What are the categories of cell signaling?

Cell signaling in multicellular organisms can be divided into 4 basic categories based on the distance the signal travels through the organism to reach the target cell. The four categories of cell signaling include paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. 

Endocrine Signaling

Endocrine signaling is a form of long-range signaling. Signals originate from endocrine cells and are released into the bloodstream, which transports them to far-away target cells in the body. Signals that are generated in the endocrine gland and travel through the bloodstream to reach distant targets are known as hormones. These endocrine signals or hormones produce a slower response but have a longer-lasting effect. 

Paracrine Signaling

Paracrine signaling is a form of short-range signaling. Paracrine signals move by diffusion through the extracellular matrix that lies between cells that are in close proximity to each other. These signals typically elicit immediate responses but they last only for a short time. One example of paracrine signaling is the inflammatory response, in which cytokines are quickly released after an injury. 

Autocrine Signaling

Autocrine signaling is a form of self-activation signaling in which the signaling cell and the target cell are one and the same. The signaling molecule originates from the target cell and sends a signal to itself. Autocrine signaling is responsible for regulating pain sensation and inflammatory responses. It is also crucial during early development, when it helps cells develop into the correct tissues and take on the correct functions. 

Juxtacrine Signaling

Juxtacrine signaling is also known as contact - dependent or direct activation signaling. Contact-dependent signaling can only take place between adjacent cells that are in physical contact with each other, either through gap junctions or through the interactions of transmembrane proteins. In this case, the signaling molecule is not free. Instead, it is bound to the cell membrane and interacts with a membrane receptor of a neighboring cell. The Delta-Notch pathway used in embryonic development is an example of juxtacrine signaling.