Paracrine signalling

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

Local action
Some signalling 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 signalling agents. The local action of growth factor signalling plays an especially important role in the development of tissues. Also, retinoic acid, the active form of vitamin A, functions in a paracrine fashion to regulate gene expression during embryonic development in higher animals. In insects, Allatostatin controls growth though paracrine action on the corpora allata.

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

Autocrine and endocrine actions
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.