27.2A: Fertilization
Fertilization occurs when a sperm and an egg have fused together to form a zygote, which begins to divide as it moves towards the uterus.
- Describe human fertilization
Key Points
- Fertilization is commonly known as conception. Once the fertilized gamete (ovum) implants itself in the uterine lining, pregnancy begins.
- The fusion of male and female gametes ( sperm and ovum, respectively) usually occurs following the act of sexual intercourse. However, artificial insemination and in vitro fertilization have made achieving pregnancy possible without engaging in sexual intercourse.
- The process of fertilization occurs in several steps and the interruption of any of them can lead to failure.
- Prior to fertilization, sperm undergo a process of capacitation in response to conditions in the female reproductive tract, which include increases in motility and destabilization of the cell membrane that allows the head of the sperm to penetrate the egg.
Key Terms
- fertilization : The act of fecundating or impregnating animal or vegetable gametes.
- capacitation : A step spermatozoa undergo in the female reproductive track that renders them capable of fertilizing an oocyte.
- implantation : The embedding of the fertilized ovum into the uterine wall.
- Nondisjunction : Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.
- zygote : A fertilized egg cell.
If pregnancy is considered to begin at the point of implantation, the process leading to pregnancy occurs earlier as the result of the female gamete, or oocyte, merging with the male gamete, or spermatozoon. In medicine, this process is referred to as fertilization; in lay terms, it is more commonly known as conception.
After the point of fertilization the fused product of the female and male gamete is referred to as a zygote or fertilized egg. For species that undergo internal fertilization, such as humans, the fusion of male and female gametes usually occurs following the act of sexual intercourse.
However, the advent of artificial insemination and in vitro fertilization have made achieving pregnancy possible without engaging in sexual intercourse. This approach may be undertaken as a voluntary choice or due to infertility.
Human fertilization : The sperm and ovum unite through fertilization, creating a zygote that (over the course of 8–9 days) will implant in the uterine wall, where it will reside over the course of 9 months.
The process of fertilization occurs in several steps and the interruption of any of them can lead to failure. At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize.
The sperm must undergo capacitation in the female’s reproductive tract over several hours, which increases its motility and destabilizes its membrane. By destabilizing the membrane, the sperm prepares for the acrosome reaction, the enzymatic penetration of the egg’s tough membrane, the zona pellucida. The sperm and the egg cell (which has been released from one of the female’s two ovaries) unite in one of the two fallopian tubes.
The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete until implantation occurs. Through fertilization, the egg is activated to begin its developmental process (progressing through meiosis II), and the haploid nuclei of the two gametes come together to form the genome of a new diploid organism.
Nondisjunction during the completion of meiosis or problems with early cell division in the zygote to blastula stages can lead to problems with implantation and pregnancy failure.