THE FERTILIZATION PAGE!

Fertilization must occur within 24-48 hours of ovulation. Fertilization in mammals is generally thought to occur in the region of the ampullary-isthmus junction or the ampulla of the fallopian tube.  There are certain barriers that sperm must overcome in order to reach the fallopian tube and fertilize an oocyte.  These barriers are discussed below as "blocks to polyspermy."  The term polyspermy refers to fertilization of an oocyte by more than one sperm cell.  While a fertile ejaculate may contain 100 million sperm, only 1 sperm will fertilize an oocyte.

Polyspermy is an undesirable condition in which more than a single sperm gains access to the cytoplasm of the oocyte, resulting in fertilization of the oocyte by more than a single sperm, a condition sometimes referred to as polyploidy (excessive genetic complement, eg. 3n=triploidy) or polyspermy.  Polyspermy is just one potential cause of polyploidy; others would include failure of the genetic material in the oocyte to successfully divide (ie. the retention of more than a single set of chromosomes in the oocyte).

Cervical and vaginal mucus, the cervix, "nooks" and "folds" in the uterine body, the uterotubal junction, the narrow fallopian tube, the corona radiata (granulosa cells surrounding and adhering to the oocyte following ovulation), the zona pellucida (the mucopolysaccharide layer separating the oocyte from the granulosa, and the oocyte membrane, itself, are all physical barriers to spermatozoa.  Sperm are like bumper cars, the direction of their movement is relatively random.  Spermatozoa are deposited in the vagina against the cervix.  As many as 25% of the sperm are killed by the acidic environment of the vagina.  Cervical mucus, produced under the influence of rising estrogens late in the follicular phase, tends to line the spermatozoa in the correct orientation to pass through the cervix and enter the uterus.  However, unhealthy or defective sperm are eliminated as they cannot pass through the cervical mucus.  Many sperm are lost or misdirected in the uterus.  As sperm are foreign cells, many are killed by the female's white blood cells.  Some sperm will not find an entrance to a fallopian tube.  Approximately 50% of the sperm that enter the fallopian tubes will enter the tube which contains no oocyte.  Finally, a small number of sperm, possibly 50, come in contact with the cumulus oophorus that surrounds the oocyte.  Enzymes (hyaluronidases) released by the acrosomal cap of the sperm are used to digest intercellular connections between the granulosa cells of the cumulus oophorus.  From among the very, very few sperm which might arrive at the zona pellucida, one will successfully be transported through the zona by the sperm "docking and transport proteins" contained within the zona.

Zona Block (The Zona Reaction)

The zona pellucida appears to contain several zona specific glycoproteins, referred to as ZP_i..... The ZP specific glycoproteins may subserve roles such as "sperm recognition particles," "sperm docking proteins," or " sperm receptors." The ZP glycoproteins appear to help one sperm achieve proper alignment to penetrate the zona pellucida.  Once a spermatozoa has successfully penetrated the zona pellucida, a poorly understood reaction known as "zona block," takes place.  It may be that "sperm channels or spermaphores" within the zona are immediately closed to other sperm.  It is clear that additional sperm will have difficulty binding to the zona and will not be allowed to penetrate the inner zona.

The Cortical Granule Reaction

Following syngamy (fusion of sperm and oocyte), cortical granules (these are secretory granules within the oocyte but close to the cell membrane) fuse with the membrane and release their contents into the membrane and perivitelline space.  Release of the cortical granule contents appears to prevent fusion of additional spermatozoa with the oocyte membrane.  Release of the contents of the cortical granule may also be responsible for initiating the zona reaction.

Fertilization (examine each stage in this diagram)

Only the head of the sperm gains access to the inside of the oocyte.  Upon penetration of the oocyte membrane by a sperm, the second meiotic reduction division (meiosis II) is completed by the oocyte and a second polar body is extruded into the perivitelline space.  Rapidly expanding DNA appears to "explode" through the nuclear pores on the head of the sperm.  The final event in fertilization is the combining of DNA from the sperm with that of the ooctye.