How Female Reproduction System Works - Getting the Great Egg Race Started

Getting the follicle-to-egg marathon running is a bit like a relay race in and of itself.

Until the primordial follicles become primary and then secondary follicles, the race primarily takes place inside the ovary on its own. But when it comes time for a group of secondary follicles to sprint to becoming the ovulated egg, hormones from outside the ovary play a more important role.

The hypothalamus gland, which is located in the brain, starts the relay race off by releasing gonadotropin-releasing hormone, or GnRH. This happens on the first day of your period.

This hormone signals the pituitary gland to generate two more hormones, follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH. 

How Female Reproduction System Works - The Great Egg Race: How Follicles Mature to an Ovulated Egg

Those millions of immature eggs that were contained in your ovaries when you were young are contained in what are known as primordial follicles, and many of them die off as time passes. (Read more about the connection between age and fertility here.) When it came time for your first menstrual cycle, the first group of some of the surviving dormant follicles began to "wake up".

As they wake up, there is a sort of competition between them as they grow. Each month some of these primordial follicles stop growing, with only the best of the group moving onto the next month of growth. Of this group, only a select group will go onto becoming primary follicles, and then onto becoming what is known as secondary follicles. It's a bit like a follicle marathon to see who will get to become the winning egg.