The corpus luteum

Normal corpus luteum function is determined by function in thefoUicular as well as the luteal phase. In the follicular phaseadequate follicle–stimulating hormone (FSH) and oestrogenstimulation are required for granulosa cell mitosis and luteinizinghormone (LH) receptor synthesis. An increase in LH pulse frequencymay also be necessary for adequate oestrogen synthesis and preparationof follicular cells for luteinization and secretion of progesterone.The nature of LH release may also influence luteal functionand pre–ovulatory progesterone may increase the responsivenessof the follicle to gonado-trophins. The thecal vascular networkbecomes extensive around pre-ovulatory follicles and may influenceaccess of gonadotrophins and/or the ability of follicular cellsto respond to them. Further vascularization is an early featureof luteinization. Angiogenic factors are found in luteal tissueand prostacyclin increases luteal blood flow. The corpus luteumconsists of large cells which secrete most of the progesteroneand have prostaglandin F₂ receptors and small cells which areresponsive to LH. In the luteal phase subnormal luteal functionhas not been associated with a reduction in LH con–centration,pulse frequency or amplitude. The number and occupancy of LHreceptors and adenylate cyclase activity do not appear to bealtered by a reduction in luteal function. Low density lipoproteinprovides the substrate and somatomedin C modulates among otherhormones‘ influences, progesterone production. In additionto the cAMP second messenger system phosphatidyl inositol metabolismmay also be associated with LH stimulation. Luteolysis is anactive process; prostaglandin F₂ or lipoxygenase products andpossibly an endogenous GnRH-like ovarian hormone may mediateit as also may oxytocin in some species.