4/7/2023 0 Comments Concentration gradientAnother explanation for concentration-dependent responses exists through secondary effects of subsequent cell interactions. A range of concentration-dependent responses could be produced by a population of cells, each of which can make only its own predetermined response in an all-or-none way. In most examples of concentration-dependent effects, it has not been clearly established whether an individual cell senses concentration in any more profound way than by making an all-or-nothing response to any concentration above a threshold value. To prove that a signal functions as a morphogen, it must be demonstrated that: (1) cells respond directly to the extracellular signaling molecule in question, and (2) the cells display at least two qualitatively different responses, apart from the nil response, at different concentrations of the signal. Morphogen Response at a Single Cell Level There have been recent advances toward a molecular understanding of this problem, and the aim of this review is to summarize facts and concepts that can explain how cells sense the concentration of an extracellular agent to which they respond by activating different genes. A third class of concentration-dependent response is that of developing and differentiating cells to a morphogen gradient. Likewise, male saturniid moths can follow a pheromone gradient toward a female, and foxhounds can follow a scent trail at extraordinarily low concentrations, but again the precise molecular mechanism of orientation is not known. However, the detailed mechanism by which such a small difference in receptor occupation is converted to polarization of a cell is not understood. In these cases a cell need not sense the absolute concentration of an external signaling molecule, but can respond to as little as a 1% difference in concentration between two sides of a cell.
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