In endocrinology, target cells can refer to the cells where hormones have their effect. A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a little amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one cell to another. Endocrine hormone molecules are secreted (released) directly into the bloodstream or simply diffuse through the interstitial spaces to nearby target tissues.
Target cells are capable of responding to hormones because they bear receptors to which the hormone can bind. Receptors are protein molecules to which one or more specific kinds of signaling molecules may attach.
Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone. In this way, hormones only affect a limited number of cells even though they are transported in the bloodstream throughout the body.
- Endocrine action: the hormone is distributed in blood and binds to distant target cells.
- Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.
- Autocrine action: the hormone acts on the same cell that produced it.