The corpus luteum is a temporary cluster of cells that forms in a female’s ovary after an egg is released during ovulation. It serves as an endocrine structure, producing and secreting hormones directly into the bloodstream. This mass plays a significant role in fertility and the early stages of pregnancy by regulating the uterine environment. It forms and regresses with each menstrual cycle unless pregnancy occurs.
Formation and Development
The corpus luteum develops from the remnants of the ovarian follicle, the follicle that released the egg during ovulation. After the egg is discharged, the follicular walls collapse, and the remaining granulosa and theca cells undergo a transformation process called luteinization. These cells proliferate, differentiate, and accumulate lipids and a yellow pigment, giving the corpus luteum its characteristic color.
This newly formed structure is located within the ovary where the dominant follicle once resided. The luteinization process, triggered by a surge in luteinizing hormone (LH), results in the formation of two main cell types: large luteal cells derived from granulosa cells and small luteal cells from theca cells. These cells mark the beginning of the luteal phase of the menstrual cycle.
Hormonal Role and Function
The primary function of the corpus luteum is the production of hormones, particularly progesterone. Progesterone is a steroid hormone that prepares the uterine lining, known as the endometrium, for potential implantation of a fertilized egg. It thickens the endometrium, increases blood flow to the uterus, and stimulates the production of glandular secretions, creating a nourishing and receptive environment for an embryo.
The corpus luteum also produces estradiol, a type of estrogen, and inhibin A. Estradiol contributes to the regulation of the menstrual cycle and, along with progesterone, helps prepare the uterus for pregnancy. Inhibin A helps to regulate the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland, which influences the development of new follicles. These hormone levels peak around six days after ovulation.
Its Fate and Significance
The fate of the corpus luteum depends on whether fertilization and implantation occur. If pregnancy does not happen, the corpus luteum regresses through a process called luteolysis around 10 to 14 days after ovulation. This regression involves a functional decline in progesterone production, followed by structural breakdown, leading to a drop in progesterone and estrogen levels. The decrease in these hormones causes the uterine lining to shed, resulting in menstruation, and the corpus luteum eventually transforms into a fibrous scar tissue called the corpus albicans.
If fertilization and implantation of an embryo occur, the developing embryo begins to produce human chorionic gonadotropin (hCG) around nine days post-fertilization. This hCG acts as a signal to “rescue” the corpus luteum, preventing its regression. The rescued corpus luteum, now termed the corpus luteum graviditatis, continues to produce progesterone and other hormones, maintaining the uterine lining and supporting the early pregnancy until the placenta develops and takes over hormone production around 8 to 12 weeks of gestation.
Common Related Conditions
Sometimes, the corpus luteum can fill with fluid or blood, forming a corpus luteum cyst. These cysts are common and benign, often resolving on their own within a few menstrual cycles or during the second trimester of pregnancy. While often asymptomatic, symptoms can include pelvic pain, ranging from mild discomfort to sharp pain, especially during ovulation or menstruation. Other symptoms may include irregular menstrual cycles, bloating, or breast tenderness. In rare cases, a cyst may rupture, causing sudden, severe pain, or lead to ovarian torsion, where the ovary twists, requiring immediate medical attention.
Another condition is a luteal phase defect (LPD), which occurs when the corpus luteum does not produce sufficient progesterone or the uterine lining does not respond adequately to the hormone. This deficiency can hinder the uterine lining’s ability to support embryo implantation and growth. LPD may manifest as a shortened luteal phase, less than 9 to 10 days, or spotting between periods. While LPD is associated with recurrent miscarriages and infertility, its direct causal link to infertility is still under investigation.