Oogenesis occurs by meiosis as well, but with some modifications. First of all, it would not be good for females to make millions of ova, since a female cannot carry millions of fetuses! Instead, no more than one ovum per month should be made. Because of this, females do not need to have constant mitosis of their germ cells occurring-- it is OK that once a germ cell is used it is not replenished. Secondly, the ova need to contain a lot of nutrients (to get the embryo through its first set of divisions), so these cells need to be big. Finally, it turns out that meiosis is only completed when producing an egg that is fertilized. Let's go through these ideas a bit more.
In an ovary are millions of oocytes. An oocyte is not the same thing as a germ cell. The germ cells develop in the fetus and before birth begin meiosis, and then freeze right at the start of the first meiotic division. These cells are stopped in prophase I of meiosis I, and are called primary oocytes. They lie within the ovaries surrounded by other cells, the follicular cells. The primary oocyte plus its follicular cells comprise the primary follicle.
Only when signalled by hormones, one single primary oocyte (of all those in both ovaries) picks up meiosis where it left off. It finishes its first meiotic division. It does this by dividing its genetic material appropriately for meiosis I, but then undergoing cytokinesis differently.
Cytokinesis is the physical separation of the daughter cells into two completely unattached cells. It occurs via a cleavage furrow. During mitosis and spermatogenesis, cytokinesis of the two daughter cells occurs right down the middle of the parent cell (symmetrical cytokinesis); but during oogenesis, cytokinesis occurs unevenly (asymmetrical cytokinesis). This is shown for you in the following two animations:
Why is it important that asymmetrical cytokinesis occurs in oogenesis? The primary oocyte is a very large cell containing many nutrients that will be important for the early mitotic divisions of the zygote (as it grows into an embryo). When the cell divides during meiosis, only one of the daughter cells can become the ovum-- we only want one ovum per month! So, the one cell that is destined to become the ovum (out of 4 potential daughter cells) is the only one that should get the nutrients. And it should get ALL of the nutrients! We want to start our zygote off well! This means that the other cells do not need much of the nutrient-rich cytoplasm. These tiny other cells are called polar bodies, and they do not last long. Polar bodies die pretty quickly. Just the ovum survives.
The large cell in the picture above is either the product of the first or second meiotic division that lives. If it is the product of the first meiotic division (where the parent cell was the primary oocyte) it is called the secondary oocyte. If it is the product of the second meiotic division (where the parent cell was the secondary oocyte) it is called the ovum. The terms secondary oocyte and ovum are NOT interchangeable-- they are different.
OK. Now let's get through all of meiosis. Females start off with millions of primary oocytes. Every month, one of these primary oocytes is hormonally stimulated to begin to enlarge and to complete the first meiotic division. (As it enlarges, the entire follicle enlarges-- this is shown in the ovulation web page... but the polar body is not included in the animation there). The oocyte is now a secondary oocyte, and its polar body will degenerate. The secondary oocyte is released from the ovary.
Once the secondary oocyte exits the ovary, it begins to travel down the uterine (fallopian) tube. If it does not encounter a spermatozoan, it never undergoes the second meiotic division. The second meiotic division only occurs when fertilization happens. That means that I, since I have never gotten pregnant, I have never had any of my oocytes finish meiosis! However, if the secondary oocyte comes in contact with a spermatozoan and fertilization begins, the secondary oocyte undergoes its second meiotic division to form the ovum (and another polar body). Now the ovum is ready to fuse with the spermatozoan.
© 2011 STCC Foundation Press