BIOLOGICAL DIFFERENTIATION. — The phenomena of biological differentiation are diverse, but all share the common condition of a gradual development 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 takes place during the development of organisms from undifferentiated embryonic elements along different lines and to a greater or lesser extent. Organisms can therefore be arranged in series according to the degree of their differentiation, and systematic botany and zoology in fact employ this criterion for classification.
Cytological and histological differentiation underlie every organic differentiation. The undifferentiated cell has general characteristics common to all undifferentiated cells of all organisms: it possesses all the fundamental properties of life, which may be summarized as excitability, movement, metabolism, growth, and reproduction. In multicellular organisms, through the division of labor, the various groups of embryonic cells become specialized in carrying out one of these fundamental properties more perfectly, while the others remain present in elementary form. Thus, through differentiation, all the specific cell types specialized in different functions are formed. These cell types gather into special tissues and special organs, which thereby acquire the status of specific tissues and organs.
On examining the process of histological differentiation, one observes that early embryonic cells are totipotent—that is, they have the possibility of differentiating along the most diverse lines—but that subsequently, with the delimitation of various territories and layers of the germ, they undergo a process of determination (which manifests itself in two successive stages: the labile phase and the irrevocable phase), which is nevertheless not yet morphologically apparent; this period is therefore also called the period of invisible differentiation. Subsequently, the first visible signs of differentiation appear, together with the gradual intensification of specific activity.
Of the various tissues, the most highly differentiated in animals is nervous tissue, in which individual cells can acquire entirely specific and particular functional and morphological characteristics.
(by courtesy of the General Curia Chancery of the Rogationist Fathers)
DI FRANCIA, ANNIBALE MARIA – Portrait.
the resumption of embryonic characteristics. It may happen that certain elements, dedifferentiated by abnormal causes, redifferentiate according to a new direction (anaplasia accompanied by metaplasia).