Endocrine, Paracrine and Autocrine Signaling

Various forms of intercellular commimication can be discerned based on the range of the signal transmission (fig. 3.5). 

In endocrine signaling the hormone is synthesized in specific signaling, or endocrine, cells and exported via exocytosis into the extracellular medium (e.g. blood or lymphatic fluid in animals). The hormone is then distributed throughout the entire body via the circulatory system so that remote regions of an organism can be reached. 

In autocrine signaling, cells of the same type commimicate with one other. The hormone produced by the signaling cell affects a cell of the same type by binding to recep- 

In autocrine signaling, cells of the same type communicate with one another. The hormone produced by the signaling cell affects a cell of the same type by binding to receptors on these cells and initiating an intracellular signal cascade. If an autocrine hormone is secreted simultaneously by many cells then a strong response occurs in the cells. Autocrine mechanisms are of particular importance in the immune response (see Chapter 11). 

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Fig. 3.5. Endocrine, paracrine and autocrine signal transduction, a) endocrine signal transduction the hormone is formed in the specialized endocrine tissue, released into the extracellular medium and transported via the circulatory system to the target cells, b) paracrine signal transduction the hormone reaches the target cell, which is found in close juxtaposition to the hormone producing cell, via diffusion, c) autocrine signal transduction the hormone acts on the same ceU type as the one in which it is produced.

Signaling by soluble extracellular molecules can be classified Into three types endocrine, paracrine, and autocrine. Describe how these three methods of cellular signaling differ. Growth hormone Is secreted from the pituitary, which Is located at the base of the brain, and acts through growth hormone receptors located on the liver. Is this an example of endocrine, paracrine, or autocrine signaling Why ... 

Hormone A chemical signal that is produced in one set of cells and directs the activity in another set of cells that can be endocrine, paracrine, or autocrine. 

Despite the diversity of hormones - endocrine, paracrine, autocrine or pheromones - they all influence intracellular events by intracellular signalling mechanisms, and the number of such mechanisms is remarkably small, hi this chapter, the signalling mechanisms, the effects and the functions of hormones are described, discussed and illustrated by reference to selected hormones. In addition, the principles underlying the mechanisms by which hormones affect the biochemistry in cells are discussed. 

Autocrine cell In animals, a cell that produces hormones, growth factors or other signalling substances for which it also expresses the corresponding receptors. See also Endocrine and Paracrine . 

Biologically, many cytokines have what appears to be an endocrine function, especially when they are released in large quantities. They also exhibit autocrine and paracrine functions because they usually have a short radius of action (Figure 22-1). However, there is an enormous difference between these molecules and the hormones. For example, hormones are produced continuously, although with variation in concentration, and cytokines are usually produced only after the action of an activation signal and for other reasons previously mentioned. In addition, the receptors for cytokines are often of high affinity but are expressed with a low density on cell membranes. 

Cytokines are extracellular signaling proteins, mostly secreted however, they could also exist as membrane-bound proteins. They are produced by many different cell types and can have an effect on adjacent cells (paracrine fashion), at a distance (endocrine), or can act on the same cells that produce them (autocrine). Cytokines mediate their action through high affinity surface receptors and can demonstrate a wide variety of actions, including the coordination of inflammatory and immune responses. They are also involved in many stages... 

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