Human fertilization: characteristics and stages
A summary of the stages through which human fertilization passes, and how it affects the gametes.
As indicated by Charles Darwin's theory of selection in The Origin of Species (1859), the diversity of life on planet Earth has arisen from the common descent of a series of key ancestors through a branching pattern of evolution. Living things exist for reproduction, for the ultimate goal of every animal is to leave its genetic imprint on future generations in the form of offspring, whatever the cost.
Human beings, as conscious and sentient animals, find pleasure and a reason for existence beyond the mere act of reproduction. Even so, we cannot deny that sex is an essential part of our lives, both as a species and as individuals. For example, data collected by the professional portal Statista show us that almost 40% of Spaniards, in 2020, had sex at least once a week. A 2% of the population practice intimate relations almost every day of their lives.
With these data, we want to reflect that sexuality is natural, and beyond stigmas and preconceptions, it follows Biological mechanisms common to many other members of the animal kingdom. Here we tell you all about human fertilization, both at a molecular and genetic level.We will tell you all about human fertilization at a molecular, genetic and physiological level.
The genetic basis of human fertilization.
First of all, it is necessary to emphasize that human cells are diploid, i.e. they have a pair of copies of each chromosome inside their nucleus (2n).. Diploidy is not a characteristic common to all living beings, since, for example, the males of many Hymenoptera have only one set of chromosomes, that is, they are haploid (n).
Diploidy is expensive, because for each cell to have a copy of a chromosome, there must have been an episode of fertilization between a male and a female of the same species. One of the chromosomes comes from the male gamete (sperm) and the other from the female gamete (egg). As each of these cell bodies are haploid (n), fusion gives rise to a diploid (2n) zygote. In the case of the human genome, the basic formula of the process would be as follows.
Fertilization: sperm (n:23) + egg (n:23) = zygote (2n:46).
Thus, each of the cells derived from the zygote (somatic line) has 23 pairs of chromosomes in its nucleus, making a total of 46, half from the father and half from the mother.. The mechanism of fertilization allows genetic variability to exist, since chromosomal permutations, overcrossing and many other events that occur during gamete formation and fertilization are the key to diversity within a single species.
In addition to all this, it is worth noting that diploidy has a clear meaning at the individual level. Since we have one copy of each gene, it is expected that if one of them fails on a given chromosome, the other one can make up for the deficiency.. Fertilization is therefore expensive, but it is always worthwhile as a biological strategy.
It all starts in the germ cells
Somatic cells, which form our entire body, divide by mitosis.. When we get a wound or suffer skin flaking, the tissues that replace the lost section are made up of exact copies of the previous cells, i.e. they are genetically the same.
To form a male or female gamete, however, there is a special cell group: the germ stem cells. These, unlike the rest, divide by meiosis and not by mitosis. The aim is to reduce the genetic information by half (from diploid to haploid) so that, when both gametes fuse, the zygote recovers its diploidy..
Let us take spermatogenesis as an example. The complete cycle lasts 75 to 90 days and begins with the spermatogonial phase, in which type A and B spermatogonia are formed from germ cells by mitosis. The B spermatogonia specialize into spermatocytes, which are the ones that will undergo meiotic division to divide their genetic information in half.
After meiosis I and II, 4 spermatids arise from each spermatocyte, which in turn have undergone chromosomal permutation and genetic overcrossing.. It is enough for us to know that the information of the spermatids is not the same as that of a diploid cell and, therefore, genotypic and phenotypic diversity is generated between parents and offspring. The last step is the maturation of the spermatids to mature spermatozoa, ready to fertilize the egg, which in turn has undergone a similar synthesis process in the female reproductive system.
The moment of fertilization
Fertilization is the act of union of the egg and the sperm that allows the beginning of pregnancy. We will skip the preliminaries, as it is easy to assume that we are all at least familiar with the mechanical component of the sexual act.
Once the male ejaculation occurs, the sperm travel through the vagina, ascend the female genital tract and meet the egg ready to be fertilized in the fallopian tubes. It is estimated that there are 15 to 200 million male gametes per milliliter of semen, but curiously, only a few hundred reach the female fallopian tubes. As you know, only one of them fertilizes the female egg, which in this case is you, the reader of these lines.
In order for fertilization to occur correctly, the woman must be in her fertile period of the month, that is, approximately between days 14-15 after the first day of monthly bleeding.. In general, a woman is considered ready to become pregnant when the endometrium (the highly vascularized layer that lines the uterine cavity) is 7 to 10 millimeters thick. If the egg is not fertilized, the excess endometrial tissue is shed and exits through the vagina, in the event we all know as menstruation.
At the microscopic level, several events take place that allow fertilization or entry of the egg to the sperm cell. Briefly, these are as follows:
- Penetration of the corona radiata: this corona consists of 2,3 or 4 thin layers of small cells surrounding the ovum. This layer protects the egg from mechanical stress during the tubal journey and, thanks to the enzyme hyaluronidase and its flagellum, the sperm can bypass it.
- Penetration of the zona pellucida: this is a translucent acellular layer covering the oocyte. The action of several spermatozoa is necessary to dissolve it.
- Membrane fusion: at this stage, a fertilization cone is formed that allows subsequent contact between the genetic information of both gametes.
- Fusion of nuclei: this process is very complex, but it is enough to know that, finally, the haploid content of each gamete fuses into a diploid cellular entity.
From this point on, a period begins that is a little beyond our current competence: implantation. At this point, the zygote begins to divide, the zygote begins to divide into 2 cells, then into 32 and then into hundreds of cells, passing through stages such as the morula or the blastocyst.. On day 14 after fertilization, the blastocyst effectively invades the endometrium and gestation proper begins.
Interestingly, during implantation about 3 out of 10 women experience an event called "implantation bleeding". This is believed to correspond to the moment when the blastocyst invades the endometrium, and manifests itself as a very light brownish spotting period a couple of weeks after fertilization.
Summary
We have tried to cover as many biological areas as possible, because human fertilization is too exciting a process to focus only on the steps that cause the sperm to enter the egg. It is essential to know why we reproduce sexually, what are the cells that give rise to the gametes and what happens at the microscopic level in the fallopian tubes of the female after male ejaculation.
With all these data in mind, we can affirm that fertilization is a slow and costly process at the biological level, but very necessary for the permanence of the species over time.. Thanks to sexual reproduction, genetic variability and evolution of living beings is possible between generations.
Bibliographical references:
- How are spermatozoa formed? ReproducciónAsistida.org. Retrieved on April 13 from https://www.reproduccionasistida.org/espermatogenesis/
- What is human fertilization and what are its stages? ReproducciónAsistida.org. Retrieved April 13 from https://www.reproduccionasistida.org/como-se-produce-la-fecundacion/#definicion-de-fecundacion
- Kinsey institute: FAQs and sex information. Retrieved April 13 from https://kinseyinstitute.org/research/faq.php
- Morier, L., Gómez, M., Rodríguez, J. J., & Pérez, L. (2004). Obtaining and characterization of a diploid human kidney cell line. Cuban Journal of Tropical Medicine, 56(1), 42-48.
- Ureña Calderón, J. F. (1971). Electron microscopic study on spermatogonial spermatocytes and sex chromosomes of the rat: rattus norvegicus (Doctoral dissertation, Universidad del Salvador).
(Updated at Apr 14 / 2024)