500 MOLECULAR MECHANISMS OF DIFFERENTIATION 



TABLE 1 



INCORPORATION OF GL YCINE - I -l** C INTO TISSUES OF II— I3 SOMITE CHICK 



EMBRYOS EXPLANTED FOR 4 h ON AGAR-EGG EXTRACT MEDIUM 



CONTAINING LABELLED GLYCINE 



^ Derived from analyses of glycine isolated chromatographically from the tissue proteins. 

 ^ Derived from nine separate determinations. 



acids which are then incorporated into the proteins of the embryo. Flickinger 

 (1954) demonstrated that in the neurula of Rana pipiens, for example, the major 

 activity is found in the neural folds, notochord and somites, but much less in the 

 remaining embryo. Cohen (1954) showed, in addition, that the rate of incorpo- 

 ration of labelled COj into the protein moiety of developing amphibian embryos 

 increased considerably from the blastula to the neurula stage. Since labelled CO2 

 can be incorporated into proteins only by first forming a part of the amino acid 

 molecules, these experiments demonstrate that amino acid incorporation is actually 

 taking place and that the rates of amino acid synthesis and incorporation vary in 

 different organs and at different developmental stages of the amphibian embryo. 

 Although labelled amino acids are not well suited for incorporation studies in 

 amphibians because of a limited permeability of the ectodermal coat of the embryo, 

 a higher uptake in the dorsal half of the gastrula could be demonstrated (Friedberg 

 and Eakin, 1949 and Eakin, Kutsky and Berg, 1951) using bisected embryos. 



During the early development of the chick embryo, the uptake of labelled 

 amino acids can be followed advantageously in explants using Spratt's technique 

 (Spratt, 1947 and 1948). In recent experiments in our laboratory, embryos at 

 5-7, or 11-13 somite stages were explanted on an agar medium containing, in 

 addition to the egg extract, labelled glycine. By isolating chromatographically, 

 after acid hydrolysis, the tracer glycine incorporated after varying times of in- 

 cubation (i-i2h.), it was possible to follow its uptake into the proteins of the 

 whole embryo and of the isolated embryonic organs. As indicated in Table I, 

 neural tube and somites are among the embryonic tissvies with a higher rate of 

 tracer incorporation while in the heart and notochord incorporation occurs at 

 a much slower rate (Schultz and Herrmann, 1958). 



In another series of experiments the uptake of tracer amino acids was allowed 

 to proceed for a limited time {e.g. 2h.) . After this period the embryos were transferred 

 to an agar-medium plate without labelled amino acid and the release of the tracer 

 from the proteins and from the free amino acid pool was measured. It was found 



