The fertilization allows the union of one egg and sperm, resulting in a single cell from which we all come. This video shows how it happens
He said the recently deceased Eduardo Galeano. “Sometimes people knew and wondered: this was the fastest sperm?”. Beyond the irony of the comment, his words describe one of the most complex, unknown and fascinating processes biology: fertilization of the egg.
As with any career, also in the life-competition does not start with the kick-off of the marshals. Athletes have to prepare, placed on the line and wait watch the shot. Something happens at the biological level. Fertilization does not begin with the union of egg and sperm. At the molecular scale, occur a series of essential preliminary steps, which serve to prepare our sexual gametes for your particular meeting.
The egg and sperm live a date very complex at the molecular level
After the release of sperm, begins a frantic race to reach the egg, located in the fallopian tube. In this way, sexual gametes undergo a series of changes at the morphological, structural and functional level. The objective is clear: only one sperm can reach and merge with the egg . Thus, the 23 chromosomes from the father manage to “mingle” with the mother 23, thus forming the zygote. This diploid cell contains the complete set of chromosomes (46 in the case of the human species), leading subsequently to the embryo.
The fertilization, therefore, has been defined as a complex process that has many steps. The three most important are, in this order, the fusion of the membranes of the sexual gametes (egg and sperm) and the resulting creation of a single cell by binding to genomic and cytoplasmic level (although virtually no sperm cytosol the fact is that some components to the zygote gives future). As an example, if we compare the fertilization with filling a glass with a bottle of water, diferenciaríamos one hand, the combination of external containers (membranes) and, secondly, the mixture of domestic content.
The goal: a single cell
The first phase of fertilization is the membrane fusion of the egg and sperm, a process that can also be seen in the response immune or communication between neurons . In all three cases, the goal is similar: the combination of phospholipid bilayers that make up the membranes
The cells must fuse membranes to create the zygote
The handshake or membrane fusion is the first requirement for fertilization to occur. Continuing the metaphor, if we pass a stranger, we greet before starting a conversation. The same applies to biological and indeed level, they have identified key molecules at this stage. For example, the gp41 protein appears to be key to viral fusion, while complex SNAREs play an important role in the release of neurotransmitters from synaptic vesicles. If any of these molecules fails, the membrane fusion will not occur . And therefore the fertilization, or also block the processes that were taking place, as the fusion of a virus or synapses between neurons –
In the first moments of the video, we can see the sea urchin eggs as large cells. Beside, sperm flutter as small bright spots, looking to win the race to fertilization and penetrate the female gamete. However, none of them can “declare victory” yet.
The maturation of gametes
As mentioned earlier, both sperm and eggs undergo important changes in shape , refers function and structure. In the case of the eggs , the female gametes are ready when they begin to reach a minimum diameter of 20 micrometers (but may reach 120 microns depending on the species). However, are the sperm sexual cells undergo further changes.
The mature sperm before reaching the ovum
Passing the female reproductive tract, male gametes must pass necessarily through two stages: the training and acromosómica reaction. The first serves as functional maturation of sperm, and it has been observed changes in the potential and membrane fluidity or induction of hyperarousal.
That is, at the time of ejaculation, Sperm are not completely ready to face the race, but end to prepare while swimming toward the egg. In addition to training, the male gametes must face known as acromosómica reaction . This allows the union of two membranes of the sperm and the plasma external of acromosoma, as shown in the following drawing. Thus, the male gamete can now enter finish line and win the race fertilization.
The arrival finish line
We have identified key proteins in anchoring the sperm to the egg Egg and sperm are ready for sex date. But how do you recognize? Research has not been able to completely unravel the mystery surrounding this particular meeting. But yes we have identified certain proteins that could be key.
The best known is Izumo1 , a protein in sperm named in honor of a Japanese temple marriage. In an article published in Nature in 2005, scientists at the Osaka University identified this molecule as the “signal” required for the recognition and binding of the two gametes.
Nine years later , researchers from the Wellcome Trust Sanger Institute of Cambridge unveiled in Nature the second key protein in this process. It was the folate receptor 4 egg, known as Juno , which in Roman mythology is the goddess of motherhood. Izumo1 therefore be anchored to Juno for effective union between sperm and egg occurs.
But who has won the race?
In track racing, often be common to use the photo-finish to discern who has won the competition. But what happens biologically? How do you know the sperm and the egg has been fertilized?
Once the male gamete has come into line, begins a stage called lock polispermia . The objective is to prevent a second sperm to penetrate the egg, and for that, it is surrounded by a protective cover, as we see in the next moment Video:
The egg appears the center surrounded by a layer that prevents the input of a second sperm. As we can imagine, creating this cover must be almost instantaneous. But actually it occurs in two phases, followed by a quick lock slower one. The first process is brought about by a change in electrical potential of the plasma membrane of the egg. In other words, the quick release occurs by varying the concentration of ions on either side of the membrane of the female gamete.
The fertilized egg forms a protective cover to prevent between a second sperm
However, this initial blockade can not last forever. And that’s when it happens the slow stage, mediated by a calcium wave. It starts right at the point where the head of the first sperm entered. Some theories suggest that, in some way, such a protein can promote the process gamete. The result is that a protective cover is generated, also known as fertilization membrane . The race is over: only able to win one
Later, the egg and sperm pronuclei be merged, then allowing the metabolic activation of the egg.. Thus, the zygote will have a complete set of chromosomes, centrioles also receiving the male gamete and mitochondria female gamete, components that form part of the new cytoplasm. The sex of the embryo is also determined, depending on whether the sperm carrying an X or Y chromosome.
Thus ends fertilization, a complex and fascinating process that will lead to the development of the embryo, the result of progressive division of the zygote. As shown in this animation published in Journal of Assisted Reproduction and Genetics , it strangely amazing to look back to where it all began. Where to begin all.
June 16, 2015
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