Differentiation

DIFFERENTIATION (from Latin differentia - difference), also differentiation (in biology and medicine), the appearance of differing parts in an initially homogeneous substrate. The term "differentiation" (English) was introduced by H. Spencer for characterizing the process of evolution of all natural systems: this process proceeds by differentiation and integration (closer union), otherwise by the transition from an unrelated homogeneous state to a related heterogeneous one. According to Spencer's teaching, differentiation in biosocial systems is always accompanied by the division of labor between the differentiating parts. Now in biological sciences, the term D. (German Differenzierung) is usually used. D. is most clearly manifested in the process of embryonic development, when initially homogeneous cells of the embryo separate into groups—primary organs—and form tissues (see Histogenesis). Analysis of this process led W. Roux, founder of "developmental mechanics," to the necessity of distinguishing between the concepts of self-differentiation (Selbstdifferenzierung) and dependent differentiation (abhängige Differenzierung). In the first case, all necessary data for the differentiation of any part or organ (usually known chemical substances) are fully present in their primordia; in the second, differentiation for its manifestation requires the presence of other parts or organs, which in one way or another determine it. An example of self-differentiation can be the development of an amphibian limb from an undifferentiated primordium, the so-called limb bud. Through the work of Braus and others, it has been established that the differentiation of skeletal parts and muscle groups of the limb proceeds independently of the nervous system and occurs in a typical manner for the given species in case of transplantation of the limb bud to another area of the body or even to an animal of a closely related species. An example of dependent differentiation is the development of the lens, which requires contact between the epidermis and the edge of the optic cup and can be induced in another location by transplanting an eye primordium. Research by Spemann showed the existence in the embryo of areas that serve as "organizers" for neighboring tissues. For example, the lip of the blastopore (see) when transplanted to any location of the embryo under the ectoderm can induce the formation of a neural tube and chord nearby. Self-differentiation of a primordium can also occur in its culture in vitro, which was demonstrated by experiments of Ph. Stohr on amphibian heart primordia. The deviations from the norm observed in this case clearly show the degree of influence of neighboring parts. But while primordia of organs consisting of various tissues continue to differentiate, embryonic tissues isolated from the organism are unable to do so, for example, fibroblasts do not produce fibers. On the contrary, already differentiated cells often lose differentiation (dedifferentiate). Thus cellular differentiation is to a large extent dependent. The terms "to differentiate, differentiation" are used 1) with respect to factors causing differentiation, for example in histological technique in regressive staining when an overstained preparation is differentiated by one method or another, 2) more often as a mathematical term for obtaining a differential.

V. Karpov.

D. in histological technique. Differentiation of a preparation consists in removing excess dye from the object and is applied in all cases when over-staining has occurred. As a differentiating agent, any substances capable of extracting dye can be used. In the simplest case, this is water (for example, washing in distilled water after Giemsa and May-Grünwald staining until the preparation turns pink is done to weaken the staining by blue basic dyes). In most cases, more energetic agents have to be used: ethyl and methyl alcohols, aniline solution in water (for example, when staining nuclei with safranin or fuchsin). Basic dyes are well differentiated by acids: acetic, trichloroacetic, hydrochloric, tannin, etc., as well as by acid dyes. Among them, Orange G, Fuchsin S and picric acid (especially in mixture with tannin) are most frequently used. For mordant dyes, either strongly differentiating agents are used, such as alcohol with 1-2% HCl (for carmine), or specific solvents—iron alum (suitable for all hematoxylin). Differentiation is the second stage of regressive staining, i.e., first the preparation is overstained, and then the dye is gradually extracted from it to the desired degree; during this process, various structures are clearly revealed, as they retain the dye to very different degrees and therefore give it up at different rates. See also Weigert's staining methods. Lit.—see literature for the article Histological technique.