Oogenesis is the biological process by which oogonia are transformed into mature oocytes. This process takes place in the ovaries and begins in the third month of intrauterine life. It ends after puberty and ceases definitively at menopause.
Oogenesis can be divided into three phases a multiplication phase a growth phase and another maturation phase however in some bibliographies we will find that oogenesis is carried out in two stages or moments in a prenatal stage or before birth and in another postnatal stage or after birth and So that it is related to the three previous phases we can say that in the prenatal stage we will find the multiplication phase, the growth phase and a first part of the maturation phase and in the postnatal stage there is only the second part of the maturation phase if not Remember these phases, don't worry , I'm going to leave you here at the top right as a card so you can review with my Gametogenesis video. During the prenatal stage, the multiplication phase begins when the primordial germ cells reach the female gonads of the embryo . to proliferate by mitosis and differentiate into oogonia, keep in mind that each oogonia is a diploid cell composed of 46 chromosomes.
During the growth phase these oogonia will increase in size due to the accumulation of nutrients in their cytoplasm and become primary oocytes. and at the same time that these primary oocytes are being formed, there are cells that surround them and form a layer of flattened follicular cells. This primary oocyte that is surrounded by this layer is called the primordial follicle.
In this video we are going to talk about oogenesis and We are going to develop follicular maturation in more depth in another video. Here we are going to talk only about the formation of the oocyte. The maturation phase begins when the primary oocytes, after being formed, replicate their DNA and enter the prophase of the first meiotic division, all the primary oocytes.
Before birth they remain in a state of latency called the diplotene stage or stage. This stage will last until puberty as a consequence of the action of the oocyte maturation inhibitor that will be secreted by the follicular cells. During the prenatal stage it is calculated that At the end of the fifth month, approximately 7 million oogonia are formed in each fetal ovary , but as the pregnancy continues, some of these will become primary oocytes and others end up degenerating.
In the postnatal stage, it is estimated that at birth the total number of oocytes will decrease. It is found between 600,000 and 800,000 in each ovary , then during childhood, most of them become atretic, which means that they degenerate and only 40,000 oocytes per ovary will be present at the beginning of puberty, however, less than 500 will ovulate each month until menopause The primary oocytes remain in the latent phase until puberty, from then on the second phase of maturation continues. In this phase, the maturation of the follicles begins and the primary oocytes increase in size before ovulation occurs, these primary oocytes resume meiosis 1 and finish it by giving rise to two haploid daughter cells of different sizes, each with 23 chromosomes.
On the one hand, we find a cell that receives most of the cytoplasm and is the secondary oocyte , and the other is the first polar corpuscle that subsequently degenerates At the time of ovulation The secondary oocyte enters the second meiotic division and stops in metaphase 2. Under these conditions, the mature follicle that contains the secondary oocyte ruptures due to the effect of luteinizing hormone and the oocyte is expelled from the ovary if the secondary oocyte is fertilized by a sperm continues and completes the second meiotic division resulting in a zygote and a second polar corpuscle that then degenerates. However, if fertilization does not occur, the cell degenerates and begins a new sexual cycle in which a few oocytes are produced each month.
Primary oocytes are stimulated to become secondary oocytes. If you liked the video, give it a like and share it with your colleagues. Don't forget to subscribe to my channel.
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