An egg cell is the germ cell of a woman. It enables her to become pregnant and produce offspring. In this process, the egg cell plays an important role in passing on genetic principles. Thus, reproduction is not possible without the existence of egg cells.
What is an egg cell?
Schematic diagram showing the anatomy and structure of the sperm cell and egg cell in humans. Click to enlarge. Oocytes arise from primordial germ cells. While cells are normally diploid, oocytes have a haploid set of chromosomes. This means that each chromosome in an egg cell is present only once. Each egg cell thus has 23 chromosomes, 22 of which are autosomes, and one gonosome, which determines the sex of the child. The haploid set of chromosomes results from the fact that each sperm also has 23 chromosomes, and the egg and sperm become one diploid cell when they unite. The eggs grow in the ovary. This requires a process called oogenesis. In a twofold cell division (meiosis and mitosis), the diploid cell set is divided in half. In this process, the second maturation division is completed only in the case of fertilization. The two divisions thus result in a total of four cells. Of these, however, two cells, the polar corpuscles, have no function.
Anatomy and structure
The female egg cell measures an approximate size of 120-150 μm. Thus, they are the largest cells found in nature and are perceptible to the naked eye. At the same time, it is surrounded by a layer called the envelope layer. This plays a crucial role in fertilization in particular. It has specific proteins that enable it to bind the sperm to itself. Only in this way can fertilization occur at all. Between the envelope layer and the cell membrane is the perivitellin space. If the sperm has been able to penetrate the envelope layer, it initially remains in this layer for a short period of time. In addition, polar bodies can be found in the interstitial space of the egg cell. These contain DNA material that is no longer needed. Behind the cell membrane lies the ooplasm and the cell nucleus. The entire DNA is located in the nucleus. In the case of fertilization, the diploid cell has already become a haploid cell during the two divisions of maturation. Thus, the DNA on the side of the egg cell is based on only 23 chromosomes. Another part of the egg cell consists of vesicles. These resemble small vesicles and contain, among other things, albumin and fat. During fertilization, these substances provide nutrition for the cell.
Function and tasks
From birth, a large amount of eggs are stored in a woman’s ovaries. While it was previously assumed that girls already had all their eggs as soon as they were born, it has now been demonstrated that they produce stem cells capable of division later in life. As soon as women are fertile, which happens during puberty, one of the eggs matures about every four weeks. This norm is subject to the fluctuations of the individual cycle. The first division produces a primary oocyte, and the second maturation division produces the secondary oocyte. Towards the end of this process, it leaves the ovary and enters the fallopian tube. This process is also called ovulation and is regulated by hormones. In some circumstances, fertilization occurs in this state, which causes the haploid chromosome set to double and become capable of dividing again. In such a case, the egg nests in the tissue of the uterus. If fertilization does not occur, the body does not need the built-up mucous membrane and rejects it along with the egg. This results in monthly menstruation. The task of the egg cell is thus reproduction. Without its existence it would not be possible to produce offspring. Through natural or artificial fertilization, the egg and sperm fuse together. After fertilization, the combined gametes give rise to the fetus.
Diseases and medical conditions
Sperm attempt to penetrate and fertilize a female egg. Click to enlarge. Hormonal conditions are crucial in the production of eggs and during the course of the cycle. Mechanical as well as hormonal problems and complications may occur. One of them is ectopic pregnancy. After fertilization, the diploid egg normally migrates to the uterus. This process takes about three to five days.If the process has been successfully completed, it is possible for the egg to implant in the uterus, creating optimal conditions for pregnancy. However, it may also happen that the egg does not manage to pass through the fallopian tube to reach the uterus. This is the case, for example, when the fallopian tubes are blocked. When this occurs, the baby grows in the mucous membrane of the fallopian tube, and an ectopic pregnancy results. Problematically, the embryo in this region cannot be supplied with the nutrients it needs. If the ectopic pregnancy is detected, abortion is often performed. If this is not performed, severe internal bleeding due to rupture of the fallopian tubes cannot be ruled out. If a woman wants to become pregnant, the eventual failure to conceive can also be caused by the eggs. In addition to stress, obesity and increased alcohol consumption, other conditions may also be present. One of these is polycystic ovary syndrome. As a result of this disease, there are often strong cycle fluctuations. Due to hormonal conditions, ovulation occurs very rarely or not at all. Instead, the eggs store in the form of cysts in the uterus.
