Cumulus oophorus

At ovulation, the ovum, surrounded by the cumulus oophorus, is released from the ovary and moves down the fallopian tubes. As described above, ovulation follows a surge in LH, which causes luteinization of the granulosa cells of the follicle destined for ovulation. During ovulation, the oocyte enters the tubal lumen and is eventually moved to the site of fertilization. Since mating can occur at any time in the cycle in humans, the spermatozoa may be at the fallopian tubes at the time of ovulation, or the ovum may be at the fallopian tubes awaiting the arrival of the spermatozoa. 

Fertilization is the fusion of the sperm and ovum. The sperm head binds to the plasma membrane of the egg (oolemma), and the entire spermatozoon enters the cytoplasm of the ovum. Only capacitated spermatozoa with intact acrosomes can enter and pass through the cumulus oophorus. The acrosome is a membrane-bound, cap-like structure covering the anterior portion of the sperm nucleus. The acrosomal reaction is the release of materials that lyse the glycoprotein coat (zona pellucida) surrounding the ovum. This is necessary for fertilization to take place. Before undergoing the acrosomal reaction, sperm go through a type of hypermotility called hyperactivation. The... 

Mammalian fertilization is the result of a precisely regulated series of cellular interactions. This process can be divided into a number of component stages, including the following (i) the early events that precede egg fusion, consisting of the preliminary event of sperm capacitation, sperm penetration of the cumulus oophorus, zona pellucida adhesion, initiation and completion of acrosome reactions, penetration of the zona pellucida, egg plasma membrane contact and fusion and (ii) the late events that occur within the egg that consist of sperm nuclear decondensation followed by pronuclear consolidation and syngamy. 

In conclusion, it is generally believed that the extracellular matrix of the cumulus oophorus functions as a selective filter permitting only those sperm that have not completed the acrosome reaction to penetrate to the zona pellucida. This matrix may also provide a unique microenvironment that regulates the final stages of capacitation. 

Another interesting system in which hyaluronan plays an important role is expansion of the cumulus oophorus during ovulation. In response to follicle stimulating hormone and a factor produced by the oocyte, hyaluronan synthesis by the cumulus cells surrounding the oocyte increases dramatically [32], A stable, gel-like matrix is formed between the cumulus cells due to crosslinking of hyaluronan by inter-a-trypsin inhibitor and TSG-6 [31, 32]. This matrix is responsible for the integrity of the cumulus cell-oocyte complex which is required for protection and transport of the oocyte during ovulation, entry into the oviduct and fertilization. Sperm-associated hyaluronidases allow penetration of this matrix at fertilization. 

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