down by microtubules so only the male pronucleus migrates towards the female pronucleus.
pronucleus