<P> The corpus luteum develops from an ovarian follicle during the luteal phase of the menstrual cycle or oestrous 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 . </P> <P> 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 . </P> <P> 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 luteinize into large luteal cells (granulosal - lutein 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 protein kinase A (PKA) activity within the cell . Small luteal cells have LH receptors that regulate PKA activity within the cell . PKA actively phosphorylates steroidogenic acute regulatory protein (StAR) and translocator protein to transport cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane . </P> <P> 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 . </P>

The hormone that is responsible for the formation of the corpus luteum
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