The ovary is made up of follicles. These tiny fluid-filled sacs each contain an immature egg (or oocyte). During the first two weeks of the menstrual cycle, hormones regulated by the hypothalamus and released by the pituitary gland trigger a few of these follicles to grow and the egg inside the follicles to mature. Eventually, one follicle becomes dominant. This is where the egg that will be released at ovulation comes from. The egg bursts from the follicle, leaving behind an empty shell of cells. Those cells go through a transformation after the egg is released, collapsing in on themselves and changing the hormones they secrete. This collapsed follicle is what becomes the corpus luteum.
The Corpus Luteum and Hormone Production
The corpus luteum is actually a temporary gland structure. It secretes the hormones estrogen and progesterone to prepare the body for the possibility of conception. These hormones help build up the lining of the uterus (to form a nice bed for the egg to implant into) and help maintain that lining. If conception does not take place, the corpus luteum begins to break down. This leads to a drop in progesterone and estrogen, which triggers menstruation. The reproductive cycle starts over again. When the corpus luteum is no longer active, it turns into the corpus albicans.
The Corpus Luteum in the Menstrual Cycle
There are two primary phases of the menstrual cycle:
Follicular phase: A select number of follicles in the ovary mature until one releases an egg.Luteal phase: Post-ovulation, the body prepares the womb to accept a fertilized egg or embryo.
Just before ovulation, there is a surge in the luteinizing hormone (LH). This hormone is vital both for ovulation and what happens just after ovulation. Before ovulation, LH triggers the follicle and the developing egg inside to speed up growth and development. LH also triggers enzymes to begin breaking down the outer walls of the follicle. Eventually, finally, the egg reaches full maturity, and the follicle wall breaks open. This releases the mature egg. Once the egg is released, LH continues to impact the cellular structure of the former follicle. Before ovulation, granulosa and theca cells in the follicle produce estrogen. However, after ovulation, LH triggers these cells to transform. They begin to release the hormone progesterone. Progesterone plays an important role in the luteal phase. First, progesterone signals the pituitary and hypothalamus glands in the brain to slow down the production of the hormones follicle-stimulating hormone (FSH), LH, and gonadotropin-releasing hormone (GnRH). This prevents additional follicles in the ovaries from developing and ovulating. Second, progesterone prepares the endometrium, or the uterine lining. Progesterone triggers the endometrium to secrete proteins. These proteins maintain the endometrium and create a nourishing environment for a fertilized egg. Progesterone also signals breast tissue to prepare to produce milk. This is why breasts can be tender after ovulation and before menstruation.
The Corpus Luteum and Conception
If an egg is fertilized and an embryo implants itself into the uterine lining, the embryo forms a very early placenta. This early placenta releases the hormone human chorionic gonadotropin (hCG, the hormone pregnancy tests detect). The presence of hCG signals the corpus luteum to continue secreting progesterone. The progesterone prevents the endometrium from being expelled and prevents further ovulation. If pregnancy doesn’t occur, the corpus luteum slowly disintegrates. This happens about 10 to 12 days after ovulation, or two to three days before menstruation starts. As the corpus luteum breaks down, the cells in the corpus luteum stop producing as much progesterone. Eventually, the drop in progesterone leads the endometrium to break down. Menstruation begins. Also, the drop in progesterone signals the pituitary and hypothalamus glands to increase the production of FSH, LH, and GnRH. This restarts the menstrual cycle, and the follicular phase starts anew.
The Corpus Albicans
When the corpus luteum breaks down, it leaves scar tissue behind. This scar tissue—which is made up of cartilage—is known as the corpus albicans. While the corpus luteum is yellow in color (corpus luteum means “yellow body” in Latin), the corpus albicans is white (corpus albicans means “white body” in Latin). The corpus albicans remains on the ovary for a few months. It doesn’t appear to serve a specific purpose. Unlike the corpus luteum, the corpus albicans doesn’t act like a gland and doesn’t secrete hormones. Eventually, immune cells known as macrophages act like an internal clean-up crew and engulf most of the cells that once made up the corpus albicans. In very rare circumstances, the corpus albicans remains and scar tissue builds up around the ovary. Not much is understood about why this happens because it is so rare.
Corpus Luteum Cysts
The corpus luteum is formed from the open follicle that released an egg during ovulation. Sometimes, the opening seals back up. Fluid fills the cavity and forms a cyst. This kind of cyst is known as a functional cyst. They are usually benign (not cancerous) and go away on their own. Usually, corpus luteum cysts are painless and harmless. If you’re going through fertility treatments, an ultrasound conducted at the beginning of your cycle may spot a corpus luteum cyst. Depending on the size of the cyst, the doctor may delay your treatment cycle or drain the cyst. If you tend to develop corpus luteum cysts, your fertility doctor may put you on birth control the cycle before treatment. This prevents ovulation in the month before treatment, which in turn prevents the potential for a cyst. Some people find out they have one of these cysts during an early pregnancy ultrasound. In these cases, the cyst will usually resolve on its own by the second trimester of pregnancy. If the cyst is unusually large, growing, or painful, it may need to be surgically drained or removed.
Corpus Luteum Cyst Pain
Sometimes, a corpus luteum cyst can cause mild discomfort. It may come as a short, sharp twinge of pain on one side. Other times it may cause a dull, more constant pain, also focused on one side of your pelvic area. If you get pregnant, this pain may persist during the early weeks of your pregnancy. If you don’t get pregnant, the pain will likely go away a few days after your period starts. As long as the pain is not severe and not accompanied by other worrisome symptoms (like vomiting or fever), there is probably nothing to worry about. In rare cases, a corpus luteum cyst can cause severe pain. In very rare cases, if the cyst grows especially large, it can cause the ovary to twist. This may lead to ovarian torsion. Surgical intervention would be required.
Corpus Luteum Deficiency or Defect
In some cases, the corpus luteum doesn’t produce enough progesterone. This can lead to abnormal spotting. Low levels of progesterone in early pregnancy may lead to what looks like a light period, making you think you’re not pregnant when you really are. When progesterone levels are low after ovulation, this may be called a corpus luteum defect or deficiency. More commonly, it’s referred to as a luteal phase defect. A corpus luteum deficiency may increase the risk of early miscarriage. Treatment may include progesterone supplementation or the use of fertility drugs, such as Clomid (clomiphene) or hCG injections. The theory is that boosting the hormones leading up to ovulation with fertility drugs may help produce a stronger corpus luteum. However, there’s no current evidence that these treatments help. Also, hCG increases the risk of developing ovarian hyperstimulation syndrome (OHSS). Proper diagnosis of a corpus luteum defect is also controversial and unclear. Based on the current evidence, the American Society for Reproductive Medicine doesn’t recognize luteal phase defect as a specific cause of infertility.
