Histochemical Studies of the Distribution of RNA in Tissues of the Developing Chick Embryo †

Quantitative studies of biochemical development in embryos frequently are inadequate without concurrent histochemical correlation because alone they offer only limited information about the distribution in tissues of the substances under study. This is especially true of the ribonucleic acids (RNA) of which only relatively few histochemical studies have been done in developing embryos. In the course of studies on the metabolism of nucleic acids and protein in the chick embryo, the distribution of RNA in various tissues of the developing embryo was investigated by adapting a fluorochrome technique to fixed histologic sections which localizes nucleic acids in cells. This report describes the fluorescent microscopy techniques employed and the findings in seven organs in the chick-heart, lung, skeletal muscle, gut, kidney, brain, and liver. No systematic histochemical study of RNA in developing organs in the embryo is known to have been reported previously. Fluorescent diaminoacridine dyes, particularly acridine orange, are used extensively to identify nucleic acids in cells, principally for the detection of cancer cells in smears by their increased fluorescence for RNA, but also for virus inclusions in cell smears and other studies of smears of cell cultures. There have been numerous criticisms of acridine orange fluorochrome techniques in fixed tissue sections, however. For example, certain other materials besides nucleic acids fluoresce with acridine orange in cartilage, elastic tissue, and mast cells and with certain acid mucopolysaccharides. More important objections have been a lack of uniformity and reproducibility of fluorescence as well as rapid fading and "unpredictable" color reversals of fluorescence, so that the same tissue areas alternately might fluoresce red or green. Controversial and paradoxical results such as these in tissues and