Human sex cells

Your body is made up of trillions of cells. All these cells were created from other cells dividing, to produce more cells. But the very first cell that divided, and in 9 months became you as a newborn baby, was actually created by two cells fusing together — an egg cell and a sperm cell. Let’s look at the egg cell first. The cells that are destined to become egg cells — primary ova — start developing in a woman’s ovaries before she is born.

By the time of birth, there are around 400 000 primary ova. Each primary ovum contains 23 pairs of chromosomes — or 46 in total, just like all other cells in the human body. Primary ova stay in the ovaries until the body reaches puberty and the menstrual cycle starts. During every cycle, one of the primary ova goes through a process that results in a unique, mature egg cell. First the chromosomes in the primary ovum, align.

Then, crossover of chromosomes happens, to create new, unique combinations of genetic material. Then, the cell divides into two. This type of cell division is called meiosis. The two new cells have 23 chromosomes — half as many as the primary ovum. The 23rd of these chromosomes is a sex chromosome, and in egg cells, it is always an X chromosome.

One of these two cells is usually larger than the other. Then, both the larger and the smaller cell split again. The smaller cell splits into two equal cells, while the larger one splits into a large cell, and a small cell. The newest large cell is a mature egg cell. The mature egg cell is spherical, about 130 micrometers big — one of the largest cells in the human body!

It’s covered in a protective coating. Special molecules inside the egg cause this protective coating to harden. This protective coating hardens when a sperm cell enters the egg, which stops additional sperm cells from entering. Like egg cells, sperm cells start off as immature cells with 23 pairs of chromosomes, or 46 in total. Sperm cells start being produced in a man’s testicles at the beginning of puberty.

Let’s look at one of these immature sperm cells. It first divides to produce two identical cells. One of these cells stays an immature sperm cell. The other divides by meiosis — just like the egg cell. This results in two cells with 23 chromosomes each.

Like in the egg cell, the 23rd chromosome is the sex chromosome. But in sperm cells, it can be an X or a Y chromosome. These two cells divide again, to produce four cells, still with 23 chromosomes in each. These four cells develop into mature sperm cells. Mature sperm cells are shaped a bit like tiny tadpoles.

The head-like structure holds the chromosomes. It is covered in molecules that can break through the protective coating of an egg to fertilise it. The tail is used for swimming. When an egg is fertilised, the new cell produced, the zygote, has 23 chromosomes from the egg and 23 chromosomes from the sperm. 46 chromosomes in total — the full number for a human!

So two different people must provide one sex cell each, to create one complete cell, which may then split and develop into a human being, after 9 months. And this is how you came to be a newborn baby too!