Transcription
Hello guys, I hope you are doing very well. In this video, I will be giving you some details about oogenesis and the phases in which this process is completed. To begin, the first thing we should ask ourselves is what is oogenesis? Oogenesis, or female gametogenesis, is a biological process that takes place in the gonads of sexually reproducing female individuals, also known as ovaries. Its main objective is to produce female gametes, or oocytes, commonly called eggs. This occurs once females reach sexual maturity, meaning when they are already capable of reproducing. But to better understand what this is about, we must know that gametes are haploid cells that are generated from diploid cells. A haploid cell is one that possesses half of the genetic information contained in the nucleus of a diploid cell. This must be the case in oogenesis because during sexual reproduction, a female gamete, an egg, fuses with a male gamete, a sperm, each contributing half of the genetic information that the new individual will possess. This is the reason why offspring inherit traits from their fathers and mothers who conceive them. That said, we must remember that the production of cells with this characteristic is carried out through a very peculiar cell division process that we know as meiosis. During meiosis, a group of diploid germ cells must undergo two consecutive cell divisions to finally become haploid gametes. These two division stages have been named meiosis I and meiosis II. Notice that this reduction in genetic load is the heart of sexual reproduction. In women, the germ cells that will eventually become eggs are called oogonia and are produced during fetal development of females. This means that absolutely all the oogonia that a girl will produce throughout her life will be formed before she is born. A very curious fact is that a large part of the oogonia produced during gestation end up dying, and only the survivors are those that will subsequently divide by mitosis to give rise to primary oocytes, which will remain frozen in this developmental stage until puberty. It is by this time that oogenesis will resume again to produce mature eggs. However, we must necessarily remember that in almost 98% of cases, the eggs produced by a female are not fertilized and are therefore eliminated monthly through her genitals by means of a bleeding event known as menstruation. Well, guys, up to this point, we have made a fairly general overview of what oogenesis consists of. Let's now look at some characteristics of the environment in which it occurs. As I mentioned earlier, oogenesis takes place in the ovaries. These organs, which in humans are two, one right and one left, are oval structures whose size is approximately that of a walnut. Each of them is located at the end of each of the fallopian tubes, which are a type of tube that connects to the uterus. Inside the ovary, two structures can be distinguished: the cortex and the medulla. The cortex, also known as the germinal epithelium, is a superficial region of the ovary that houses the ovarian follicles, structures that keep trapped the eggs that have not yet reached maturity, meaning they are not yet reproductively functional. The ovarian medulla, on the other hand, is an area of loose connective tissue filled with millions of blood vessels responsible for nourishing the oocytes located in the follicles. In mammals, such as humans, once females have reached puberty, the ovaries begin to grow rapidly and the follicles they contain begin to mature cyclically. The maturation of the follicles that culminates in the maturation of the oocytes is not carried out all at once. The most common thing is that the maturation of a single follicle occurs per month, and therefore, the release of a single functional egg. This released egg will travel through the uterine tubes to the uterus and will remain waiting for a sperm to fertilize it. If not, it will be eliminated during the menstrual cycle. This process, which undoubtedly involves oogenesis, occurs every month until the reproductive period of females ceases, which is characterized by the total absence of the menstrual cycle. This period is commonly known as menopause. It is quite curious to mention that it has been scientifically proven that out of the almost 400 million immature eggs a woman has at birth, only 400 reach maturity. Well, after reviewing these very important data, we are ready to start looking at the biological stages that an egg must go through to mature and become viable for reproduction. Oogenesis occurs in a series of steps that take place in two major periods: the prenatal development period, meaning during gestation, and the postnatal development period, once the female has been born. Let's begin by describing the events that occur during prenatal development. During this period, the female embryo develops, and the diploid germ cells that have the potential to become eggs, known as oogonia, multiply rapidly by mitosis, producing all the millions of oogonia that a female will use throughout her life. They also begin the first phase of meiosis, meiosis I, thus transforming into primary oocytes. These primary oocytes do not continue meiosis but rather enter a state of pause once they have reached prophase I, meaning their genome has been duplicated but they have not completed the first of the meiotic divisions. This pause state lasts for at least another decade until the juvenile female reaches fertile age, experiencing the menstrual cycle for the first time. During this developmental phase, the primary oocytes are surrounded by a layer formed of connective tissue made up of follicular cells. Each of the oocytes surrounded by this layer forms what is known as a primordial follicle. There are as many primordial follicles as there are primary oocytes. Now, as I told you earlier, this developmental stage remains paused until puberty, where the postnatal development phase of the primary oocytes begins. By this date, every month, an ovulation occurs, meaning an oocyte is released from one of the follicles into the uterus. During this process, the primary oocytes that were suspended in prophase I of the first meiotic cycle are activated during this period, and as one of the follicles matures, a primary oocyte completes the first meiotic division. However, although during this division the chromosomes are naturally divided, the cytoplasm of the primary oocyte is not. That is, most of the cytoplasm remains in only one of the daughter cells. The daughter cell that receives half of the chromosomes but a tiny portion of the cytoplasm is known as a polar body, while the other receives the name of secondary oocyte. The polar body will not become a functional oocyte but will be degraded. Its formation simply serves for the initial primary oocyte to reduce its chromosomes by half during this division, thus becoming a haploid cell, the genetic characteristic that every gamete must fulfill. Now, only the primary oocyte will advance to the second meiotic division, provided that a sperm fertilizes it. This division is also unequal, like the first, thus forming a fertilized secondary oocyte and another polar body. The complete maturation of this oocyte into a mature egg is completed when the two polar bodies formed have degenerated, meaning they have died and subsequently been degraded. The production of a single functional gamete during oogenesis is an important difference from spermatogenesis, which takes place in males. In the latter, all the cells resulting from the two meiotic divisions end up being functional male gametes. Well, guys, with this, we have reached the end of this video. If you liked it, I invite you to give it a like and subscribe to our channel so you can continue enjoying our selection of educational videos. Until a next video.