Chemically stimulated differentiation of post-nodal pieces of chick blastoderms.

Earlier studies on the role of sulfhydryl (-SH) groups in morphogenesis have indicated that in chick and amphibian embryos, an -SH-containing substance can act as an inducing stimulus in the action of the primary organizer. Thus, blocking -SH groups by chloroacetophenone (CAP) has specific developmental effects and the inducing capacity of the Hensen's node is affected (Lakshmi, 1962öf, b). Reversal of the effects of-SH block by cysteine indicated that whatever is blocked by the -SH inhibitor is restored by cysteine (Mulherkar, Rao & Joshi, 1965; Mulherkar, Rao, Joshi & Joshi, 1966). It appears, however, that not all -SH-containing substances can reverse the effect of-SH block by CAP. Thus the teratogenic effects of CAP could not be ameliorated by a subsequent treatment with cysteamine, a decarboxylation product of cysteine, while CAP and cysteamine mixed in equimolar proportions had no effect on development (Rao, 1969). Inhibition of developing neural tissue and of inducing capacity of the organizer may be due to block of -SH groups of proteins or of the -SH in free cysteine and/or glutathione in the inducing tissue. Since the effects of -SH block can be reversed effectively by cysteine and not by cysteamine, it is likely that the action of cysteine in reversing the effects of-SH block is not the restoration of-SH groups. On the other hand, it is quite likely that an inducing stimulus can be given by substances of low molecular weight, probably cysteine or reduced glutathione. Waheed & Mulherkar (1967) have shown that postnodal pieces (PNPs) treated with cysteine and reduced glutathione, when used as organizer grafts, can induce neural tissue and that the grafts are neuralized or differentiate into somites. Thus there is good evidence that -SH-containing substances like cysteine and glutathione have the ability to stimulate differentiation of axial structures. In the present investigation an attempt has been made to study the inductive effects of cysteine and glutathione on post-nodal pieces of the chick embryo.