Corpus Luteum - Development and Structure

Development and Structure

The corpus luteum develops from an ovarian follicle during the luteal phase of the menstrual cycle or estrous cycle, following the release of a secondary oocyte from the follicle during ovulation. The follicle first forms a corpus hemorrhagicum before it becomes a corpus luteum, but the term refers to the visible collection of blood left after rupture of the follicle that secretes progesterone. While the oocyte (later the zygote if fertilization occurs) traverses the Fallopian tube into the uterus, the corpus luteum remains in the ovary.

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 5 cm in diameter.

Its cells develop from the follicular cells surrounding the ovarian follicle. The follicular theca cells luteinize into small luteal cells, (thecal-lutein cells) and follicular granulosa cells (granulosal-lutein cells), luteinize into large luteal cells forming the corpus luteum. Progesterone is synthesized from cholesterol by both the large and small luteal cells upon luteal maturation. Cholesterol-LDL complexes bind to receptors on the plasma membrane of luteal cells and are internalized. Cholesterol is released and stored within the cell as cholesterol ester. LDL is recycled for further cholesterol transport. Large luteal cells produce more progesterone due to uninhibited/basal levels of PKA activity within the cell. Small luteal cells have LH receptors that regulate PKA activity within the cell. PKA actively phosphorylates StAR (steroidogenic acute regulatory protein) and PBR (peripheral benzodiazepine receptors) to transport cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane.

The development of the corpus luteum is accompanied by an increase in the level of the steroidogenic enzyme P450scc that converts cholesterol to pregnenolone in the mitochondria. Pregnenolone is then converted to progesterone that is secreted out of the cell and into the blood stream. During the bovine estrous cycle, plasma levels of progesterone increase in parallel to the levels of P450scc and its electron donor adrenodoxin, indicating that progesterone secretion is a result of enhanced expression of P450scc in the corpus luteum.

The mitochondrial P450 system electron transport chain including adrenodoxin reductase and adrenodoxin has been shown to leak electrons leading to the formation of superoxide radical. Apparently to cope with the radicals produced by this system and by enhanced mitochondrial metabolism, the levels of antioxidant enzymes catalase and superoxide dismutase also increase in parallel with the enhanced steroidogenesis in the corpus luteum.

Like the previous theca cells, the theca lutein cells lack the aromatase enzyme that is necessary to produce estrogen, so they can only perform steroidogenesis until formation of androgens. The granulosa lutein cells do have aromatase, and use it to produce estrogens, using the androgens previously synthesized by the theca lutein cells, as the granulosa lutein cells in themselves do not have the 17α-hydroxylase or 17,20 lyase to produce androgens.

Once the corpus luteum regressed the remnant is known as corpus albicans.