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 plasma membrane of the oocyte. As you can see, the first sperm to reach the oocyte is never the one to fertilize it. The digestive enzymes released by this reaction digest the extracellular matrix of the corona radiata. How do sperm penetrate the corona radiata? Some sperm undergo a spontaneous acrosomal reaction, which is an acrosomal reaction not triggered by contact with the zona pellucida. The plasma membrane of that sperm then fuses with the oocyte’s plasma membrane, and the head and mid-piece of the “winning” sperm enter the oocyte interior. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte’s plasma membrane. These enzymes clear a path through the zona pellucida that allows sperm to reach the oocyte. 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. Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida. The sperm first burrow through the cells of the corona radiata. To reach the oocyte itself, the sperm must penetrate the two protective layers. The underlying zona pellucida (pellucid = “transparent”) is a transparent, but thick, glycoprotein membrane that surrounds the cell’s plasma membrane.Īs it 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. 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. As you will recall from your study of the oogenesis, this oocyte (specifically a secondary oocyte) is surrounded by two protective layers. Fertilization must occur in the distal uterine tube because an unfertilized oocyte cannot survive the 72-hour journey to the uterus. Upon ovulation, the oocyte released by the ovary is swept into-and along-the uterine tube. If they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte’s thick outer layer of cells. Sperm must undergo the process of capacitation in order to have the “capacity” to fertilize an oocyte. 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.
The fluids improve the motility of the spermatozoa. Intercourse more than a day after ovulation will therefore usually not result in fertilization.ĭuring the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called capacitation, or priming.
In comparison, an oocyte can survive independently for only approximately 24 hours following ovulation. Thus, fertilization can still occur if intercourse takes place a few days before ovulation. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. Their journey-thought to be facilitated by uterine contractions-usually takes from 30 minutes to 2 hours. Thus, the race into the uterine tubes, which is the most typical site for sperm to encounter the oocyte, is reduced to a few thousand contenders. Of those that do enter, thousands are destroyed by phagocytic uterine leukocytes. 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. During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the vagina.
Transit of Spermįertilization is a numbers game. This new single cell, called a zygote, contains all of the genetic material needed to form a human-half from the mother and half from the father. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. Summarize the events that occur as a sperm fertilizes an oocyteįertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse.Explain capacitation and its importance in fertilization.Describe the obstacles that sperm must overcome to reach an oocyte.By the end of this section, you will be able to:
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