SODIUM DEPENDENCE OF EMBRYONIC INDUCTION

Induction of nerve and pigment cells in presumptive epidermis of the frog gastrula has been found to be dependent upon the concentration of the sodium ion. This sodium dependence is shown by small aggregates of presumptive epidermis cells induced by sucrose or by ions (Li+, Ca2+, Mg2+, Mn2+) to differentiate into nerve or pigment cells. Following these treatments, induction takes place when the external NaCl concentration is 0.088 M, but does not occur at 0.044 M. This range of concentrations is within the physiological range for continuous culture of cell aggregates and explants of amphibian embryos commonly used for studies of induction. Similarly, normal induction of nerve and pigment cells by mesoderm in small explants, prepared from the dorsal lip and lateral marginal zones of the early gastrula, is dependent upon the external concentration of sodium. Nerve and pigment cells are induced by the mesoderm when the culture medium contains 0.088 M NaCl. At 0.044 M NaCl, however, the mesoderm differentiates into muscle and mesenchyme, but induction of nerve and pigment cells does not occur. The sodium dependency of aggregates prepared from early gastrula is restricted to the period of induction. The end of this period corresponds in duration to the end of gastrulation. Cell aggregates prepared directly from presumptive neural plate at the end of gastrulation similarly have become independent of sodium. The sodium dependency of induction in presumptive epidermis cell aggregates by ions is compared to sodium dependency of induction of ectoderm by mesoderm in cell aggregates from early gastrula. The working hypothesis presented is that normal embryonic induction depends upon an endogenous source of ions, and that there is an intracellular release of ions within the embryo during late gastrulation. Preliminary experiments have revealed a threefold increase in exchangeable Ca2+ within the cells and a forty-fold increase in intracellular Na+ during gastrulation. The sodium dependence demonstrated in the present experimental system and postulated for normal embryonic induction is compared to other sodium-dependent systems in transport at tissue, cellular, nuclear, and organelle levels. © 1969.