Learning

Endocrine System : Target Cells
  1. Hormones and Target Cells
  2. Finding a Partner

Hormones and Target Cells
Hormones are powerful messenger molecules that control essential body functions by carrying messages from endocrine glands to target cells and tissues. Some hormonal actions cause short-term changes, such as a faster heartbeat or sweaty palms. Others dictate long-term development, such as bone and muscle growth. Still other hormones control continual body functions, such as maintaining body fluids, heart rate and metabolism.

Hormones have many unique features and interact with target cells in specific ways.
  • Natural hormones are potent. That is, very small amounts cause a response.
  • The same hormone can be made by different glands. For instance, both the ovaries and the adrenal gland release estrogens.
  • A hormone can have different effects depending on the target cell's location, the gender of the individual and the species. For instance, estrogen released from a women's ovaries prepares the uterus for monthly mentrual cycles, while the same molecule binds with bone cells to maintain bone strength.
  • Hormones influence gene expression by binding DNA in a cell's nucleus. That is, hormones turn on certain genes that are preprogrammed to make specific proteins. These proteins cause a cell to respond in a new way (grow, secrete, metabolize, etc.).






Finding A Partner
The endocrine system is a complex communication network made up of specialized cells, glands and hormones. The glands release hormones into the blood or the fluid surrounding cells in response to stimuli from inside and outside the body.

Once released, hormones travel throughout the body looking for target cells that contain matching receptors. The hormone binds with the receptor, something like how a key fits a lock to unlock a door. Hormones, like keys, need to have a compatible receptor, or lock, in order to work. In the same way that a skeleton key cannot open a car door, a male sex hormone cannot produce masculine features if the target cell does not have receptors, or locks, that can read the hormone, or accept the key.

The protein receptor, depending on the type of hormone and its specific message, carries out the messenger's instructions by either altering the cell's existing proteins or turning on genes that will build a new protein. Both actions create a wide array of body responses.

Estrogen Receptor BindingFor instance, steroid hormones, like the sex hormone groups estrogens and androgens, seek out specific target cells and bind to receptor proteins located inside the nucleus of the cell, as shown below. This lock and key binding triggers the cell's DNA to start building certain proteins, such as another hormone or an enzyme.

CAPTION: One way steroid hormones function is by binding to specific hormone receptors inside a cell. (click image for animation) CREDIT: Tulane/Xavier Center for Bioenvironmental Research

Each hormone-receptor unit produces different cellular and body responses because each unit turns on distinct genes that code for a specific protein. Different proteins, in turn, cause unique biological responses: estrogens can stimulate uterine growth and androgens can stimulate muscle growth.