One problem has intrigued embryologists for many decades: In the absence of strictly defined developmental fates, how does a cell "know" where it is in an embryo, in order to know what to become? An early suggestion, which has been borne out by experiments, is based on the concept of a concentration gradient—a variation in concentration of a substance across a region of space.
A concentration gradient is formed whenever a substance is created in one place and moves outward by diffusion. When this occurs, there will be a high concentration of the substance near its point of origin, and increasingly lower concentrations further away. This provides positional information to a cell anywhere along the gradient. Cells pick up this chemical signal, and its strength (concentration) determines the cell's response. Typically, the signal is a transcription factor, and the response is a change in gene transcription.
Because such a signalling substance helps to give form to the embryo, it is termed a morphogen ("morph-" meaning "form," "-gen" meaning "to give rise to"). Morphogen gradients are a key means by which originally identical cells are exposed to different environments and thus sent along different developmental paths. In humans and other mammals, one morphogen that acts early in development is retinoic acid, a relative of vitamin A.
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