Oogenesis

Need help preparing for the Biology section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Oogenesis of the Reproductive System. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Oogenesis is the differentiation of egg cells or ovum into competent cells that can develop further once fertilization occurs. In humans, this process begins during gestation before the female has been born from the mother. One stem cell called an oogonium will undergo mitosis and form two identical daughter cells: Another oogonium and a primary oocyte. This entire process is known as oocytogenesis.
The Primary Oocyte
After the second oogonium and the primary oocyte are formed, the second oogonium will repeat the process of forming new daughter cells in order to replenish the stem cell population continuously. The primary oocyte, however, will undergo meiosis and will eventually form the ovum. It is important to note that the process of forming an ovum does not happen all at once. When the primary oocyte starts meiosis I, it will not complete it right away. Instead, it will halt the process during prophase I. In this way, when the female is born, she will have several primary oocytes that have halted development at prophase I of meiosis I.
Also, the process of the female forming a primary oocyte and oogonium, and the oogonium renewing itself only happens during gestation. Thus, once the female is born, the number of primary oocytes she has will not increase. In fact, the number decreases as the female grows due to the natural death cycle of the cells. Only when the female enters puberty will the primary oocytes finish meiosis I. In other words, the primary oocytes will complete meiosis I beginning at menarche or the first menstrual cycle. Every subsequent menstrual cycle once every month or about 28 days, more primary oocytes will complete meiosis I.
The Secondary Oocyte
The primary oocyte is diploid throughout meiosis I, and when the homologous chromosomes eventually separate, two haploid cells are formed. Note, however, that unlike the oogonium, when the primary oocyte undergoes meiosis I, two identical daughter cells are not formed. Instead, because of an unequal division of the cytoplasm, one substantially smaller cell is formed called the polar body.
The polar body is essentially a nucleus with DNA and will be discarded. The other cell that is formed is the secondary oocyte. The secondary oocyte will retain a majority of the cytoplasm and will proceed to meiosis II. However, once it does so, it will halt once again during metaphase II. This same process will repeat every menstrual cycle. The primary oocytes will complete meiosis I to form secondary oocytes, and the secondary oocytes will begin meiosis II and halt development during metaphase II.
Fertilization
For the secondary oocytes to complete meiosis II, fertilization must occur. If fertilization does not happen, the secondary oocytes will simply be shed. If a sperm fertilizes the secondary oocyte, then meiosis II will finish, forming another polar body and a daughter cell. Like the previous polar body, this one consists of a nucleus with DNA and minimal cytoplasm and will be discarded. The daughter cell is called the ovum and will be able to fuse with the sperm cell to form a zygote.
Differences Between Spermatogenesis & Oogenesis
There are several differences between spermatogenesis & oogenesis to understand. First, note that the ovum is the largest diameter cell in the human body, and significantly larger than the sperm cell. Also, the sperm has a head and a flagellum, which is important for motility. At the head of the sperm is a specialized organelle called an acrosome. The acrosome contains special enzymes for breaking down the outer membrane of the ovum, which is essential because the outer membrane of the ovum, called the zona pellucida, is not easily penetrated. The sperm, by way of the acrosome, can initiate the acrosomal reaction in which enzymes are released that break down the zona pellucida.
Another difference between spermatogenesis & oogenesis is in the production of sperm versus eggs. Spermatogenesis is a constant process of sperm production that happens from puberty until the male dies. For females, the process starts before birth and stops at birth. Whatever oogonia and primary oocytes the female has at birth will be all she has, and no additional oogonia or primary oocytes will be generated.
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[@laurastrong7585]

Thank you for your whole biology series of videos. You've been a huge help

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You're very welcome!

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I just finished the biology series and I am excited to watch the other playlists you have made. These videos are incredible and such helpful review.