BIOLOGICAL DIFFERENTIATION. - The phenomena of b. d. are diverse, but all share the common condition of a gradual evolution of specific physiological and morphological characters. There is a cytological d., a histological d., an organological d., and a d. of botanical and zoological species.
Through d., the cell, or the tissue, or the organ or the organism as a whole, acquires structural and functional characteristics that distinguish it from others that are undifferentiated or differentiated along a different line or to a different degree.
D. is accomplished during the development of organisms from undifferentiated embryonic elements along different lines and to a more or less advanced degree. Organisms can therefore be arranged in series according to the degree of their differentiation, and it is indeed on this criterion that systematic botany and zoology rely for classification.
Cytological d. and histological d. form the basis of every organic d. The undifferentiated cell has general characteristics that are common to all undifferentiated cells of all organisms: it is endowed with all the fundamental properties of life, properties that can be summed up in excitability, movement, metabolism, growth and reproduction. In multicellular organisms, with the division of labor, the various groups of cells of the embryo specialize in carrying out one of these fundamental properties in a more perfect manner, while the others also remain in an elementary form. Thus through d. all the specific cell types specialized in the different functions are formed. These types of cells gather into special tissues and special organs, which take on the value of specific tissues and organs.
Examining the process of histological d., one observes how the early embryonic cells are totipotent, that is, they have the possibility of d. along the most diverse directions, but how, subsequently, with the delimitation of various territories and various layers of the germ, they undergo a process of determination (which manifests itself in two successive stages: labile phase and irrevocable phase) that nevertheless is not yet morphologically manifest, for which reason this period is also called that of invisible differentiation. Subsequently the first visible signs of d. appear, and with these the gradual heightening of specific activity.
Of the various tissues, the most highly differentiated in animals is nervous tissue, where it is possible for individual cells to acquire entirely specific and particular functional and morphological characters.
For certain differentiated elements a return to the undifferentiated condition is possible (anaplasia). Even by isolating differentiated elements from the organism in vitro and immersing them in embryonic juices, dedifferentiation occurs and the resumption of embryonic characters. It can happen that certain dedifferentiated elements, for abnormal causes, redifferentiate along a new direction (anaplasia accompanied by metaplasia).
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Alberto Stefanelli