Oogenesis: definition and summary

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To carry out reproduction, humans need sex cells to unite (gametes) masculine (spermatozoon) and the female sex cell (Ovum). These cells are specialized in carrying out reproduction and have very special characteristics. Therefore, to make them, the body follows a series of complex processes called gametogenesis.

In the case of women, since the gametes are called ovules, these processes of egg formation are collectively called oogenesis (egg formation). In this lesson from a TEACHER we will see the definition and summary of oogenesis so that you know all the processes that are carried out. If you are interested in knowing more, do not hesitate, keep reading!

Index

1. What is oogenesis? Simple definition
2. Prenatal oogenesis
3. Postnatal oogenesis

The oogenesis is defined as the set of training processes (or genesis) of Ovum, the female gamete. Oogenesis only takes place in females (or females) and takes place in the ovaries.

The ovaries They are two almond-shaped organs, about 4 to 5 centimeters in diameter, located in the upper part of the pelvic cavity, in a depression in the lateral wall of the abdomen. To hold on, heThe ovaries are attached to several ligaments, that serve as an anchor to other nearby viscera and to the abdominal wall.

In the outer region of each ovary are tiny masses of cells called primary follicles. The follicles act as a protective bag and each of these follicles contains an immature egg. At the beginning of each ovarian cycle, which lasts approximately 28 days, at least 20 follicles begin to develop, but only one follicle reaches full development. The rest will be eliminated, that is, they will degenerate.

Stages of oogenesis

Unlike spermatogenesis, oogenesis it is a very long process. Oogenesis begins during embryonic development of the female but does not continue throughout the life of the female if not that, when she is born, there is an interruption in which the cell enters a state of hibernation.

When the female reaches puberty, a series of physiological changes causes oogenesis to reactivate and continue. As you can see, oogenesis is summarized in two stages:

1. First stage: stage prenatal, namely, before birth (while the female is still an embryo)
2. Second stage: stage postnatal, namely, after birth (when the female reaches the puberty and hormonal changes occur)

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As we saw in the previous summary, the prenatal oogenesis are all those processes that cells undergo that will give rise to female sex cells, the ovules, before the birth of the individual. In this case, before the birth of the female.

During the formation of the embryo, the stem cells that generate the different human tissues migrate, from their place of origin to the place where the future organ will be generated. In our case, the stem cells that will give rise to the eggs are called germ stem cells and they move (migrate) to the future ovary. Once in the future ovary, these cells divide by mitosis and give rise to more cells by mitotic division or mitosis. These cells are called oogonia. As they are generated by mitosis from germ cells, we say that oogonia are stem cells of the ovary with all the genetic endowment of the species (diploid).

These cells undergo some modifications and divide again by mitosis, to generate other cells called primary oocytes. Again, these cells have been formed by a process of mitosis, so they remain diploid. Primary oocytes surround themselves with other cells, called flat epithelial and follicular cells, that protect and nourish them. This complex formed by primary oocyte, follicular cells and flat epithelial cells is called primordial follicle.

Around the seventh month of gestation, the primary oocytes begin to divide by meiosis. As it is the first meiosis they suffer, this is called meiosis I. Upon reaching one of the phases of meiosis (the diplotene phase of prophase I), meiotic division stops and the oocyte is "hibernating". This long period of inactivity is called dictyotena.

The dichthyotene is broken when puberty is reached, at which time the oogenesis process is restarted by hormonal action.

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The postnatal oogenesis are all those processes that cells undergo that will give rise to female sex cells, the ovules, after the birth of the individual. In this case, after the breakdown of the dictytin that occurs during puberty in females.

Let's remember how we had left the oocytes before birth: the primary oocytes were surrounded by other cells, forming the primordial follicle. These oocytes had become blocked in meiosis I (in the diplotene phase of prophase, specifically). Each female has about a million primary oocytes in dichotomy when she is born, until she reaches sexual maturity. At that time the number of oocytes is reduced to about 400,000-500,000, since a large number are eliminated.

Upon reaching sexual maturity, the follicles begin to mature and the primary oocytes increase in size. Shortly before the woman ovulates, meiosis I ends and a haploid secondary oocyte and the first polar body. The polar body, which only carries a small part of cytoplasm, is eliminated (atrophied) while the secondary oocyte (which carries DNA and most of the cytoplasm) undergoes a second meiosis and is stopped in metaphase II. East secondary oocyte it is expelled from the ovary (ovulation). Once expelled, two things can happen: it is fertilized or it is eliminated (menstruation).

Note that only 400 to 500 primary oocytes will transform into secondary oocytes throughout the reproductive life, since for each ovulation in each cycle, about a thousand oocytes are lost due to atrophy. This process is very important, since defective oocytes are eliminated that would not give rise to a healthy embryo. Furthermore, not all the secondary oocytes released by a woman are fertilized and only some of the fertilized oocytes (zygotes) go on to develop a healthy embryo.

If you have any questions, concerns, contributions or comments, we would love to hear from you in our comments section!

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