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ONE of the objectives pursued actively for the past 80 years by cell biologists has been to reconstruct from single cells maintained in vitro the corresponding functional tissue. This not only requires that cells obtained from a given tissue and maintained singly in vitro proliferate actively but also that upon reaching confluence they reconstruct the tissue to which they belong in vivo in such a way that both the morphological characteristics of the tissue and its functions are preserved. In recent years, the discovery of mitogenic factors that maintained cells in an active stage of proliferation, thereby preventing their precocious senescence in vitro, has made it possible, in the case of a few tissues, to achieve these objectives. The discovery of growth factors is in a sense not a recent occurrence. The first growth factor to be identified was erythropoietin. This factor, first identified by Carnot and Deflandre in 1906, controls the proliferation of erythroblasts, the progenitors of red blood cells (...