Spermatogenesis: what it is and what are its phases

Sexual reproduction is defined as the process in which a new descendant living being is created from the combination of genetic information of two organisms. parental, giving rise to the mechanisms of heredity, 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 the genetic variability across generations: a child will never be exactly the same as any of the other two parents. Based on this premise, we can understand how natural selection works. Since living beings in a population are not exactly the same as each other, there are certain mechanisms that can come to favor the persistence of a specific character in the same species, allowing its expansion throughout the time.

Giving 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, can reproduce more ease. If the trait is heritable, their children will come out with longer necks as well, which will end up encouraging the spread of that positive trait in the species.

To understand all these biological mechanisms, it is necessary to be clear about how offspring is produced, that is, the process of generating life from the formation of parental gametes to the development of a new individual. Today we address one of those complex topics: spermatogenesis.

• Related article: "Main Cell Types of the Human Body"

What is spermatogenesis?

spermatogenesis is the process by which sperm (male gametes) are formed. This essential mechanism for the production of life is carried out in the testicles, in round-shaped structures that are called seminiferous tubules. These tubes, about 200 micrometers in diameter and 50 centimeters long, produce 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 of interest knowing that gametes (both male and female) have half the genetic information of the rest of our cells bodily. Now you will better understand what we mean.

sperm and haploidy

The cells that make up all of 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 sexual pair), or what is the same, a total of 46. This condition is called diploidy (2n).

On the other hand, Genes have a number of variations, called alleles.. The important thing you should know about this topic is that, for the same gene, one allele is inherited from the other. father and one from the mother, so each of our traits is encoded by two different alleles, such as minimum. This allows us to be more "effective" at an evolutionary level, because if an allele from one of the parents fails or does not perform its function correctly, it is expected that the other parent can counteract this mistake.

For half of the genetic information that makes us up 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 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 only have one set of 23 chromosomes. How is this accomplished?

• You may be interested in: "The 4 types of sex cells"

The phases of spermatogenesis

Spermatogenesis and meiosis are two sides of the same coin, since one cannot be conceived without the other. Next, we briefly present each of the phases that occur during spermatogenesis.

1. proliferative phase

spermatogonia are specialized stem cells that give rise to spermatozoa by differentiating. Spermatogonia are still diploid, which means they have a total of 46 chromosomes, half from the mother and half from the father (remember: diploid, 2n), like the rest of our cells somatic

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

2. meiotic phase

It is the process of generating spermatozoa per se, and for this reason it is also called spermatocytogenesis.. This mechanism is set in motion by the release of the hormone GnRH (gonadotropin-releasing hormone), which is produced in the hypothalamus and which, in turn, stimulates the anterior pituitary for the production of gonadotropins (luteinizing hormone and folliclestimulating).

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

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

In summary, in genetic recombination (of the homologous type) the paired chromosomes of both parents (remember spermatocytes are still diploid) align so that similar DNA sequences cross over they. So, there is an exchange of genetic material and the recombined chromosomes are not the same as the father's or the mother's.

3. spermiogenesis

In this part of the mechanism, the spermatids are transformed into the spermatozoa proper. There are various phases within this block (Golgi, Cap, Acrosome 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 until 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 sperm every 24 hours.

As a curious fact that serves to close everything shown, it is incredible to know that a man expels from 15 to 200 million spermatozoa with each milliliter of ejected semen. Each ejaculation, therefore, can be composed of up to 300 million sperm..

Summary

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

The mechanisms of evolution and natural selection fall on spermatogenesis and oogenesis, because thanks to they are given processes such as genetic recombination and the creation of a living being from "2 halves genetics”. Without these very specific biological mechanisms, understanding diversity on Earth would be impossible.

Bibliographic references

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• Andrade, c. TO. T. (2018). The gluconeogenic enzyme fructose-1, 6-bisphosphatase and its participation in the spermatocyte-spermatid transition.
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• Correa, Y. R. M., Nunez, D. TO. O., Marín, I. H., Tovar, J. M., & Ruíz, A. TO. (2005). Arrest of spermatogenesis. Ginecol Obstet Mex, 73, 500-8.
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• Molfino, H. m. G., & Figueroa, H. g. (2017). THE SPERMATOGONIAS OF MAMMALS. Biotempo, 14(2), 233-243.