Hormones are chemical messengers that travel throughout the body coordinating
complex processes like growth, metabolism, and fertility. They can influence
the function of the immune system, and even alter behavior. Before birth, they
guide development of the brain and reproductive system. Hormones are the
reason why your arms are the same length, why you can turn food into fuel, and
why you changed from head to toe at puberty. It is thanks to these chemicals
that distant parts of the body communicate with one another during elaborate,
and important, events.
In response to a signal from the brain, hormones are secreted directly into the
blood by the glands that produce and store them. These glands make up what is
known as the endocrine system (endocrine means "secreting internally").
Chemicals that interfere with the function of hormones are therefore known as
The testes and ovaries, or "gonads", are perhaps the most familiar endocrine
glands. In males, testes produce sperm and secrete the male sex hormone
testosterone; in females, ovaries produce eggs and the female hormone estrogen.
It is these hormones that determine secondary sex characteristics like muscle
mass and facial hair. They also help to orchestrate sperm production,
menstruation and pregnancy. Other endocrine glands include the thyroid,
pancreatic islets, and adrenal glands. These are involved primarily in growth,
metabolism, and the "fight or flight" response to stress.
While all cells are exposed to hormones circulating in the bloodstream, not all
cells react. Only a hormone's "target" cells, which have receptors for that
hormone, will respond to its signal. When the hormone binds to its receptor,
it causes a biological response within the cell. If we liken a hormone to a
radio signal, then a receptor is the antenna. Without the antenna, no signal
would be received and no music would exit the radio. Signaling ends when the
circulating hormones are broken down and excreted by the body.
Hormone disrupting chemicals, or "endocrine disruptors," can act at any point
along this hormone signaling pathway. Some are hormone mimics: they bind the
receptor and send a false signal - one not commanded by the brain. Others
block the action of a natural hormone, keeping it from binding its receptor and
having an effect. Endocrine disruptors can also alter the amount of hormone
synthesized, how fast it is degraded, or the way in which a target cell
responds. All of these mechanisms of action can have the same effect:
disrupted development in an embryo, or altered functioning in an adult.
To learn more about how hormones work, visit Tulane's
comprehensive website: "Environmental Concepts Made Easy".