3H-actinomycin D binding to ultrathin section of plastic-embedded Locusta migratoria testicular tubules. Improvement of the technique and further characterization of the reaction.

Semi-thin and ultrathin sections of locust testes have been incubated in 3H-actinomycin D solution and submitted to radioautography. The improved technical conditions described allow the reproducible obtainment of cell radioautographs with a moderate nuclear labelling and a very low nonspecific background which are usable for semi-quantitative results. Extraction with enzymes (DNase, RNase, pronase) or concentrated salt solution have been carried out before 3H-Actinomycin D treatment in order to characterize the reaction. The semi-quantitative results obtained at the light microscope level suggest that, in relation to the structural and chemical changes which occur in chromatin during spermiogenesis, some proteins may be easily hydrolysed in early spermatids. In ultrathin sections of spermatocytes the X chromosome is heavily "stained" with 3H-Actinomycin D, while 3H-uridine is not incorporated into the sex chromatin. These results are discussed in the light of current ideas on the constitution of active chromatin.