Spermatogenesis: What It Is And What Are Its Phases

spermatogenesis

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

Asexual reproduction generates copies identical to a single parent, while sexual reproduction allows genetic variability throughout generations: a child will never be exactly the same as either parent. Based on this premise, we can understand how natural selection works. As the living beings in a population are not exactly the same, there are certain mechanisms that can favor the persistence of a specific character in the same species, allowing its expansion over 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, it can be reproduced more easily. If the trait is heritable, their children will come out with longer necks as well, which will end up encouraging the expansion of that positive character 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

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 rounded structures 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.

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

Sperm and haploidy

The cells that make up 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 sexual pair), or in other words, a total of 46. This condition is called diploidy (2n).

On the other hand, Genes have a series of variations, which are called alleles The important thing you should know regarding this topic is that, for the same gene, one allele is inherited from the father and another from the mother, so each of our characters is coded by at least two different alleles. 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 one from the other parent can counteract this error.

So that half of the genetic information that makes up us can come from the father and the other half from the mother, it is clear that The primitive cells that form us must contain half the genetic information as 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 achieved?

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.

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1. Proliferative phase

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

Spermatogonia, by mitosis (generation of 2 exactly identical cells from the original one), give rise to 2 types of cells, type A and type B. It is the type B that interests us, since these will be those responsible for generating a primary spermatocyte. On the other hand, A cells can continue dividing by mitosis.

2. Meiotic phase

It is the process of generating sperm per se, and that is why 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 adenohypophysis to produce gonadotropins (luteinizing and follicle-stimulating hormone).

We are not going to focus on the underlying processes due to their complexity, but you should keep a clear idea in mind: 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 primitive cell gives rise to 4 haploid cells, based on 2 successive divisions (meiosis I and meiosis II). Furthermore, in this process the previously mentioned 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 (homologous type) the paired chromosomes of both parents (remember that spermatocytes are still diploid) align, so that similar DNA sequences cross over between them. So, an exchange of genetic material occurs and the recombined chromosomes are not the same as those of the father or the mother

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3. Spermiogenesis

In this part of the mechanism, the spermatids are transformed into the sperm itself. There are various phases within this block (Golgi, Cap, Acrosomal and Maturation phases), but it can be summarized in the following premise: the flagellum of the sperm 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 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 15 to 200 million sperm with each milliliter of semen ejected. Each ejaculate, therefore, can be composed of up to 300 million sperm

Summary

As you have seen, in the end it all boils down to a game of genetic exchange. Since living beings that reproduce sexually have to reduce our genetic information in the gametes by half, it is necessary for the sexual cells to go through a process called meiosis, which gives the eggs and sperm the haploidy essential to understanding the life. Thus, a whole one emerges from two halves, the zygote that will give rise to an adult individual after pregnancy.

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