If you’re trying for a baby, you’re probably already used to tracking your ovulation date. You know that you have the best chance to get pregnant when you plan around this important time. But have you ever wondered how your body gets ready to release an egg right on schedule? It starts in the follicles.
On average, a woman’s ovaries contain about 400,000 follicles at the start of puberty. Each follicle is a collection of cells including a single immature egg cell, or oocyte. Over a woman’s life, only a small fraction of these follicles will mature to the point where they can release their eggs. It’s this journey we’re interested in exploring!
The Stages of the Follicles
At any given time, many follicles will be in different stages of development, so that each month, one will be ready to release its egg during ovulation. Each follicle develops through several stages, growing and maturing along the way.
This is the stage all follicles start in before they begin to develop. At this point, the follicle consists of an oocyte and a flat layer of follicular, or granulosa, cells.
Each month, about 20 follicles are “recruited” or begin to develop, in response to levels of follicle-stimulating hormone (FSH). The flat granulosa cells become cuboidal, the oocyte and follicle grow to almost 0.1 mm, and a capsule called the zona pellucida forms around the oocyte. This capsule stays with the egg cell after ovulation and allows sperm to penetrate the egg.
The follicle recruits other cells to form layers called theca around itself. Capillaries develop between the layers to circulate blood. Sometime in this stage or the next, a fluid-filled cavity called an antrum forms. A follicle with an antrum is called an antral or Graafian follicle.
During this stage, the follicle is in its final configuration, but it continues to increase in size. The granulosa cells differentiate into four types that react differently to FSH. Some respond by producing estrogen.
Most follicles that began development together have died by this stage. A follicle that will ultimately release its egg can grow to as much as 20 mm. Follicles that are less responsive to FSH die off until there is one dominant follicle that moves on to the preovulatory stage.
The follicle has reached its maximum size. Once ovulation begins, it develops an opening, or stigma, and releases its egg. In about 1-2% of cases, two eggs are released, possibly resulting in fraternal twins. This chance increases with age.
What Happens During Ovulation?
During ovulation, the egg leaves the follicle and moves into the fallopian tube. As the egg is carried away, the follicle folds in on itself and becomes the corpus luteum. This structure releases hormones that thicken the uterine lining – the endometrium – and prepare the uterus for the fertilized egg. If the egg isn’t fertilized, of course, the endometrium and the empty follicle are excreted during menstruation.
This entire process has one crucial goal: to release a healthy egg ready for fertilization. Many follicles develop together, but in almost every case, only one will make it through the entire process to ovulation.
To have the best chance of getting pregnant, you want to understand as much as you can about the process of ovulation. The development of follicles is an important part of that process, but if you want to learn more, we’ve assembled a page that has answers to common questions plus a quiz to test your knowledge as you learn.