What's in the ovaries?
If you look inside an ovary, you will find many, many primordial follicles, as you discussed (probably) in lab. These are the primary oocytes, encapsulated by follicular cells.
A portion of a cross-section through an ovary has been schematized and is shown to the right. The blue circles represent a single layer of follicular cells, and the orange inside represents the primary oocyte arrested in prophase I of meiosis I (the nucleus is the dark brown dot).
Below we will look at the development of a single follicle in more detail, so I have drawn a black box around the one we will examine further.
How does ovulation occur?
As you have seen, the primary follicles can be influenced by gonadotropins. In fact, each month, one follicle from one of the ovaries is influenced to begin to develop due to the gonadotropins (specifically, FSH). When this happens, the follicle enlarges and matures. Then the secondary oocyte is released... this is called ovulation. I have tried to illustrate follicular development, maturation, and ovulation in this animation. The only things I left out of the animation are completion of meiosis I, and degeneration of the corpus luteum into the corpus albicans. I know that you saw the various stages of the follicles in lab (at least in the model), so some of this may be review for you.
The actual step of ovulation is not so complicated. The follicle fuses with the wall of the ovary... this is assisted by the high levels of LH that exist at the time. After oocyte release, the follicle reforms into a corpus luteum. The corpus luteum is then around to continue the production of estrogens started by the developing follicle.
© 2011 STCC Foundation Press