Acrosome reaction
The acrosome is a membrane-bound organelle of Golgi apparatus origin, commonly located at the tip of the head of the mature spermatozoon, which contains a variety of enzymes and antigens required for fertilization.
In some lower animal species, a protuberance (the acrosomal process) forms at the apex of the sperm head, supported by a core of actin microfilaments.
In some echinoderms, including starfish and sea urchins, a significant portion of the exposed acrosomal content contains a protein that temporarily holds the sperm on the egg's surface.
The cortical granules inside the oocyte then fuse to the outer membrane, and a quick fast block reaction occurs.
Furthermore, the cortical granules inside the oocyte fuse with the outer membrane, initiating a rapid block reaction.
[3] In other cases, such as in the wood mouse Apodemus sylvaticus, premature acrosome reactions have been found to cause increased motility in aggregates of spermatozoa promoting fertilization.
[4] The acrosomal reaction typically occurs in the ampulla of the fallopian tube, the site of fertilization, when the sperm penetrates the secondary oocyte.
[5] The first stage is the penetration of corona radiata, by releasing hyaluronidase from the acrosome to digest cumulus cells surrounding the oocyte and exposing acrosin attached to the inner membrane of the sperm.
The cumulus cells are embedded in a gel-like substance made primarily of hyaluronic acid, and developed in the ovary with the egg and support it as it grows.
In the mouse, it has been demonstrated that ZP3, one of the proteins that make up the zona pellucida, binds to a partner molecule (to the β1,4-galactosyl transferase receptors) on the sperm.
[6] As seen in mouse studies, IZUMO1 binds to the oocyte protein JUNO and once bound together, the sperm and the egg fuse together to form two pronuclei.
The acrosome reaction is induced by passage through the cumulus oophorus cells, mediated by the hormones they secrete (such as progesterone, LPA, LPC).
[6][9][10] However, the physiological role of truly spontaneous acrosomal reaction, occurring well before this point in the female reproductive tract, or in vitro, is a separate phenomenon.
[6] Furthermore, the rate of spontaneous acrosome reaction is higher in more promiscuous species such as Apodemus sylvaticus, which face a high level of sperm competition.
[13] The acrosome reaction can be stimulated in vitro by substances a sperm cell may encounter naturally, such as progesterone or follicular fluid, as well as the more commonly used calcium ionophore A23187.
