The beginning of a new life

 What really happens when an egg is fertilized by a sperm? We tell you the secrets of a fascinating process.

This is how conception occurs. A succession of surprising events helps the sperm fertilize the egg

Leaving the vagina, ascending the cervix, passing through the uterine cavity, and reaching the fallopian tubes where, hopefully, an egg waits to be fertilized. The sperm's task does not end here. It must still be able to break the envelope of the egg and fertilize it, and then it must reach the uterus and nest successfully...

Fertilization seems like an obstacle course. And in a way it is. But sperm and egg have an advantage: the fact that pregnancy finally occurs is not the result of chance. Recent research has revealed a series of factors that, linked together, make the miracle of life possible.

The sperm are

To go from the vagina to the fallopian tubes, the movement of their tails is not enough for sperm. Today we know that a kind of suction mechanism is activated naturally inside the uterus: the uterine orifice pushes up everything that is at the bottom of the vaginal cavity. Prostaglandins , substances present in semen, promote this suction ; so do the contractions that the uterus experiences in the female orgasm.

The next challenge that sperm face is entering the "correct" fallopian tube, that is, the one that houses the egg. Thanks to the production of estrogen and increased blood flow, the tube in question is wide open to facilitate the passage of sperm, while the empty tube remains "closed."

Another factor that helps the sperm in their feat: the woman's body temperature rises slightly after ovulation due to the influence of the hormone progesterone. This benefits the sperm cells, as they reach their greatest mobility at a body temperature of 37 degrees.

Attracted by the smell

It is known that the egg emits stimulating substances to attract sperm to meet it. These, for their part, are equipped with olfactory receptors (recently discovered) with which they capture those "signals" that the egg sends and that help them find the correct path.

Scientists have also proven that there are other substances capable of attracting sperm. A recent experiment showed how the smell of a certain flower (specifically, the lily of the valley) attracted them so much that they even doubled their speed of movement.

This finding suggests that there are aromatic substances that induce sperm to move in a certain direction. This could have future applications in the field of artificial insemination, to increase the chances of pregnancy.

Researchers have made another interesting discovery: the existence of a blocking olfactory substance that makes the stimulus sent by the egg ineffective. Its discoverers believe that in the future this could lead to new contraceptive methods: it would be enough to "cover the nose" of the sperm and thus prevent them from finding the egg.

Teamwork

Once the egg and sperm meet in the tube, the most difficult thing remains fertilization. Luckily, sperm can survive up to 120 hours in the female genital tract. Thus, if you have sexual intercourse five days before ovulation, there is a possibility that the woman will become pregnant.

Like all large single-celled organisms, the egg is surrounded by a protective layer, called the zona pellucida, which is quite thick and poorly penetrable. A large number of sperm are required to "erode" this envelope for a time, in a kind of teamwork, until the membrane "breaks" at one point. Only then can the sperm located in front of the tear penetrate and fertilize the egg.

The process could be compared to the assault on a fortress. That is, a minimum concentration of sperm is required to achieve natural fertilization. If this is scarce, teamwork is not possible and fertilization does not occur.

These discoveries debunk a popular belief: contrary to what was previously thought, the process has nothing to do with a selection of sperm, nor is it the fastest sperm that fertilizes the egg.

Implantation in the uterus

After fertilization, the embryo moves to the uterus and, after about five days, begins to nest there. Days later, the embryo begins to produce HCG (gonadotropin hormone) which, together with progesterone and estrogen, allows the process to continue.

If all goes well, the pregnancy will progress. On the contrary, if the embryo does not implant in the uterus, after about 12 days a hemorrhage will announce that the pregnancy has not occurred.