Sperm may follow a trail of progesterone released by the egg
Eggs release progesterone, and sperm carry calcium channels called CatSper that respond to it, so researchers suspect sperm use the hormone as a homing signal. That has prompted an idea for male contraception: block the progesterone binding site on those channels. It is one of many surprising jobs for the hormone of pregnancy.
Progesterone is the body's main progestogen, a steroid sex hormone central to the menstrual cycle, pregnancy and embryonic development. It is also a building block for other steroids, including sex hormones and corticosteroids, and it acts in the brain as a neurosteroid; some of its breakdown products boost the calming GABA-A receptor. As a medicine it was first prescribed in 1934 and is now used in contraception and hormone therapies.
Each cycle it turns the lining of the uterus into a secretory layer ready for implantation, while thickening cervical mucus so it becomes hard for sperm to cross. If no pregnancy follows, levels fall and the lining is shed: ordinary menstrual bleeding is, in effect, progesterone withdrawal. During pregnancy the hormone appears to damp the mother's immune response so the embryo is accepted, and it relaxes the muscle of the uterus. A drop in progesterone is thought to be one step towards labour, and its fall after delivery helps trigger milk production, which it holds back during pregnancy.
Its effects reach well beyond reproduction. Progesterone binds the mineralocorticoid receptor more strongly than aldosterone itself, blocking that hormone's salt-retaining action so the body sheds sodium and fluid. When levels drop, aldosterone rebounds and fluid is briefly retained. In the fetus, placental progesterone can be converted into the potent androgen DHT, including by a so-called backdoor route, which makes it a contributor to male sexual development.
Breast tissue shows how it teams up with oestrogen, which switches on progesterone receptors. Together with prolactin, progesterone drives the growth of milk-producing lobules in pregnancy; mice lacking a downstream signal called RANKL fail to develop them at all. Its role in breast cancer remains unsettled: synthetic progestins combined with oestrogen in menopausal therapy have raised risk, while natural oral progesterone has been linked with less, possibly because it produces only low hormone levels.
Source: Progesterone
