BIOLOGICAL DIFFERENTIATION. — The phenomena of biological differentiation are varied, but all share the common condition of a gradual evolution of specific physiological and morphological characteristics. There is cytological differentiation, histological differentiation, organological differentiation, and differentiation of botanical and zoological species.
Through differentiation, the cell, tissue, organ, or 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.
Differentiation occurs during the development of organisms from undifferentiated embryonic elements along various lines and to varying extents. Organisms can therefore be arranged in series according to the degree of their differentiation, a criterion indeed used by systematic botany and zoology for classification.
Cytological and histological differentiation form the basis of all organic differentiation. The undifferentiated cell possesses general characteristics common to all undifferentiated cells of all organisms: it is endowed with all the fundamental properties of life, which can be understood in terms of excitability, movement, metabolism, growth, and reproduction. In multicellular organisms, with the division of labor, the various groups of embryonic cells specialize in performing one of these fundamental properties more perfectly, while retaining the others in an elementary form. Thus, through differentiation, all types of specific cells specialized in different functions are formed. These types of cells gather into specialized tissues and organs, which assume the value of specific tissues and organs.
Examining the process of histological differentiation, it is observed how early embryonic cells are totipotent, meaning they have the potential to differentiate in the most diverse directions. However, subsequently, with the delimitation of various territories and layers of the germ, they undergo a process of determination (which manifests in two successive phases: a labile phase and an irrevocable phase) that is not yet morphologically evident, hence this period is also called invisible differentiation. Later, the first visible signs of differentiation appear, accompanied by the gradual intensification of specific activity.
Among the various tissues, the most highly differentiated in animals is nervous tissue, where individual cells can acquire entirely specific and particular functional and morphological characteristics.
For certain differentiated elements, a return to the undifferentiated condition (anaplasia) is possible. Even when isolated differentiated elements are cultured in vitro and immersed in embryonic juices, dedifferentiation and the resumption of embryonic characteristics occur. It can happen that certain elements, dedifferentiated due to abnormal causes, redifferentiate along a new direction (anaplasia accompanied by metaplasia).