EFFECT OF THE POTASSIUM ION ON INDUCTION OF NOTOCHORD FROM GASTRULA ECTODERM OF RANA PIPIENS

¹ Marine Biological Laboratory, Woods Hole, Massachusetts 02543

Small aggregates of cells prepared from explants of ventral ectoderm of Rana pipiens gastrula were treated for 3-4 hrs in 71 mM Li⁺ added to standard solution. When cultured in standard xsolution containing 1.3 mM K⁺, the aggregates of ventral ectoderm cells differentiated into nerve and pigment cells. As the K⁺ concentration in the culture medium was raised through 2.3, 3.3 and 5.3 mM, the frequency of pigment cells declined and networks of vacuolated cells were formed, which later fused to form small notochordal masses. Raising the K⁺ to 7.3 and 9.3 mM gave nerve and pigment cells again, as in the lower potassium concentrations. The effect of high K⁺ on lithium-induced cells becomes irreversible during the first 18 hrs. Cells may be returned to 1.3 mM K⁺ after 18 hrs and notochord differentiation will occur.

Induced notochord cells first appear as somewhat larger cells which become filled with very small vacuoles by 7-8 days of culture. These small vacuoles then coalesce to form larger and larger vacuoles (9-12 days). The vacuolated cells aggregate into small clumps. Cultures were fixed and stained at various intervals for photography.

Cultures of small explants of notochord in 5.3 mM K⁺ were observed to undergo the same type of differentiation. In both cases the notochord cells and vacuoles were smaller than in the whole embryo. Larger explants of notochord from archenteron roof gave large masses of notochord cells about the same size as in the whole embryo. When lithium-treated aggregates of the ventral ectoderm were fused together in 5.3 mM K⁺ to form a composite of 10-15 aggregates, the notochordal masses were larger and the notochord cells were about the same size as obtained in comparable sized explants of notochord from the roof of the archenteron.

These findings are discussed in relation to the effects of high potassium on membrane depolarization, protein synthesis, DNA synthesis, and increase in intracellular Ca²⁺.