Oocyte, ovum
General information
The egg cell is the female germ cell of the human being. It is haploid. This means that it contains only one set of chromosomes. In women, egg cells develop from the original germ cells and are used for reproduction and the transfer of genetic characteristics from mother to child.
Origin
Oocytes are produced in the ovary during a process called oogenesis. During oogenesis, two maturation divisions take place: However, the second maturation division only takes place when the egg is fertilised by a sperm cell. At the end of oogenesis, there are four cells, of which only two really develop into full-fledged oocytes, the others are released as so-called polar bodies.
In contrast to their precursor cells, the resulting cells are then no longer diploid but haploid, i.e. they contain only one set of chromosomes. When an egg cell is reached by a sperm cell (sperm) during fertilization, which is also haploid, and these two cells fuse together, a diploid cell with a complete set of chromosomes is created again, from which a divisible zygote and finally the embryo can develop. While the male germ cell exclusively supplies DNA to the zygote, the egg cell contributes, in addition to its DNA, cell water (cytoplasm) and other cell organelles, especially the mitochondria, which also contain DNA, but which can only be passed on on the mother’s side.
- The first division corresponds to a meiosis,
- The second of a mitosis.
Structure of the egg cell
Oocytes are the largest cells found in nature. For this reason, even the human egg cell, which tends to be one of the smaller ones among all vertebrates, is still visible to the naked eye. The eggs of mammals are not as large as those of birds or reptiles, for example, because mammalian embryos are nourished via the placenta and therefore no yolk or egg white within the egg is needed as additional food.
A human egg cell has an average diameter of 0.11 to 0.14 millimeters. On the very outside of the egg cell there is a covering layer called zona pellucida, the outer egg membrane. This layer plays an important role in fertilisation, as certain proteins contained in this envelope can bind the sperm cell to the egg.
Once this has taken place, the zona pellucida dissolves. The so-called perivitelline space adjoins the outer egg membrane. In this area, the sperm stays for a short time once it has succeeded in penetrating the zona pellucida.
This is also where the polar corpuscles are located, which contain the excess DNA that the full-fledged egg cells now no longer need. On the other side of this liquid-filled space is the inner egg membrane, which is the cell membrane of the egg cell (oolem). Inside the egg cell is the ooplasm, which contains the cell nucleus with the haploid DNA. In addition, the ooplasm contains several vesicles, which contain fat and albumin, among other things. In the first phase of embryonic development, these vesicles serve to nourish the cell.