A Modern Concept of the "Cell Theory": A Perspective on Competing Hypotheses of Structure

Abstract

According to the "cell theory" in its original form, morphogenesis in multicellular organisms results from oriented cell divisions and cell growth. This view is based on the still-valid fact that cells represent the smallest structural and functional units exhibiting all fundamental properties and manifestations of life. Each cell stores, replicates, and partially expresses the genetic information of an organism. Recombination and syngamy take place at the cellular level, and sexual reproduction of multicellular organisms depends on single cells. Nevertheless, morphogenesis in multicellular organisms is governed by supracellular factors. This basic tenet of "organismal theory" is demonstrated best by plasmodial organisms such as siphonal algae that can be structured to a certain, if limited, extent in the absence of individualized cells. It seems improbable, though, that "cell theory" and "organismal theory" represent true alternatives. Multicellular organisms are not merely aggregates but nonlinear systems of cells with emergent characters. Typological and finalistic "explanations" are not, however, intended to clarify causal connections. The formulation of a unifying concept appears to be hindered not only by obsolete historical reasons but also by conceptual differences and also by methodological barriers. Concerning the balance of genetic and epigenetic factors in morphogenesis, it appears unlikely that information stored in one-dimensional nucleic acid molecules could suffice to direct ordered three-dimensional development.