Gametes are the fertilizable male and female gametes or germ cells. Their diploid (twofold) set of chromosomes has been reduced to a haploid (single) set by prior meiosis (maturation division), resulting in a diploid cell with a twofold set of chromosomes after fertilization, the union of a female and a male gamete. The female gamete corresponds to the egg capable of fertilization and the male gamete corresponds to the sperm capable of fertilization.
What is a gamete?
Fertilizable female or male gametes or germ cells are called gametes. In humans, as in all other mammals, male and female gametes look very different. The female gamete is the egg capable of fertilization and the male gamete is the sperm capable of fertilization. Despite very different appearance and size, their common feature and characteristic is their haploid (single) set of chromosomes. The simple chromosome set is formed by previous meiosis (maturation division) of the primordial germ cells, which are separated from the somatic body cells at a very early stage, still during the embryonic phase. The process of egg formation, including the first part of meiosis, begins in the embryonic phase and is completed shortly after birth. This means that after sexual maturity has occurred, females have only a limited supply of gametes capable of fertilization, even though the total supply of about 500 eggs capable of ovulation may seem generous. In the male, meiosis of the primordial germ cells takes place throughout his life after puberty, so that sperm capable of fertilization are repeatedly produced and supplied “fresh”. After the union of a female and a male gamete, i.e. a female egg with a male sperm, a diploid cell, the zygote, is formed by the union of the two haploid chromosome sets. It embodies the primordial cell from which the genetically preprogrammed individual emerges through innumerable divisions (mitoses) and cell differentiations.
Anatomy and structure
The female gamete, the egg cell, has a spherical shape with a diameter of 0.12 to 0.15 millimeters. The oocyte is surrounded by a proteinaceous envelope layer, which plays an important role in the docking of a sperm. Between the envelope layer and the cell membrane of the oocyte is the perivitellin space, which contains three so-called polar bodies, each with a haploid set of chromosomes. The polar bodies are formed during the first and second meiosis, are no longer needed by the body and are therefore degraded later. They play a role in in vitro fertilization because their chromosome set can be examined for possible hereditary damage before the egg cell containing the identical chromosome set is implanted. The cytoplasm of the egg contains organelles (e.g. mitochondria) and lysosomes, which contains nutrients for the time after fertilization. The nucleus, also located in the egg, contains a complete haploid set of chromosomes. The male gamete, also called sperm or sperm filament, is much smaller than the egg and consists of a head with a nucleus and also a haploid set of chromosomes and a middle piece or neck with attached mitochondria and the subsequent flagellum, which provides for self-motion of the sperm. On the head is the so-called acrosome, the head cap, which contains enzymes for penetrating the egg membrane.
Function and tasks
Gametes serve what is known as sexual reproduction, allowing recombination of genes within a population so that different individuals can develop in the same population. In combination with possible gene mutations that arise spontaneously during cell division or due to mutagens, a population or society can adapt to changing environmental conditions. If certain traits are advantageous in a changed environment, a so-called gene shift within the population in favor of the advantageous trait occurs over the course of many generations. This possible adaptation process is so important for survival and of such great advantage that it outweighs the disadvantages of lethal mutations, which can also occur. In contrast to this is the so-called asexual or asexual reproduction by sprouting or similar processes.This is cloning, the production of genetically identical individuals, the development of which involves significantly fewer risks than sexual reproduction, but offers no possibilities for adaptation to a changing environment. During the fusion of the male sperm with the female egg cell, a special feature must be taken into account. Upon penetration of the egg, the sperm loses the neck and the flagellum, both of which remain outside the egg membrane, and thus also the male mitochondria. This means that on the paternal side, only the DNA located in the nucleus is inherited. The independent mitochondrial DNA is inherited exclusively on the maternal side.
Diseases
Diseases, ailments, and dysfunctions can arise during the formation of the gametes, during gametogenesis. A spontaneous mutation may occur during maturation division, or an error may occur during chromosome division. For example, parts of a chromosome may be missing or a chromosome may be duplicated in the haploid set, resulting in a so-called trisomy after fusion. Relatively well known is trisomy 21, also called Down syndrome, in which the diploid chromosome set contains a triple chromosome 21. Turner syndrome is caused by the absence of an X chromosome in the diploid chromosome set. In such cases, it is a gametopathy, a germline damage corresponding to a pre-damage of the female egg or the male sperm. As a rule, defective chromosome divisions during meiosis are lethal for the subsequent cells, especially for the cell that is missing a whole chromosome or parts of a chromosome. That is, they are usually unable to survive and no new individual can arise.
