The Follicular Phase
The ovarian cycle can be divided into two phases:
- The follicular phase
- The luteal phase
It is during the follicular phase that an egg begins to develop and mature inside a fluid-filled sac within the ovary called the follicle. During its development, the follicle will increase in size from a microscopic dot to two centimeters or more at the time of ovulation.
Although only one egg is typically released each month, many eggs actually begin to develop. This development begins near the end of the previous menstrual cycle, even before the period has started. As these eggs continue to develop, fewer and fewer of them progress until only one remains. The remainder of the eggs that don't develop undergo a process known as atresia and are lost forever. The one egg that is destined to ovulate is usually selected as early as day five or six of the menstrual cycle. This follicle then continues to develop while the rest of the ones that had started to develop regress.
The follicle in which this egg develops is primarily responsible for the production of estradiol, the principal hormone responsible for the development of the lining of the uterus, the changes in the cervical mucus, and so on.
Egg and follicle development in the ovaries are under the control of a pituitary hormone known as follicle-stimulating hormone (FSH). Early in the follicular phase, FSH is produced by the pituitary in relatively large amounts, and this signals the eggs and their follicles to begin to develop. FSH continues to control the development of the egg(s) and follicle(s) throughout the follicular phase. When the one egg destined to ovulate has reached maturity and is ready to be ovulated, a second pituitary hormone known as luteinizing hormone (LH) triggers the release of the egg from the ovary. FSH primarily stimulates development and maturation of the follicles and eggs. LH primarily stimulates ovulation.