The traditional means by which progesterone exerts its effects within the human body, also known as the genomic mechanism, is through its interaction with the progesterone receptor, PR. The progesterone receptor, typically described as a nuclear transcription factor, acts on ribosomal RNA to regulate ribosomal transcription, resulting in the production of the corresponding proteins. There are two primary forms of progesterone receptors that are located within the body namely PR-A and PR-B. PR-A and PR-B share the same DNA binding domain but have differing amino acid sequences with PR-A having 164 more amino acids than PR-B. Some of the effects of progesterone on organs within the body through its binding with the progesterone receptors PR-A and PR-B include:
- Thickening of the uterine endometrium, getting it ready for the implantation of a fertilized ovum. Progesterone is released by the corpus luteum from the granulosa cells in the ovary for this purpose.
- Maintenance of pregnancy, helping to ensure that a successfully implanted fertilized ovum is carried by the mother until delivery. After the development of the placenta, one of its roles is to continue the production of progesterone for the duration of the pregnancy, making sure that serum progesterone levels are sufficient to maintain the fetus within the uterus.
- In the female breast, progesterone stimulates the growth in number and size of the alveoli and lobes during pregnancy. In addition, progesterone inhibits the production of breast milk throughout the duration of the pregnancy. As progesterone levels fall after delivery, milk production within the breasts is triggered.
- In the central nervous system, progesterone plays a role in the development of the myelin sheath surrounding the nerves. It has also been shown to play role in the neurological recovery from after a traumatic brain injury or a hypoxic-ischemic injury within the brain.
In addition to the genomic mechanism, progesterone can also exert its effects through non-genomic means. The non-genomic pathway involves the activation of intracellular signaling pathways, which occurs through ion channels as well as second messenger cascades. This pathway normally occurs in the central nervous system; however, it can also take place in other areas of the body.