CSEC Biology: The Mechanism of Fertilization in Humans

Fertilization in humans is simply how the two gametes (the sperm and the egg cell) fuse to form the zygote.

Before the whole process of fertilization can begin, two gametes from two completely separate people need to get 'up close and personal.' This happens through sexual intercourse, which involves inserting the erect penis into the vagina. 

  • When stimulated, spongy tissue in the penis filled with blood and becomes erect. 

  • At the climax, semen is ejaculated from the penis into the neck of the vagina. 

  • Muscles in the wall of the sperm duct help to propel the semen forward

  • The sperm cells in the semen, with their tails, swim from the vagina, through the cervix and uterus, into an oviduct.

If you remember from the previous post on the menstrual cycle, ovulation is the release of an egg cell from the ovary.

After ovulation, the egg is caught in the funnel of the oviduct (fallopian tube). Very slowly, the egg travels towards the uterus. If the egg is not fertilized by a sperm within 8-24 hours after ovulation, it will die. By this time, it has only traveled a short way along the oviduct. So a sperm cell must reach the egg while it is quite near the top of the oviduct if fertilization is to be successful.

The two gametes are haploid (as we discussed in this post) meaning that they only have half of a full set of chromosomes. So, in fertilization, where the sperm gamete fuses with the egg gamete, they form a diploid zygote (which has a full set of chromosomes, half of the set of chromosomes from each parent's gamete).

The process of fertilization has two main phases:

  1. Acrosomal reaction- After swimming up the uterus to the oviduct, the sperm cell comes into contact with the ovum (egg). The ovum sends signals to the sperm cell, which causes the sperm cell to release digestive hydrolytic enzymes from its acrosome. These enzymes break down the outer layer of the egg cell (zona pellucida).

  2. Cortical reaction- The first sperm cell to get through the zona pellucida then fuses with the egg plasma membrane. The cortical granules of the ovum fuse with the egg plasma membrane, causing the vitelline layer to be impenetrable to other sperm cells. The nucleus of the sperm now fuses with the nucleus of the ovum in its cytoplasm thus combining two sets of haploid genetic material and forming a diploid zygote.

The process is best described using a diagram:

After fertilization, the diploid zygote travels out of the fallopian tube and down the uterus and slowly begins several mitotic divisions known as cleavage.

These cell divisions produce the inner cell mass (ICM), which will become the embryo, and the trophoblast, which surrounds the ICM and interacts with maternal tissues. This structure takes several hours to reach the uterine wall, at which point it is made up of 16 or 32 cells, and is called a blastocyst. The blastocyst then implants itself in the endometrium in a process known as implantation.

The implantation of the blastocyst in the endometrium occurs in three phases: apposition, adhesion and the embedding in the endometrium.

The apposition can only occur during a certain time period within the course of the menstrual cycle, known as the "implantation window". The apposition is connected with the maturation of the endometrium. As soon as the adhesion on the endometrium is complete, the cells that lie on the periphery of the blastocyst - the trophoblast - differentiate into two cell types: the syncytiotrophoblast (ST, on the outside) and the cytotrophoblast (CT, on the inside). Through the secretion of certain enzymes, the ST cells break down numerous structures of the endometrium and cause the decidual reaction. This process leads to the embedding of the blastocyst into the endometrium, at which point it is completely surrounded by ST cells. During the second week of the blastocyst's development, vacuoles appear in the ST. They combine into structures that later become filled with maternal blood, which comes from vessels broken down by the ST's enzyme activity. This is how placenta is formed.

After numerous mitotic divisions and the development of certain structures, the embryo develops to this point:

CSEC requires that you know the functions of the following parts of the fetus in utero:

Amniotic sac- This thin-walled sac surround the fetus and contains a fluid produced by the fetus (amniotic fluid). It also contains the membrane that covers the fetal side of the placenta (amnion). This protects the fetus from injury. it also helps to regulate the temperature of the fetus.

Placenta- The placenta provides oxygen, nutrients, and other substances to the fetus from the mother and gets rid of carbon dioxide and other wastes.

Umbilical Cord- This is a rope-like connective tissue that runs from the fetus to the mother and contains 2 arteries and a vein. This cord transports oxygen, nutrients, and other substances through the mother's blood to the fetus and gets rid of carbon dioxide and other wastes. (Your belly button/navel is the point at which you were connected to your mother during her pregnancy).


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