The Ovulation Cycle, Egg Follicle Development and Hormones. Every month throughout her reproductive years, a woman’s ovulation cycle causes eggs to mature and leave the ovaries, after which either conception or menstruation occur. A description of ovarian cyst formation begins with the ovulation cycle and egg development.The Ovulation Cycle and Hormones
At the beginning of the ovulation cycle, a part of the brain called the hypothalamus releases GnRH (gonadotropin releasing hormone). GnRH triggers the pituitary gland that ovulation is ready to occur.
The pituitary gland releases two hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LH). These hormones trigger the beginning of follicle maturation. The follicle contains an egg, or ovum. When the egg matures it produces estrogen telling the pituitary gland that it is ready to be released from the ovary.
The pituitary gland releases an extra amount of luteinizing hormone; this release is called an LH surge. Within a day of the LH surge, the follicle containing the egg ruptures, and the egg is sent through the adjacent fallopian tube to the uterus.
A natural cyst called a corpus luteum forms over the ruptured tissue. The corpus luteum produces progesterone, which keeps the uterine wall thick, and ready to accept a fertilized egg. If the egg is fertilized, it attaches to the uterine wall, and the corpus luteum continues to produce the progesterone required for the early part of pregnancy, until the placenta is fully developed and takes over this role.
If the egg is not fertilized, the corpus luteum ceases progesterone production and dies off, leaving a corpus albicans, or small scar, in the ovary. As progesterone levels fall, the uterus sheds it’s lining, resulting in menstruation.
A woman’s ovaries contain a lifetime supply of oocytes, or immature eggs. Every month an oocyte develops into a mature egg. Each oocyte is encased in a sac called a follicle. A number of these sacs begin to develop at the beginning of each cycle, but normally only one “dominant” egg fully matures each month.
Primordial Follicle: The starting point for egg maturation, this structure contains an immature, or primary oocyte and a single layer of flat follicular cells. About fifteen to twenty primordial follicles begin to mature every month.
Primary Follicle: FSH and LH signal the beginning of the ovulation process. The maturing follicles become several layers thick, and the flat cells begin to develop into cuboidal or columnar cells (the cells cease to be flat and fill out). Yolk granules develop to nourish the oocyte, and the follicle begins to produce estrogen. Only a few maturing eggs will continue to the next stage.
Secondary Follicle: By the “secondary phase” several layers of cells, known as the zona granulosa, have developed. These cells secrete follicular fluid that builds up in pockets between the cells. One secondary follicle matures faster than the others, and, in most cases, will be the only one to enter the mature phase.
Secondary Vesicular Follicle: The pockets of fluid begin to merge into a larger pocket called the follicular antrum.
Mature (Pre-Ovulatory) Follicle: Also known as agraafian follicle. As the follicle matures, it becomes much larger, and is filled with a large amount of fluid. Maturation causes a rise in estrogen levels, signaling ovulation.
Corpus Luteum: After the follicle ruptures, a corpus luteum forms over the ruptured tissue. This “natural cyst” produces progesterone that keeps the uterine lining thick. If fertilization occurs, the corpus luteum will remain throughout the pregnancy.
Corpus albicans: A small scar that forms on the ovary after the corpus luteum dies away.