CELLULAR, THEORY. - With the discovery of the cell, the idea of the cellular constitution of all animals and plants matured. However, only in 1838 did Schleiden for plants and in 1839 Schwann for animals formulate this conception, which was defined as the cellular theory or doctrine, namely: metazoa and metaphyta must be considered as constituted by a colony of simple organizations, the cells, and the life of the whole is given by the sum of the elementary lives. Protozoa and protophyta represent organizations formed by a single cell. In 1850 Milne-Edwards introduced the concept of division of labor, whereby the cells of the various tissues specialize for a determined task, assuming the form most suited to its accomplishment (histological differentiation). With the discovery of cell division, made in 1850 by Mohl in filamentous algae and then confirmed by Naegeli in higher plants and by Vogt, Kolliker, Remak, etc. in animals, it was intuited that the colony originated through the successive division of an initial ancestral cell, which is the egg. Virchow, the great German pathologist, formulated the well-known aphorism: "Omnis cellula a cellula" and considered diseases as cellular alterations that manifest themselves during divisions.
The cellular theory as it was formulated found in Haeckel a fervent supporter, since it accorded very well with the transformist evolutionary idea, placing the protozoon and man on the same constitutive plane.
The colonial conception of the higher forms was, however, soon criticized by great minds of biology such as Wolfgang Goethe and later Driesch. In fact, the life of the whole that constitutes a higher individuality is something more than the sum of the lives of the elementary components. There is a life of a higher order that is not constructed by the parts but rather superimposes itself upon them, dominates them and regulates them. De Bary, among botanists, said that it is not the cells that make the trees, but the trees that make the cells, meaning precisely the dominion of the higher unity over the activities of the individual cells that compose it. But the cellular theory, toward the end of the 19th century, was also attacked on the purely structural plane. First of all, the various cellular organelles began to be criticized, being considered artifacts of histological technique, and thus it was demonstrated that the structure of the cytoplasm (granular, filamentous, alveolar) assumed the various appearances depending on the fixative used (Hardy and Fischer, 1899). Finally, the very unity of the cells was attacked, though this led to excesses. Thus Rhode, at the beginning of this century, wrote a histology without ever speaking of cells. Plasmodesmata (protoplasmic bridges between cell and cell), plasmodial tissues (in which there is division of the nucleus without division of the cytoplasm), syncytial tissues (formed by the fusion of cells into undivided protoplasmic masses), tissues in which the intercellular parts prevail (such as the connective tissues), were considered as evidence against the cellular doctrine.
Furthermore, organisms not divided into cells are described, such as certain algae (Caulerpa), and acellular organisms are produced experimentally, such as the larvae of Chaetopterus of Lillie.
Thus protozoa are considered not a single cell but a much more complex, non-cellular organization. Other acellular conditions of living beings, such as bacteria and viruses, come to limit the universal concept of this doctrine. This extremist point of view against the cellular theory is, however, today to be considered superseded by the evidence of other facts.
If there is no longer any doubt that there exists a higher life that is by far more than the sum of the lives of the cells and that is not formed by these but dominates and governs them, there is likewise no doubt that the cells composing the organism have their own independent life, which manifests itself especially if their relations with the higher unity cease.
The first demonstrations in this regard can be considered those given in 1897 by Tirelli, who studied the survival of tissues in the cadaver. After the death of the whole, after the extinction of the higher life, the various cells continue to live for a certain time and to a varying extent depending on the tissues. They end up dying as well, because the cessation of the collective organic functions no longer permits the exchanges at the level of the individual cells necessary for their life.
But even more demonstrative is the method of tissue culture, which consists in isolating from an organism a fragment of tissue in vitro and cultivating it, feeding it, and washing it to remove the products of metabolism. This method, perfected by Carrel, has made it possible to keep alive and in continuous reproduction the cells of a chicken embryo for about twenty years, that is, for a time much longer than they would have lived had they remained part of the chicken itself. And the extinction of the strain depended on purely accidental causes. Thus it has been possible to cultivate still-living cells taken from a cadaver and make them multiply actively.
It has thus been seen that the proliferative activity of cells is much greater in culture, isolated from the higher organism, than in the organism itself. This confirms the regulating action of the higher individuality on the activity of the individual elements. Theoretically the life of cells is indefinite: it is their mutual bond in the soma of the higher individuality that condemns the cells to die with it. The cellular theory is therefore not dead, but must be modified in its formulation.
It must be admitted that there exist organisms with a non-cellular organization, such as bacteria, viruses and some other lower form (blue-green algae). Protozoa, although they have a cellular organization, are something more than a cell, and this in relation to their independent life. Metazoa and metaphyta are multicellular insofar as they arise from the successive division of an initial cell (egg and oosphere). In the early embryonic stages one can perhaps speak of a true colony of cells, but as development proceeds, a higher individuality is established that dominates the reproduction and activity of the components. With development the various cells, through the division of labor, differentiate into the various tissues. In this process the cells may even lose their anatomical individuality and unite into more extensive masses, as, for example, in striated muscle fibers, if functionality requires it, or produce extracellular structures that, in the end, come to have a preponderance over the cells themselves, as in the connective tissues, but they are formations always dependent on the cellular condition.
The pluricellular organism is therefore not a colony of cells but rather an association. One may conclude with Cotronci (p. 9 of the op. cit. in the bibliography): "The living organism is therefore an association of elements endowed with relative autonomy that constitute an entity of a higher order that dominates and directs the life of the individual elements. This dependence of the life of the individual elements on the life of the organism, we may hold to be all the greater the higher the degree of organization."