Information Exchange Among Cells

Short-Distance Signaling

Instead of entering the bloodstream, some signals may diffuse directly to neighboring cells, travel between adjacent cells, or even target the same cell that released them.

Not every signal molecule, a molecule that elicits a response from another cell or within the cell itself, needs to travel long distances from the signaling cell to the target cell. Sometimes signals only need to work on nearby cells. Short-distance signaling is a type of communication whereby a signaling molecule acts on a target cell located close to the signaling cell. One type of short-distance signaling is also known as paracrine signaling, which is when signal molecules only diffuse through a short extracellular space before they reach the target cell. Paracrine signal molecules are designed to affect only nearby target cells, so paracrine signal molecules are kept from diffusing very far by being quickly taken up by nearby cells, broken down by extracellular enzymes, or immobilized on the extracellular matrix, which is the noncellular portion of a tissue. Neurons use a type of paracrine signaling because they are in close proximity to each other, therefore, the neurotransmitter signals spend very brief moments in the extracellular matrix. The development of multicellular organisms also involves different types of short-distance signaling molecules.

Some signal molecules do not diffuse through extracellular space at all but remain attached to the surface of the signaling cell, even while bound to the receptor on the target cell. This type of signaling is called contact-dependent signaling, which is sometimes referred to as juxtacrine signaling. Yeast cells exhibit a classic example of contact-dependent signaling when preparing for sexual mating. The cell that is ready to mate will secrete a peptide known as mating factor. In response to this signal, nearby cells stop proliferating and prepare to conjugate with the first cell.

Finally, some signaling molecules act on the cell that secretes them so that the same cell acts as both the signaling cell and target cell. This is autocrine signaling, which occurs when a cell secretes a signaling molecule that acts on itself. Because the signal molecules can bind to neighboring cells that also present the correct receptor, autocrine signaling is effective at getting neighboring groups of cells to behave in the same way. For example, autocrine signals are common during cell differentiation, when cells of a multicellular organism develop into their specialized roles in the organism. Cellular responses to wounds and foreign intruders are also often regulated by autocrine signaling. One of the roles of prostaglandins, for example, is to regulate platelets, which are cells used in blood clotting. This is an autocrine signal and explains why prostaglandins are broken down so quickly in the extracellular space. In addition, autocrine signaling is also common in tumor development, when too many growth-factor molecules can be released, causing out-of-control growth.

Types of Short-Distance Signaling

The three main types of cell signaling over short distances are paracrine signaling, contact-dependent signaling, and autocrine signaling. Paracrine signaling happens when signal molecules only diffuse through a short extracellular distance before they reach the target cell. Contact-dependent signaling requires physical contact between cells, and autocrine signaling occurs when signals are sent to receptors within or on the surface of the same cell.
It is possible for a given signal molecule to be involved with more than one type of signaling process. Epinephrine, for example, is a small amino acid derivative and can act as a neurotransmitter or can be distributed widely through the body by the bloodstream. Epidermal growth factor, a protein hormone, can be part of a contact-signaling cascade or can be secreted into the bloodstream to act as an endocrine signaling molecule.