Studies on Embryonic Induction using Radioactive Tracers

T H E nature of the agent which brings about induction during the early development of vertebrates has been extraordinarily difficult to determine. As yet we are not even certain whether there is a passage of a comparatively massive quantity of material from the organizer into the reacting competent tissue, as is probably most usually assumed, or whether the action is primarily located at the surface of intercellular contact, as suggested by Weiss (cf. 1947, 1950). Waddington & Goodhart (1949) showed that certain unnatural evocators (steroids) certainly penetrate the competent tissues, and can be recognized inside the reacting cells by their ultraviolet fluorescence. This rinding does not by any means solve the problem as far as concerns the substances involved in natural induction. A possible method of attack here would seem to be the use of radioactive tracers. Abercrombie & Causey (1950) used P32 to label chick grafts and showed that the diffusion of the tracer into the reacting tissues was sufficiently restrained for the graft to remain easily distinguishable from the host, but they did not attempt to ascertain whether any minor passage of tracer occurs. Waddington (1950) attempted to use P32 to follow the transfer of substances in induction, the inducing agent employed being dried yeast which had been labelled. It was found that the tracer passed into the induced tissues in easily recognizable quantities; but it was pointed out that although a negative result would have been strong support for the hypothesis of surface action, the fact that tracer is transferred does not justify any far-reaching conclusions of a positive kind, since it remains unknown whether the P32 moves in the form of simple radicals such as the phosphate ion or of large macromolecules such as nucleic acids. The availability of amino-acids and purines labelled with CM or S35 has made possible further work along similar lines. It is particularly desirable to discover whether such tracers can give any useful information about the transfer of proteins and/or nucleic acids, since Brachet (1945,1955) has argued that induction probably involves a rather massive passage of nucleoprotein microsomes from 1 Authors' address: Institute of Animal Genetics, West Mains Road, Edinburgh 9, U.K.