Human Fertilization: Where And How It Happens, Process

The fertilization or  human fertilization is the process of recognition and fusion of male and female sex cells that occurs during sexual reproduction. It fulfills the main function of originating the zygote, that is, the cell with half the chromosomal load of each parent, which will then give rise to the embryo and later the fetus.

Fertilization is one of the first events that takes place during human embryonic development, but it can only occur when certain previous conditions are satisfied or, better said, after another series of processes such as gametogenesis (oogenesis and spermatogenesis) have occurred. and intercourse, for example.

Nowadays there are different in vitro techniques that allow to control and ensure fertilization without the need for sexual contact between a man and a woman. These techniques are included in the field of assisted reproduction and consist of the union of an egg and a sperm under laboratory conditions.

Events prior to fertilization

Gametogenesis

Gametogenesis, as its name implies, has to do with all the events that lead to the formation and development of sex cells or gametes, which are specialized generative cells. The female gametes are known as ovules and the male ones as sperm.

While the other cells of the human body are produced by mitosis, gametes are produced by meiosis, which is a type of reductive cell division where daughter cells are characterized by having half the number of chromosomes compared to the cells from which they are derived. .

The male gametogenesis is known as spermatogenesis, while the female one is called oogenesis or oogenesis. Male gametes are small and mobile, while female gametes are larger and immobile.

Spermatogenesis begins in males during puberty and continues into old age. It occurs in special areas of the testicles (male reproductive organ) known as “seminiferous tubules.”

Oogenesis, on the other hand, takes place during the last stages of a woman’s embryonic development, that is, before her birth, which means that a girl is born with all the eggs she will have in her life.

This process occurs in the ovaries, which are at the ends of the fallopian tubes, one of the characteristic structures of the vagina, the female reproductive organ.

The ovules derive from the maturation of their precursor cells, which are known as oogonia. This takes place from the beginning of puberty until menopause.

When puberty begins, one oogonia matures in women at a time, a fact that occurs periodically and is evidenced as the menstrual cycle.

Intercourse, copulation, or sexual intercourse

Unless it is a case of assisted reproduction, fertilization can only occur if an egg comes into contact with a sperm in a woman’s body, and for this to happen it is necessary that intercourse or sexual intercourse takes place (known to other animals as mating).

If it is necessary to define it precisely, we can say that copulation consists of the conscious act of introducing the man’s penis into the woman’s vagina, which may or may not occur for reproductive purposes and that allows the sexual cells of both to approach. individuals to produce offspring.

Where does fertilization occur?

Fertilization or fertilization in humans is internal, which means that it occurs inside the mother, in the womb, to be more exact.

Recall that female sex cells (gametes) are immobile, that is, they always remain in the organ that produces them, while male gametes are mobile and must be deposited in the female reproductive tract for the fusion of both cells to occur.

What is the uterus like?

The uterus is part of the female reproductive system. It is a pear-shaped organ that measures between 7 and 8 cm long and between 5 and 7 cm wide at its widest part. It has very thick muscular walls, which can ask up to 3 cm.

This organ has two parts: the body and the cervix. The body represents the upper part of the uterus and corresponds more or less to two thirds of this, it is the largest part. The cervix, on the other hand, consists of a small cylindrical portion at the bottom of the uterus.

The uterus has two tubes called uterine tubes (uterine tubes or fallopian tubes), which are about 10 cm long and 1 cm in diameter. These tubes project to each side of the uterus and consist of four parts that, from the furthest to the closest, are called: infundibulum, ampulla, isthmus and uterine part.

Fertilization occurs inside these tubes, specifically in the region of the ampulla, where an ovum, which is transported from one of the ovaries located in the final portion of the tube, and a sperm, which was released into the vagina from the penis and that it is transported by the same tube.

The ovaries are the female gonads. This means that they are the sites where female sex cells are produced and matured: the ovules. The uterus is associated with two ovaries, one on each side of the two uterine tubes.

It may be the case that fertilization does not occur specifically in the blister, but rather “later”, but it never occurs in the body of the uterus.

When a mature ovum is released from the ovary into the uterine tubes and this is not achieved with a sperm that fertilizes it, then it is transported to the body of the uterus, where it is degraded and reabsorbed.

Fertilization process

Fertilization is a fairly complex process and is highly controlled by different physical and hormonal factors, both men and women. It is important to remember that it occurs only between mature sex cells, but we will not discuss this process here.

Fertilization can last about 24 hours. It begins when a sperm comes into contact with an egg and ends with the first division of the zygote, which is the cell that results from the fusion of female and male sex cells, where the genetic material of both is combined.

The zygote, after many cell divisions, will develop into an embryo, a fetus, and finally a baby.

The phases of human fertilization are 4, let’s see what they are called and what they consist of:

1- Preparation of the sperm and approach to the ovule

The meeting between an egg and a sperm in vivo , that is, inside a woman’s body, occurs only when the woman is in the fertile days of her cycle (when an egg begins to travel through the uterine tube) and when a man ejaculates semen inside his reproductive system.

Sperm ejaculated with semen, which is the fluid produced by man to “transport” sperm, are not fully mature, that is, they are not ready to fuse with an egg.

Once in the vagina, they undergo various physical changes that together are known as training. Training includes increased swimming speed, modification of the biochemical characteristics of your plasma membrane, etc.

When the sperm are closer to the egg, they have already been “trained” and that is when the union of one of these with the egg can occur.

However, human eggs are covered by a very thick layer called the zona pellucida, which in turn is surrounded by a set of follicular cells known as the corona radiata.

Only sperm that manage to cross the corona radiata can come into contact with the ovum and for this they secrete a special enzyme called hyaluronidase.

2- Union of the ovum and the sperm

Once a sperm reaches the zona pellucida of an egg, the union between the two cells can occur and for this, what experts call the acrosome reaction takes place.

The acrosome is an intracellular organelle of sperm that derives from the Golgi complex and contains many hydrolases (proteins that hydrolyze chemical bonds of other proteins). Its interior is characterized by having a very low pH, that is, acidic.

The acrosomal reaction is triggered by the zona pellucida after the “detection” of a sperm. In this, the region of the plasma membrane that covers the acrosome fuses with the zona pellucida and releases different enzymes that manage to make a “hole” so that the sperm can reach the interior of the ovum.

When the plasma membranes of the two cells irreversibly come into contact, changes occur in the membrane of the ovum that make it impenetrable by another sperm.

3- Fusion of the sperm with the ovum

During this stage the membrane of the ovule recognizes that of the sperm, so that both can fuse completely. When this happens, the sperm stops moving its tail; in fact, the tail is lost, as only the organelles in the body of the sperm are discharged into the cytosol of the ovule.

During this phase of fertilization, the sperm nucleus undergoes some changes, becoming a “pronucleus” that will later fuse with the female pronucleus to form a diploid nucleus (2n).

4- Completion of meiosis of the ovum and formation of the zygote

The female eggs are in what is known as a “meiotic arrest”, which means that they have not completed this division.

When a sperm fuses with an ovum, the latter completes meiosis and becomes a mature ovum, whose pronucleus is the one that fuses with the male pronucleus.

The fusion of both nuclei produces the zygote, which is now a cell with half the genetic load of one individual (the father) and half the genetic load of another (the mother). Shortly after nuclear fusion, the zygote undergoes the first divisions, which ultimately produce an embryo and then a fetus.

In vitro fertilization

In vitro fertilization consists of the fusion of an egg with a sperm under controlled conditions in a laboratory.

This is a common practice in assisted reproduction programs, which seek to help men and women who have problems conceiving, either due to infertility or lack of donors, and who want to do so.

A woman can donate eggs and for this she undergoes a treatment to stimulate superovulation. If the treatment works, the eggs produced are “aspirated” by laparoscopy, an outpatient procedure, for example.

A man can also donate sperm and the sperm undergoes special treatments to achieve their qualification.

The eggs of a donor, when they are aspirated, are placed in a Petri dish with a special medium containing the trained sperm of another donor, which promotes fertilization.

The first divisions of the resulting zygote are monitored for a few days and, depending on the age of the woman who wants to conceive, one to three embryos are transferred to her reproductive system, hoping that one of them is successful and is implanted correctly in the uterus.

References

  1. Dudek, RW, & Fix, JD (2005). Embryology. Lippincott Williams & Wilkins, Philadelphia.
  2. Gardner, DK, Reed, L., Linck, D., Sheehan, C., & Lane, M. (2005, November). Quality control in human in vitro fertilization. In Seminars in reproductive medicine (Vol. 23, No. 04, pp. 319-324). Copyright © 2005 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA.
  3. Monroy, A. (2020). Encyclopaedia Britannica. Retrieved August 18, 2020, from britannica.com
  4. Moore, KL, Persaud, TVN, & Torchia, MG (2018). The Developing Human-EBook: Clinically Oriented Embryology. Elsevier Health Sciences.
  5. Paulson, RJ, Sauer, MV, & Lobo, RA (1990). Embryo implantation after human in vitro fertilization: importance of endometrial receptivity. Fertility and Sterility, 53 (5), 870-874.
  6. Schoenwolf, GC, Bleyl, SB, Brauer, PR, & Francis-West, PH (2014). Larsen’s human embryology Ebook. Elsevier Health Sciences.

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