Concept

Contact inhibition

Summary
In cell biology, contact inhibition refers to two different but closely related phenomena: contact inhibition of locomotion (CIL) and contact inhibition of proliferation (CIP). CIL refers to the avoidance behavior exhibited by fibroblast-like cells when in contact with one another. In most cases, when two cells contact each other, they attempt to alter their locomotion in a different direction to avoid future collision. When collision is unavoidable, a different phenomenon occurs whereby growth of the cells of the culture itself eventually stops in a cell-density dependent manner. Both types of contact inhibition are well-known properties of normal cells and contribute to the regulation of proper tissue growth, differentiation, and development. It is worth noting that both types of regulation are normally negated and overcome during organogenesis during embryonic development and tissue and wound healing. However, contact inhibition of locomotion and proliferation are both aberrantly absent in cancer cells, and the absence of this regulation contributes to tumorigenesis. Contact inhibition is a regulatory mechanism that functions to keep cells growing into a layer one cell thick (a monolayer). If a cell has plenty of available substrate space, it replicates rapidly and moves freely. This process continues until the cells occupy the entire substratum. At this point, normal cells will stop replicating. As motile cells come into contact in confluent cultures, they exhibit decreased mobility and mitotic activity over time. Exponential growth has been shown to occur between colonies in contact for numerous days, with the inhibition of mitotic activity occurring far later. This delay between cell-cell contact and onset of proliferation inhibition is shortened as the culture becomes more confluent. Thus, it may be reasonably concluded that cell-cell contact is an essential condition for contact inhibition of proliferation, but is by itself insufficient for mitotic inhibition.
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