Cytochemical Reactions of Nucleic Acids

BY ETHEL SUE LUMB Zoology Department, Washington University, St. Louis (Present Address: The Biological Laboratories, University ofRochester) IN current biological nd biochemical research a great deal of attention is being directed toward the nucleic acids and nucleoproteins and the role, or roles, which they play in life processes. Through the efforts of the cytochemist the nudeic acids may be identified, localized, and quantified within individual cells. Such cytochemical observations afford an important link between the chemistry of tissue extracts and the physiology of the living cell. As Danielli has emphasized (1946, 1947), the validity of the observations, and consequently of the hypotheses drawn from them, depends on the reliability of the methods by which the material is prepared. Although some of the older papers dealing with the cytochemical techniques have been summarized in the reviews by Gersh (1941) and Dempsey and Wislocki (1946), there has been no comprehensive review of the more recent literature. The purpose of this paper is to review the most pertinent literature concerning the cytochemical techniques for studying the nucleic acids and to discuss their validity as specific tests. When the cytochemist attempts to interpret he results of qualitative or quantitative tests run on tissue sections, he is confronted with many unique and baffling problems. Chemical reactions within the cell are seldom, if ever, subject to the controlled conditions of test-tube chemistry, but take place within the peculiar chemical and physical conditions found in protoplasm. Furthermore, most of the present cytochemical reactions are run on dead cells. Since very little is known of the effect of death by various causes on the physical and chemical condition of protoplasm, any attempt to extrapolate from the dead to the living state is fraught with danger. Thus, the very nature of the material imposes on the cytochemist limitations which must be kept in mind in evaluating the techniques and in interpreting the results. More specifically, in considering the validity of any cytochemical technique, special consideration should be given to the following points: (1) the specificity of the chemical reaction between the substance to be identified and the test reagents, (2) the accuracy and the ease with which the reaction product may be detected within the cell, (3) the degree to which the protoplasmic onstituents are retained with a minimum distortion from the living conditions, (4) the extent to which the protoplasmic constituents or the reaction products are diffusible, (5) the degree to which the reacting groups come into contact with each other and are free to react, that is, are not bound in such a way that the expected reaction cannot occur. If quantitative determinations are desired, consideration must be given to such additional factors as: (6) the number of reacting groups which may be involved in the dye binding or the color reaction, (7) the orientation and aggregation of the reacting protoplasmic material or the reaction products, (8) the exact thickness of the cytological preparation, and (9) the sensitivity and accuracy of the means of determining the amount of light absorbed by the specific ompounds being analyzed.