Introduction: Cell Controls and Signal Molecules

Abstract

All cells are submitted to a variety of specific and unspecific extracellular signals (Fig. 1.): photons, ions, metabolites, neurotransmitters, and local or general hormones. At all moments, the number, size, and activity of the cells result from their complex present and past network of interactions with these signals. The action of these signals may be defined by several of their characteristics: their nature, their kinetics, their biochemical mechanisms. Signals may control the level of activity of the target cell, either stimulating this activity (positive control) or inhibiting it (negative control). They can influence the nature of the cell activity, e.g., by changing the program of the cell; this is a differentiating action. They may also increase the number of functional units (e.g. organelles) per cell or the number of cells (hypertrophy and hyperplasia) or decrease it; these are positive or negative trophic actions. The kinetics of the action of signals varies from a few microseconds to days. The biochemical mechanisms of action also vary from the simple opening of an ion channel to the complex activation of sets of genes. It is obvious that these three characteristics are related; the opening of an ionic channel is almost immediate and can directly influence very defined enzymatic or transport functions. Differentiation which implies the alteration of the expression of sets of genes, and the consequent modification of the cell protein pattern, will necessarily take at least hours and will involve controls at the level of transcription.