Fertilization: Our discreet birth

Fertilization raises a metaphysical dilemma: if any one of the other millions of sperm had been the first to reach the Ovum, Would you have been left without existing, or would you have simply been another, with another body?

Let's leave the philosophy and describe what can be observed. The fertilization is the union of two cells to generate a new organism, in beings with sexual reproduction. Each cell contributes approximately the same amount of genetic material and the result is an organism that looks like the parents, but is not identical to either of them. At that moment it is largely determined how you are going to be (although many subsequent interactions between environmental factors and genes will also play a very important role).

Not long after the origin of the sexual reproduction, both cells involved in the union began to differentiate from each other. One ended up becoming a large ball full of food, completely immobile (the ovule), and another (the sperm), a small vehicle of gene transport, with the only functions of moving until exhaustion (many times erratically, because only in some cases it is guided by attractive substances) or until finding the right Ovum, break down his protective barriers and inject his genetic makeup into him.

This differentiation was driven by the efficiency obtained from the division of labor. The Ovum it is a costly cell in energy terms, while sperm are cheap and can be produced by organisms in huge quantities. This different investment makes the interests of the producers of ovules and those of sperm are very different. At the moment of evolution when one of the two cells became a little larger than the other, the war of the sexes began, which manifests itself in so many ways in nature (and in our society).

The sperm of the animals they consist of a front deposit, called acrosome, which has the function of releasing substances that disintegrate the protective layers of the ovule; of a reduced cytoplasm (with many
mitochondria to generate the energy necessary for movement); of a nucleus with the genetic material and a long tail that moves thanks to protein contractions. A Ovum is the result of a special meiosis, in which three of the 4 cells which in theory should be produced from the original degenerate, and give up their cytoplasm with its food reserves to the final cell. In addition to the plasma membrane, the ovum is surrounded by another external envelope, the pellucid membrane, and, in some cases, like that of women, surrounded by another protective layer of cells, the follicular.

When the spermatozoon find the Ovum, it separates the weak junctions between the cells that surround it with the movement of its flagellum and makes its way to its outer envelope. It must be recognized as belonging to the same species as the Ovum by a pellucid membrane protein, ZP3. Galtase in the sperm cell membrane must bind to this molecule, in the same way as antigens and antibodies in the sperm. immune system. Another protein, ZP2, non-specific, it serves as an anchor to the acrosome. Molecular recognition alters the permeability of the sperm membrane to sodium ion and calcium ion, which enter it, and cause the membrane spermatozoon merges with the pellucid.

The acrosome It then releases the same types of substances as many poisonous animals to destroy tissues and cells (such as hyaluronidase and lysine). Thus the pellucid envelope is disintegrated and the spermatozoon it reaches the cytoplasmic membrane, where it must be recognized by molecules of the integrin family (as we can see, the ovum is very well protected from being invaded by sperm from other species). After recognition, cytoplasmic membranes of the cells fuse and the sperm nucleus, together with its cytoplasm and the large amounts of calcium ion that had accumulated, penetrate into the ovum.

The most urgent thing at this time is to prevent others sperm enter the egg (causing genetic chaos). The massive influx of calcium generates a practically instantaneous depolarization of the cytoplasmic membrane of the ovum, a process very similar to that of the electrical impulse in nerve cells, which also requires great speed (as we see, living beings use the basic mechanisms they possess in very different ways). This blocks the entry of new sperm for a few minutes. The influx of calcium also causes cortical granules below the cytoplasmic membrane of the ovum to be brought closer to it by the action of the cytoskeleton. They merge with it and release their content to the outside, which blocks the ZP3 protein recognition of the pellucid membrane. Thus, no new spermatozoon is recognized and the pellucid membrane becomes rigid and impenetrable.

The next steps are the unpacking of the chromosomes from the spermatozoon and the disintegration of its nucleus. These chromosomes are surrounded by the envelope nuclear ovum and the packaging process of both the father and mother chromosomes begins (these had not been packed before in case fertilization did not occur) by histones produced by the Ovum, to be organized and distributed in the following mitotic divisions (in the initial stages both groups of chromosomes are still separated). The synthesis of proteins and other molecules, which had been almost stopped before fertilization, feverishly resumes, to produce everything that the developing embryo needs.

In most cases, like humans, the two gametes do not contribute exactly the same amount of genetic material. Mitochondria, which contain DNA own, they are inherited from the mother, since the sperm hardly contributes. Furthermore, in the case of mammals, in which sex is determined by a pair of sex chromosomes, the male chromosome contains far fewer genes than its female counterpart.

In reality, we are not independent embryos (in the human case) until one day after fertilization. In that interval, we have not yet created the
RNA that serve us to synthesize our first proteins and we depend on the RNA maternal It is at that moment, when we first acquire some independence from our mother: in the words of the Spanish biologist Antonio Giráldez, "that is our first puberty ".

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