Paracrine signalling

Paracrine signaling is a form of cell signaling in which the target cell is close to ("para" = alongside of or next to, but this strict prefix definition is not meticulously followed here) the signal releasing cell.

The signal chemical is called the paracrine agent or paracrine hormone.

The distinction is sometimes made between paracrine and autocrine signaling. In both types of signalling, the signal is limited to other cells in the local area. However, paracrine signalling affects cells of a different type than the cell performing the secretion, while autocrine signaling affects cells of the same type.

Reasons for degradation
Sometimes, the reason that the effects are limited to a local area is because the signal chemical is broken down too quickly to be carried to other parts of the body.

Alternatively, the signal may only reach nearby cells for one of the following reasons:
 * (1) the nearby cells take up the signal at a very high rate, leaving little signal free to travel further.
 * (2) the signal gets stuck in the extracellular-matrix, or structure surrounding the signal releasing cell, and thus the signal is unable to travel far from the signal releasing cell.

Examples
Examples of paracrine signaling agents include growth factor and clotting factors. Growth factor signalling plays an important role in many aspects of development.

In mature organisms paracrine signaling functions include responses to allergens, repairs to damaged tissue, formation of scar tissue, and clotting.

Overproduction of some paracrine growth factors has been linked to the development of cancer.

Other examples of paracrine agents are somatostatin and histamine.

Autocrine and endocrine actions
Some paracrine agents also have autocrine, intracrine or endocrine actions as well.

For example, testosterone secreted from the testes acts in an endocrine manner to stimulate peripheral events (e.g. muscle growth), and in a paracrine manner to stimulate spermatogenesis in the adjacent seminiferous tubules.