Paracrine signalling

Paracrine signaling is a form of cell signaling in which the target cell is near ("para" = near) the signal-releasing cell.

A distinction is sometimes made between paracrine and autocrine signaling. Both affect neighboring cells, but whereas autocrine signaling occurs among the same cell, paracrine signaling affects other cells. Two neurons would be an example of a paracrine signal.

Local action
Some signaling molecules degrade very quickly, limiting the scope of their effectiveness to the immediate surroundings. Others affect only nearby cells because they are taken up quickly, leaving few to travel further, or because their movement is hindered by the extracellular-matrix.

Examples
Growth factor and clotting factors are paracrine signaling agents. The local action of growth factor signaling plays an especially important role in the development of tissues. In insects, Allatostatin controls growth though paracrine action on the corpora allata.

In mature organisms, paracrine signaling is involved in responses to allergens, tissue repair, the formation of scar tissue, and blood clotting.

The overproduction of certain paracrine growth factors has been implicated in the pathology of cancer.

Somatostatin and histamine are paracrine agents.

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

For example, testosterone secreted from the testes acts as an endocrine agent to stimulate peripheral events, such as muscle growth, and as a paracrine agent to stimulate spermatogenesis in the adjacent seminiferous tubules.