16.1 Fertilisation
Learning Objectives
By the end of this section, you will be able to:
- Describe the obstacles that sperm must overcome to reach an oocyte to enable fertilisation
- Explain capacitation and its importance in fertilisation
- Summarise the events that occur as a sperm fertilises an oocyte in order to form a zygote
- Explain the artificial reproductive technologies (ART) that are used for assisted reproduction in humans
Fertilisation occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. 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. This new single cell, called a zygote, contains all the genetic material needed to form a human – half from the mother and half from the father.
Transit of Sperm
Fertilisation is a numbers game. 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. 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. Their journey—thought to be facilitated by uterine contractions—usually takes from 30 minutes to two hours. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. Thus, fertilisation 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. Intercourse more than a day after ovulation will therefore usually not result in fertilisation.
During the journey, fluids in the female reproductive tract prepare the sperm for fertilisation 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 in order to have the “capacity” to fertilise 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. Fertilisation must occur in the distal uterine tube because an unfertilised oocyte cannot survive the 72-hour journey to the uterus. As you will recall from your study of the oogenesis, this oocyte (specifically a secondary oocyte) is surrounded by