Differentiation of cells of the Rana pipiens gastrula in unconditioned medium.

INSTANCES of differentiation in absence of an inductor have multiplied to such extent that the classical picture of embryonic induction is in process of serious re-examination (Grobstein & Parker, 1958; Nieukoop, 1955; Trinkaus & Groves, 1955). Questioned on the basis of the occurrence of neural differentiation without organizer in explants of presumptive epidermis (Barth, 1941; Holtfreter, 1947, 1955; ltd, 1952), the necessity for a specific inductor has proved difficult to demonstrate in other inductive systems. Grobstein & Parker (1958) have shown that tubule formation in metanephrogenic mesenchyme may occur in abnormal sites in vivo without contact with its normal ureteric inductor and even in the anterior chamber of the eye without contact with any other tissue. Niu (1958/?) has recently reported a certain percentage of neural differentiation in explants of presumptive epidermis cultured as controls in his salt medium without addition of inductor substances. Such observations not only emphasize the difficulty of finding an adequate neutral environment for the study of inductors but also suggest the necessity for revision of such concepts as induction and self-differentiation. The present experiments were undertaken with the objective of working out a simple system for culturing presumptive epidermis in a situation that would permit easy alteration of the culture medium and facilitate observation of the effects on differentiation of specific additions to the medium. The work of Niu (1956) had demonstrated in elegant fashion the advantages of using very small explants which could attach to glass and spread out in a single cell layer eliminating the need for sectioning. The success of Niu's medium (Niu-Twitty solution) in cultures of Ambystoma and Triturus explants offered new hope that a similarly modified Holtfreter's solution might support in vitro differentiation of early embryonic cells from Rana. R. pipiens was chosen as the source of presumptive