Corpus Luteum
The corpus luteum is a temporary endocrine gland in the ovary necessary for the production of progesterone to prepare the endometrium (lining of the uterus) for implantation of a fertilized egg (blastocyst) and sustain pregnancy. The corpus luteum maintains a healthy and nutritional endometrial environment during embryonic development until the placenta is developed enough to produce progesterone in sufficient quantities to take over the maintenance of pregnancy. If the amount of progesterone secreted by the corpus luteum is insufficient or continues for too short a period, the developing fetus is unlikely to survive. This is called a luteal phase defect (LPD).
The corpus luteum—literally translated as yellow body—is actually the remnant of a follicle, a protective group of special cells which surrounds each individual oocyte housed within the ovaries. The metamorphosis from follicle to corpus luteum takes place immediately following the ovulation stage of a woman's sexual cycle but begins about 14 days earlier during the follicular phase. In the follicular phase, luteinizing hormones (LH) and follicle stimulating hormones (FSH) released by the pituitary gland cause follicles, housed within the ovaries, to secrete estrogen. Several follicles begin to grow quickly, secreting increasing amounts of estrogen. One follicle, however, will grow more quickly than the rest, starving them of estrogen and maturing in approximately 14 days. It is from this follicle--the Graafian follicle--that the mature egg bursts forth and moves down the fallopian tube to uterus. Once the egg is released, the empty follicle--previously containing follicular fluids--now fills with clotted blood. The follicle undergoes luteinization, changing dramatically in shape and color, and luteal cells begin to form.
This transformation of cells is called differentiation. The follicle has now become the corpus luteum which consists of many blood vessels (70-80% of the blood which flows through the ovaries flows to the corpus luteum), and both small and large cells. Small cells contain receptors for LH, large cells do not. This means the large cells cannot be stimulated to secrete progesterone, even though they actually produce most of the progesterone secreted by the corpus luteum. The corpus luteum reaches peak performance within four days of development, continuing to grow for another four or five days. It stores cholesterol, secretes the steroid hormones estrogen and progesterone, and produces the peptide hormones oxytocin (which stimulates uterine muscle contractions and the production of breast milk), IGF-1 (a growth stimulator), and relaxin (which allows ligaments to relax during childbirth). Because estrogen causes uterine contractions, the amount of progesterone secreted by the corpus luteum must be great enough to completely override this effects of estrogen. The progesterone quiets contractions, allowing implantation of the blastocyst. Once implanted, the blastocyst begins to secrete a special hormone, chorionic gonadotropin, a signal that it is now nestled in the endometrium and ready to grow. This signal stimulates the corpus luteum to continue its production and secretion of progesterone.
When an egg remains unfertilized, the corpus luteum regresses, stops producing hormones, shrivels up, and decomposes. This process takes approxiately 12 to 16 days. The termination of luteal function and subsequent lowering of hormone levels once again triggers the release of LH and FSH, which trigger menstruation, marking the beginning of the next sexual cycle.
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