Inductors

in embryology, the rudiments of organs and tissues of the developing embryo whose influence on other parts of the embryo in contact with them is necessary for the latter’s development in a particular direction. For example, in the embryos of chordates the rudiment of the notochord and muscles (chordamesoderm) serves as the inductor of the central nervous system, and the rudiment of the eye, as the inductor of the lens.

The action of inductors is not species-specific: an inductor taken from the embryo of one species of animal can bring about the induction of the corresponding rudiment of an organ in embryos of another species. Often inductors do not lose their inductive properties after being killed (by fixation in alcohol, boiling, and so forth). In experiment the action of an embryo’s own inductors may be replaced by the action of a number of organs and tissues (living or dead) of adult animals (foreign, or heterogenous inductors). Foreign inductors may differ in the character of their inductive action; thus, the liver of a guinea pig induces in amphibians the formation in the ectoderm of the gastrula of structures of the forebrain, and its bone marrow induces the formation of the notochord, muscles, and other mesodermal derivatives. Both isogenous and foreign inductors can be effective not only in direct contact with the cells of the reacting system but also through a filter with microscopic pores.

The transfer of substances from inductors to the cells of the reacting system has been shown by cytochemical, biochemical, and immunological methods. The substances transferred from inductors to the cells of the reacting system are localized in the cytoplasm of those cells. In a number of cases substances possessing various inductive properties have been isolated from the inductors: ribonucleoprotein, which induces formation of the brain; a protein with a molecular weight of between 25,000 and 30,000 that induces formation of the notochord, muscles, and other mesodermal derivatives. The inductive influence of the complex chemical substances whose source are inductors can be imitated by treating the cells of the reacting system with simpler chemical compounds, such as sucrose and lithium chloride, or certain harmful agents, or by changing the pH of the medium.

The term “inductors” also designates the factors necessary for the morphogenesis of animals and plants during the postembryonic period of development (for example, hormones and phytohormones).