Spermatogenesis: what is spermatogenesis and what are its phases?

Let's see what spermatogenesis is and how it enables reproduction.

Sexual reproduction is defined as the process in which a new descendant living being is created from the combination of genetic information of two parental organisms, giving rise to the mechanisms of inheritance, genetic variability and evolutionary processes that have allowed species to reach where they are today.

Asexual reproduction generates copies identical to a single parent, while sexual reproduction allows genetic variability over generations: a child will never be exactly the same as either parent. Based on this premise, we can understand how natural selection works. Since living beings in a population are not exactly the same among themselves, there are certain mechanisms that can favor the persistence of a particular character in the same species, allowing its expansion over time.

To give a theoretical example: if a giraffe is born with a longer neck than the rest (due to a mutation or recombinant action of the DNA of both parents), it may be able to reach more food, become stronger than the rest and, therefore, be able to reproduce more easily. If the trait is heritable, their offspring will have longer necks as well, which will end up promoting the expansion of this positive trait in the species.

To understand all these Biological mechanisms, it is necessary to have a clear understanding of how offspring are produced, that is, the process of the generation of life from the formation of the parental gametes to the development of a new individual. Today we address one of these complex topics: spermatogenesis.

What is spermatogenesis?

Spermatogenesis is the process by which spermatozoa (male gametes) are formed.. This essential mechanism for the production of life takes place in the testicles, in rounded structures called seminiferous tubules. These tubes, about 200 micrometers in diameter and 50 centimeters long, produce the sperm and the hormone testosterone, essential for the growth of the penis and scrotum, the depth of the voice and body hair in men.

Before continuing with this fascinating process, we must clarify a series of genetic terms of great importance, since it is interesting to know that the gametes (both male and female) have half the genetic information of the rest of our body cells. Now you will better understand what we mean.

Sperm and haploidy

The cells that form all our tissues and divide by mitosis to maintain our organs and structures are known as "somatic". Each of these cell bodies contains in its nucleus 23 pairs of chromosomes (two complete sets, 22 autosomal pairs and one sex chromosome), or in other words, a total of 46. This condition is called diploidy (2n).

On the other hand, genes have a number of variations, which are called alleles.. The important thing to know in this regard is that, for the same gene, one allele is inherited from the father and another from the mother, so each of our traits is encoded by at least two different alleles. This allows us to be more "effective" at the evolutionary level, because if an allele of one of the parents fails or does not perform its function correctly, it is expected that the allele of the other parent will be able to counteract this error.

In order for half of the genetic information that shapes us to come from the father and the other half from the mother, it is clear that the primordial cells that form us must contain half the genetic information of the somatic cells.. Otherwise, with each generation more chromosomes would be added to the cells, making life impossible (2n + 2n:4n, 4n + 4n: 8n, etc.). Based on this premise, we can assume that sperm cells are haploid (n), that is, they possess only one set of 23 chromosomes. How is this achieved?

The phases of spermatogenesis

Spermatogenesis and meiosis are two sides of the same coin, as one cannot be conceived without the other. Here is a brief description of each of the phases that occur during spermatogenesis.

1. Proliferative phase

The spermatogonia are the stem cells specialized in giving rise to spermatozoa as they differentiate into sperm.. Spermatogonia are still diploid, which means that they have a total of 46 chromosomes, half from the mother and half from the father (remember: diploid, 2n), like the rest of our somatic cells.

Spermatogonia, by mitosis (generation of 2 exactly equal cells from the primordia), give rise to 2 types of cells, type A and type B cells. It is the type B cells that are of interest to us, since they will be in charge of generating a primary spermatocyte. On the other hand, the A cells can continue to divide by mitosis.

2. Meiotic phase

This is the process of generation of spermatozoa per se, which is why it is also called spermatocytogenesis.. This mechanism is triggered by the release of the hormone GnRH (gonadotropin-releasing hormone), which is produced in the hypothalamus and which, in turn, stimulates the adenohypophysis for the production of gonadotropins (luteinizing hormone and follicle-stimulating hormone).

We are not going to focus on the underlying processes due to their complexity, but you should keep in mind one clear idea: in this case, the secondary spermatocytes (product of the primary ones, in turn coming from the B spermatogonia) divide by meiosis, not by mitosis.

In mitosis, a cell duplicates its genetic information and gives rise to 2 identical cells.. On this special occasion, a diploid primordial cell gives rise to 4 haploid cells, based on 2 successive divisions (meiosis I and meiosis II). In addition, in this process the previously mentioned genetic recombination takes place, so that the descendants are not the same as the initial one. After meiosis, the spermatids appear, which are already haploid.

In summary, in genetic recombination (of the homologous type) the paired chromosomes of both parents (we recall that spermatocytes are still diploid) are aligned, so that similar DNA sequences cross-link with each other. Thus, an exchange of genetic material occurs, an exchange of genetic material takes place and the recombined chromosomes are not the same as those of either the father or the mother..

3. Spermiogenesis

In this part of the mechanism, the spermatids are transformed into the spermatozoa themselves. There are several phases within this block (Golgi, Capping, Acrosomal and Maturation phase), but it can be summarized in the following premise: the flagellum of the spermatozoon grows, which allows it to move, and the length of its head decreases.to acquire the pointed shape that we all know.

Figures and times

Human spermatogenesis lasts from 62 to 75 days, and extends from sexual maturation in adolescence to the death of males. All these processes occur constantly in the testicles because, without going any further, a healthy man produces about 100 million viable spermatozoa every 24 hours.

As a curious fact that serves to close all that has been shown, it is incredible to know that a male expels from 15 to 200 million spermatozoa with each milliliter of semen ejaculated. Each ejaculation, therefore, can be composed of up to 300 million spermatozoa..

Summary

As you have been able to see, in the end it all comes down to a game of genetic exchange. Since living beings that reproduce sexually have to halve our genetic information in the gametes, it is necessary for the sex cells to go through a process called meiosis, which gives the eggs and sperm the haploidy essential to understand life. Thus, from two halves emerges a whole one, the zygote that will give rise to an adult individual after gestation.

Spermatogenesis and ovogenesis are responsible for the mechanisms of evolution and natural selection, since it is thanks to them that processes such as genetic recombination and the creation of a living being from "2 genetic halves" take place. Without these very specific biological mechanisms, understanding diversity on Earth would be impossible.

Bibliographical references

• How are spermatozoa formed? Reproducción asistida.org. Retrieved March 13 from https://www.reproduccionasistida.org/espermatogenesis/#fase-proliferativa
• Aguilar, J., López, M. C. G., Gilabert, A. C., Ortiz, A., González, E., Galisteo, J. O., ... & Castilla, J. A. (2004). Spermatogonial stem cells. International journal of Andrology: sexual and reproductive health, 2(2), 54-59.
• Andrade, c. A. T. (2018). The gluconeogenic enzyme fructose-1, 6-bisphosphatase and its involvement in the spermatocyte-spermatid transition.
• Bassas, L. (2001). Spermatogenesis and Infertility. Revista Iberoamerica de Fertilidad, 18, 11-17.
• Correa, Y. R. M., Núñez, D. A. O., Marín, I. H., Tovar, J. M., & Ruíz, A. A. (2005). Arrest of spermatogenesis. Gynecol Obstet Mex, 73, 500-8.
• Marina, S. (2003). Advances in the knowledge of spermatogenesis. Clinical implications. Revista Iberoamericana de Fertilidad, 20(4), 213-225.
• Molfino, H. M. G., & Figueroa, H. G. (2017). MAMMALIAN SPERMATOGONIA. Biotempo, 14(2), 233-243.