Cells constantly communicate with each other by direct contact (juxtacrine signaling), over short distances (paracrine signaling), and long distances (endocrine signaling).
Direct communication between adjacent cells underlies tissue organization. Contact signals use a simple sugar (oligosaccharide), protein, or lipid component of the cell membrane to message.
Nerve cells are juxtacrine signalers: translating internal electric pulses into chemical communiqués that are passed cell-to-cell over gap junctions. These conversations are overheard by nearby cells.
Brain stem cells monitor activity, listening in on nearby nerve and glia cells. This chemical eavesdropping regulates new cell growth.
Paracrine signals are used for cell growth and repair, including blood clotting and scar tissue formation. Allergen responses are initiated by paracrine signals.
Insects and crustaceans control growth through paracrine signals: allatostatins, which are neuropeptide hormones. Retinoic acid, the active form of vitamin A, is an allatostatin which regulates gene expression during embryonic development in higher animals.
Short-distance statements are typically designed to degrade quickly, as diffusion could lead to disruption rather than appropriate action.
Long-distance communication is big business in a multicellular organism; the medium to living large.
Neurons communicate over long distances via long fibers – axons – that send electrical signals. These cells provide the intelligence network known as the nervous system.
Other cells have cellular extensions – filopodia – which are used for sensing, cell-to-cell interactions, and migration. Cytonemes are long, thin filipodia that are specialized for exchanging signaling proteins between cells over long distances. Filopodia form a dedicated network by directly extending from cells that receive signaling proteins to cells that make them. Physical connection between cytonemes completes a private communication channel between cells.
The endocrine system is the glandular network. Glands produce hormones that traverse the bloodstream with regulation instructions.
Hormone is a general term for any endocrine signaling compound. All multicellular organisms produce hormones. Plant hormones are termed phytohormones. Animals typically transport hormones in the blood.
A hormone hits home on a certain cell receptor by binding to its receptor protein. This reception is then translated into cell-speak by a 2nd messenger. The cell then decides on an internal response.
In contrast to the glands of the endocrine system, exocrine glands are doers, not talkers: secreting their products exclusively through dedicated ducts directly into the external environment. Saliva, sweat, mucus, and mammary are mammal exocrine glands.