Fertilization is the process by which gametes (an egg and sperm) fuse to form a zygote. Through it, a number of steps and reactions occur. During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the vagina. Almost immediately, millions of these sperm are overcome by the acidity of the vagina (approximately pH 3.8), and millions more may be blocked from entering the uterus by thick cervical mucus. Of those that do enter, thousands are destroyed by phagocytic uterine leukocytes. The race into the uterine tubes, the most typical site for sperm to encounter the oocyte, is reduced to a few thousand contenders. The sperms must make their way into one of the two uterine tubes (fallopian tubes) to meet the egg, and overcome the cilia that work only in the direction of the uterine cavity. Approximately 100–1000 sperms reach the ampulla, the site of fertilization, where about 20–200 sperms meet the oocyte cell mass.

The journey through the female reproductive system to reach the oocyte is thought to be facilitated by uterine contractions which usually take around 30 minutes to 2 hours. A healthy sperm could reach the ampulla within 30 minutes. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 48–72 hours. Thus, fertilization can still occur if intercourse takes place a few days before ovulation. In comparison, an oocyte can survive independently for only approximately 24 hours following ovulation. Therefore, intercourse more than a day after ovulation will usually not result in fertilization.

During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called capacitation, or priming. The fluids improve the motility of the spermatozoa. They also deplete cholesterol molecules embedded in the membrane of the head of the sperm, thinning the membrane in such a way that will help facilitate the release of the lysosomal (digestive) enzymes needed for the sperm to penetrate the oocyte’s exterior once contact is made. Sperm must undergo the process of capacitation to have the “capacity” to fertilize an oocyte. If they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte’s thick outer layer of cells.

Contact Between Sperm and Oocyte

Upon ovulation, the oocyte released by the ovary is swept into—and along—the uterine tube. Fertilization must occur in the distal uterine tube because an unfertilized oocyte cannot survive the 72-hour journey to the uterus. The released oocyte is a secondary oocyte surrounded by two protective layers. The corona radiata is an outer layer of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation. The underlying zona pellucida (pellucid = “transparent”) is a transparent, but thick, glycoprotein membrane that surrounds the cell’s plasma membrane.

As the oocyte is swept along the distal uterine tube, the oocyte encounters the surviving capacitated sperm, which stream toward it in response to chemical attractants released by the cells of the corona radiata. To reach the oocyte itself, the sperm must penetrate the two protective layers. The sperm first burrow through the cells of the corona radiata. Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida. This initiates a process called the acrosomal reaction, in which the enzyme-filled “cap” of the sperm, called the acrosome, releases its stored digestive enzymes. These enzymes clear a path through the zona pellucida, allowing sperm to reach the oocyte. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte’s plasma membrane. The plasma membrane of the sperm then fuses with the oocyte’s plasma membrane, and the head and mid-piece of the “winning” sperm enter the oocyte interior.

How do sperm penetrate the corona radiata? Some sperm undergo a spontaneous acrosomal reaction, an acrosomal reaction that is not triggered by contact with the zona pellucida. The digestive enzymes released by this reaction digest the extracellular matrix of the corona radiata. As you can see, the first sperm to reach the oocyte is never the one to fertilize it. Rather, hundreds of sperm cells must undergo the acrosomal reaction, each helping to degrade the corona radiata and zona pellucida until a path is created to allow one sperm to contact and fuse with the oocyte’s plasma membrane.

When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to prevent polyspermy, or penetration by more than one sperm. Preventing polyspermy is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes, and incompatible with life.

The first mechanism is the fast block, which involves a near-instantaneous change in sodium ion permeability upon the binding of the first sperm, depolarizing the oocyte plasma membrane and preventing the fusion of additional sperm cells. The fast block sets in almost immediately and lasts for about a minute.

At the same time, an influx of calcium ions following sperm penetration triggers the second mechanism, the slow block. In this process, referred to as the cortical reaction, cortical granules sitting immediately below the oocyte plasma membrane fuse with the membrane and release zonal inhibiting proteins and mucopolysaccharides into the space between the plasma membrane and the zona pellucida. Zonal inhibiting proteins cause the release of any other attached sperm and destroy the oocyte’s sperm receptors, thus preventing any more sperm from binding. The mucopolysaccharides then coat the nascent zygote in an impenetrable barrier that, together with the hardened zona pellucida, is called a fertilization membrane, or eggshell. 

Recall that at the point of fertilization, the oocyte has not yet completed meiosis; all secondary oocytes remain arrested in metaphase of meiosis II until fertilization. Only upon fertilization does the oocyte complete meiosis. The unneeded complement of genetic material that results is stored in a second polar body that is eventually ejected. At this moment, the oocyte has become an ovum, the female haploid gamete. The two haploid nuclei derived from the sperm and oocyte and contained within the egg are referred to as pronuclei. They decompress, expand, and replicate their DNA in preparation for mitosis. The pronuclei then migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material intermingles. This step completes the process of fertilization and results in a single-celled diploid zygote with all the genetic instructions it needs to develop into a human. Sex, hair and eye color determination happen at this point.

Clinical Correlation

Fraternal Twins

Fraternal twins, also known as dizygotic twins, are a type of twins that result from the simultaneous release and fertilization of two different eggs (oocytes) by two different sperm cells. Fraternal twins are essentially like any other siblings, with the key difference being that they share the same womb during pregnancy.

Because they develop from separate fertilized eggs, they have their own unique genetic makeup and can be of the same gender (two brothers or two sisters) or of different genders (a brother and a sister). They share approximately 50% of their genes, like any other siblings born at different times.

Early Pregnancy Factor

Early pregnancy factor (EPF) is a protein that has been used to describe a substance in the mother’s blood that seems to be present within hours after conception (fertilization), before the embryo even reached the uterus. This factor might be involved in preventing the mother’s immune system from rejecting the developing embryo. Ongoing research is attempting to identify and characterize the specific molecules or factors involved in early pregnancy, although a universally accepted explanation has yet to be established.