A hormone is a chemical that is produced by endocrine glands. These chemical messengers course through the bloodstream and enter the body's tissue, where they activate genetic switches that trigger physical and mental development, sexual reproduction, emotions, and general health. For example, the growth hormone produced by the pituitary gland controls the growth of long bones in the body. In women the ovaries produce estrogen, which is responsible for the development of breasts and controlling the menstrual cycle.
Estrogen belongs to the steroid family of hormones, which is derived from cholesterol; specific chemical messages from the brain dictate what type of steroid hormone is produced. In women, the brain sends follicle stimulating hormone to the ovaries to tell them to produce estrogen, while luteinizing hormone is sent during the second half of the menstrual cycle to signal the body to produce progesterone, which balances and slows estrogen's effects. Cells in the breast, uterus and brain respond to estrogen, while muscle cells do not.
Estrogen is largely recognized for its influences on a woman's reproductive functions. But it may also boost a variety of brain functions, including memory, with important implications for aging women, particularly those prone to Alzheimer's disease, because their estrogen supply tapers off during menopause. Estrogen's role in memory may stem from its effect on the tiny knobs that protrude from a neuron's message receiving ends, known as dendrites, where important connections are made. Recent research shows that in mature animals the hormone can foster new connections between neurons to strengthen the communication web, improving the way the brain processes information.
Scientists have also found that a group of neurons often damaged in Alzheimer's have receptors for both estrogen and a neuron-protecting chemical known as a nerve growth factor, suggesting an interaction that possibly prevents Alzheimer's. Estrogen may also affect mood and brain blood flow.