Radioautographic analysis of the incorporation of protein and nucleic acid precursors into various tissues of early chick embryos cultured in toto on medium containing LiCl

Abstract

Chick embryos at presomite and early somite stages with extraembryonic area partially trimmed were cultured on cellulose acetate rafts on liquid medium with glucose as nutrient, glycine-C 14 or adenine-C 14 as precursors for proteins and nucleic acids, and normal NaCl-containing medium, or various inhibitors, including LiCl, substituted for a portion of the NaCl. Track counting of radioautographs indicated that LiCl inhibited synthesis of proteins, and probably of RNA, most in the lateral portions of the bilateralizing optic vesicles, and of proteins, least of any head region in the dorsal portion of the diencephalon. Inhibition in the telencephalon was fairly high. Dissected embryos showed that this effect was not mediated by the mesoderm. In presomite embryos, lateral epiblast (region that would form neural folds) was inhibited as much as medial or lateral hypoblast in regard to synthesis of proteins, and probably of RNA. The LiCl experiments on somite stages were repeated on agar clot medium with albumen + glucose as nutrients, entire blastoderms explanted, and methionine-H 3 or uridine-H 3 as precursors. Grain counting indicated that LiCl inhibited protein synthesis in the lateral portions of the optic vesicles most strongly, with inhibition in the telencephalon and lateral prechordal substrate almost as high. Again the dorsal portion of the diencephalon was inhibited more weakly. RNA synthesis was most inhibited in the telencephalon, with optic vesicles and medial substrate the only other head regions examined that showed significant inhibition. Protein determinations on embryos cultured on the same agar clot as above, compared with embryos grown in ovo, indicated that over a 2-day culture period, embryos on NaCl-control medium increased their protein content about two-thirds as much as embryos in ovo, and that, in the best instances on 11.8 m M LiCl medium, embryos doubled their protein content during 41–42 hours of culture. Since in a separate study perfect cyclopia was produced in the chick on LiCl-containing medium, the present results together with recent work of others on amphibian embryos are interpreted as supporting the viewpoint that the lithium effect produces cyclopia by acting continuously over a period of time on presumptive neural and neural tissue, at least as importantly as, and possibly more importantly than, on prechordal substrate.