Normal Ovarian Function
The ovaries are two small organs, about the size of your thumb, that are located in the female pelvis. They are attached to the uterus, one on each side, near the opening of the fallopian tube. The ovaries contain the female gamete cell, called the oocyte. In non medical terms, the oocyte is called the “egg”.
The ovaries are filled with follicles. Follicles are fluid-filled structures in which the oocyte (also called egg) grows to maturity. Current knowledge indicates that females are born with their entire lifetime supply of gametes. At birth, the normal female ovary contains about 1-2 million/oocytes (eggs). Females are not capable of making new eggs, and in fact, there is a continuous decline in the total number of eggs each month. By the time a girl enters puberty, only about 25% of her lifetime total egg pool remains, around 300,000. Over the next 30-40 years of a female's reproductive life, the entire egg supply will be depleted. Although no one can know with absolute certainty the number of eggs remaining within the ovaries at any given time, most women begin to experience a significant decrease in fertility (the ability to conceive a child) around the age of 37. At the time of menopause, virtually no eggs remain.
The large supplies of eggs within each ovary are immature, or primordial, and must undergo growth and maturation each month. The eggs are stored within follicles in the ovary. Within a woman's lifespan, large numbers of follicles and oocytes will be recruited to begin the growth and maturation process. The large majority, however, will not reach full maturity. Most will die off in a process called atresia. Thus, only about 300-500 of these eggs will mature over a women's life span.
The maturation of eggs typically takes about 14 days and can be divided into 2 distinct periods. During the initial period, many eggs, as many as 1000, begin to develop and mature. The second phase of development requires gonadal hormone stimulation to stimulate further development. However, even though hundreds of eggs have begun to mature, most often only one egg will become dominant during each menstrual cycle, and reach its' fully mature state, capable of ovulation and fertilization. The remaining eggs/follicles will wither and die. Pre-pubertal girls do not produce the gonadal hormones that are necessary for the second phase of development, so the many eggs that started to mature will simply wither away. The large number of eggs that are used each month account for the steady decline in the female's total egg pool that occurs from birth to menopause.
In post-pubertal females, the dominant egg continues to develop, relying on hormones for growth and stimulation. When the egg becomes fully mature, the follicle surrounding the egg bursts, and releases a mature egg which travels through the fallopian tube toward the uterus. The egg is capable of being fertilized for a short period, about 48 hours. If the egg is not fertilized during this time, it will die, and in another week or so, a new cycle of egg maturation will begin.
This cyclic process of development continues through out a female's life until most or all of the eggs are depleted. This is the period of life known as menopause. This occurs sometime in the 4th or 5th decade of life, with the average age in the US being 51. Depletion of the egg pool anytime prior to age 40 is referred to as premature ovarian failure. Any female who receives treatment with drugs that damage the ovarian follicles is at risk to develop premature ovarian failure--even many years after the treatment has ended. The majority of young girls treated with chemotherapy will retain fertility initially, but may be at risk to develop premature ovarian failure. This knowledge may be important to consider for family planning.