... diagrams of each can be found in Appendix 'A'. Endocrine signalling is the release of signal molecules from their synthesis site to a distant region of the organism. This is usually achieved in animals by using circulatory systems (such as the bloodstream in mammals) to transport the signal molecules to their intended destination. With paracrine signalling it is only target cells within a close proximity of the synthesis site of the signalling molecule that are affected. It is paracrine signalling that accommodates the conduction of electrical impulses between nerve cells and from nerve cells to muscle cells. The release of signal molecules by cells that then react to the signal molecules that they release is autocrine signalling: this type of signalling is often used by growth factors and tumour cells. Contact dependent signalling allows adjacent cells to connect via signalling molecules in their plasma membranes. This type of signalling is ...

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