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in embryology, the emergence of qualitative distinctiveness of parts of the developing organism (during the stages preceding the appearance of morphologically distinguishable rudiments of tissues and organs) that to a certain degree determines the paths of subsequent development of the parts of the embryo.
The term “determination” is used both for the evaluation of the morphogenetic properties of cellular material and to designate the processes by which that material reaches the state of determination. Cellular material is considered to be determined from the state when, upon transplantation to a foreign site, it first manifests the capacity to differentiate into the organ that will be formed from it, given normal development.
The term “determination” was proposed in 1900 by the German embryologist K. Heider. Operations on the living embryos of animals (isolating and culturing their parts in saline solution, removing and transplanting to an abnormal site during the stages before the formation of morphologically distinguishable rudiments of organs) have made it possible to accumulate data on the stages of determination and on the determining factors in the development of the various tissues and organs in embryogenesis and regeneration. Thus it was established that in the dorsal ectoderm of the vertebrate embryo, at the site where the neural plate (the germ of the nervous system) forms, morphologically elusive changes occur and the plate gradually acquires the capacity to develop into neural structures. Transplantation of a section of the ectoderm from the dorsal side of the embryo to the ventral yields varying results, depending on the stage of development at which it is performed. Only in the late gastrula stage does the dorsal ectoderm of the embryo acquire the capacity to develop into a neural plate at a new site; upon transplantation at earlier stages, it is subject to local influences and forms only integumental epithelium. Morphologically undetectable changes in the embryo’s dorsal ectoderm occur under the influence of material of the chordamesoderm, which, in the process of gastrulation, turns inside through the dorsal lip of the blastopore and lines it. When material of the dorsal lip of the blastopore is transplanted under the ventral ectoderm in the early gastrula stage, it induces, even there, the formation of a neural plate. An analogous sequence of changes has also been discovered in the material of future rudiments of other organs (for example, the eyes, inner ear, and mouth) and tissues. The concept of determination made it possible to describe these changes in comparable form for various organs and to elucidate their common principles. At first, determination is labile, the fate of the material may still be changed under other conditions, and differentiation of the developing organ is still weak; later, determination becomes fixed and stable, and differentiation of the organ becomes more complete.
The process of determination also includes autonomic changes in cellular properties (on the basis of ooplasmic segregation and the interaction of the nuclei with the qualitatively differentiated cytoplasm in various blastomeres) and the influences of certain groups of cells on one another. The relative significance of these processes varies in various organogeneses and in various groups of animals. In animals with spiral cleavage, ooplasmic segregation is more strongly expressed and determination of body parts is manifested as early as the cleavage stage. In chordates the cellular interactions have relatively greater significance; at certain stages of their embryonic development their cells acquire so-called competence, or the capacity to react to the inductive influence of other cells by the formation of certain structures. However, without the influence of an inductor, this competence is not realized and is replaced in time by a new competence—that is, by the capacity to form other structures. In normal development, latent differentiation occurs in competent material under the influence of an inductor, as a result of which the cellular material acquires at first labile, and then stable determination. Only after this does morphologically detectable differentiation begin; the rudiment of an organ appears and its differentiation begins. In successive stages of morphological differentiation, new systems of interaction and new processes of determination take part, in the course of which (simultaneous with the determination of the fate of the cellular material) there occurs a limitation of the possible paths of its differentiation—that is, a limitation of morphogenetic potential.
The activation of certain genes in various cells and the synthesis of various messenger ribonucleic acids and proteins are at the basis of determination. Thus, in cells that later form the rudiment of the lens of the eye, after intensive synthesis of deoxyribonucleic acid and proliferation of cells there occurs synthesis of messenger RNA and specific lens proteins——α-,β-, and γ-crystallines, whose appearance either precedes or coincides with the beginning of morphological differentiation of the lens cells. The means of differential activation of the genes, the role of inductive influences in this process, their nature, the synthesis of specific proteins, and the paths of transition from the genetic and biochemical differentiation realized in the process of determination to the morphological differentiation of the rudiments of organs and tissues are the objects of intensive investigation.
T. A. DETLAF