Corpus luteum



The corpus luteum (Latin for "yellow body") is a 1 mm, temporary endocrine structure in animals. It develops from an ovarian follicle during the luteal phase of the estrous cycle, following the release of a mature egg from the follicle during ovulation. While the egg traverses the Fallopian tube into the uterus, the corpus luteum remains in the ovary.

Function
It is essential for establishing and maintaining pregnancy in females. In the ovary, the corpus luteum secretes estrogens and progesterone, which are steroid hormones responsible for the thickening of the endometrium and its development and maintenance, respectively. If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 14 days in humans). It then degenerates into a corpus albicans, which is a mass of fibrous scar tissue. The uterine lining sloughs off without progesterone and is expelled through the vagina (in humans and some great apes).In an estrus cycle the lining degenrates back to normal size.

If fertilized, however, the placenta secretes the hormone human chorionic gonadotropin (hCG) or a similar hormone in other species. This hormone signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining (endometrium) of the uterus, and providing an area rich in blood vessels in which the zygote(s) can develop. From this point on, the corpus luteum is called the corpus luteum graviditatis. The introduction of the hormone prostaglandin at this point causes the degeneration of the corpus luteum and the abortion of the fetus.

Structure
The corpus luteum is typically very large relative to the size of the ovary; in humans, the size of the structure ranges from under 2 cm to 6 cm in diameter. Its cells develop from the follicular cells surrounding the ovarian follicle. The granulosa cells become the outer granulosa lutein layer and secrete progesterone. Theca interna cells become the inner theca lutein layer which secretes estrogen.

The granulosa cells which are stimulated by FSH mainly secrete estrogen whereas Theca interna cells are stimulated by LH to produce androstenedione, which via a few steps, gives the granulosa the precursor for estrogen manufacturing. (Boron & Boulpeap Medical Physiology updated edition 2005 p1153-1155)

Reference

 * Bagnell, C. 2005. "Animal Reproduction". Rutgers University Department of Animal Sciences.

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