Hormones

A hormone is a chemical produced in the body that interacts with a receptor in a target tissue to cause a change in the function of that tissue. There are various types of hormones in the body and include the exocrine (glands with ducts that release hormones for local action), and endocrine hormones.

Hormones control a number of essential functions in the body, including growth and development. For example, thyroxine, produced in the thyroid gland influences metabolic rate. Insulin and glucagon are produced in the pancreas and regulate glucose levels in the blood.

Of the endocrine hormones we have learned that there are those that follow the classical pattern of being released into the circulation to induce effects at distant target sites. An example of a classical endocrine hormone would be growth hormone (GH) that is produced by the pituitary gland, released into the general circulation and transported throughout the body to cause tissues to grow.

We now realize that there are also hormones produced in endocrine glands that are released into the circulation to signal to other cells in the originating gland how to behave. These hormones are called paracrine hormones. Still others act directly on the cells that produced the hormone and are called apocrine hormones. In all cases endocrine hormones function as signaling molecules in a manner parallel to the nervous system. Whereas the nervous system functions to communicate quickly, the endocrine system employs hormones to tell cells and tissues throughout the body how to behave over more prolonged periods of time. Examples include development or regulation of the menstrual cycle in women or the production of sperm in men.

Protein hormones bind with receptors on the surface of cells. Interaction with cell surface receptors triggers a cascade of post receptor chemical reactions that ultimately result in interaction with the genetic machinery of the cell to stimulate the appropriate response.

Steroid hormones are small molecules that are fat-soluble and thus easily diffuse through cell walls to bind with receptors inside of cells. Once bound with cells these receptors then form complexes, which enter the cell nucleus where the complex then binds with the genetic machinery of the cell to stimulate a response in the cell. These interactions a tremendously complex involving in some cases slightly different receptors of the same family as in the case of the estrogen receptor and the cooperation of coactivator and repressor molecules. Complicating the process even further is the recognition that some growth factors such as epidermal growth factor interact with the same genetic regions as the steroid receptor complex to stimulate the same response. Therefore, it is possible that a chemical can interact like a steroid without having any functional similarity to the hormone.