The Measurement of Desoxyribonucleic Acid in Individual Thyroid Cells Prebicusly Subjected to +°4I Radiation As this project has progressed, emphasis has shifted purely physiologic toward the morphologic changes caused by hat more From the I radiatio he Zhe large bizarre nuclear forms originally found and described at the beg ine c= Since ordinary stainizg ne=nods Suggest the presence of excessive amounts of chromatin material in the Sifarre te (DT this project have received increasing attention. forms that were produced by small doses of 431] plus thiouracil stimlati¢My 2 nmcczar seemed appropriate to try to measure the amount of DNA actually containedjin such muclei. Therefore, a histochemical method for measuring desoxyribonucleig acid in these large nuclei of irradiated thyroids was adapted to this objective. The method of Pollister for the quantitative microspectr sphotometrig method of measurement of Feulgen stain within a nucleus was set up in our laborajory with the suggestions and advice of Dr. Pollister of Columbia University. Si the Feulgen stain combines with DNA in a quantitative fashion, the amount of jtain contained within a nucleus could be measured by absorbance of transmitted} orzochronm light having a specific wave length at which there was maximal absorpticn[far this stain. Rat thyroids were used in these studies. The staining procedure yas carried out under highly controlled conditions. The irradiated and nonirradiated [tissues which were to be compared were placed side by side on the same microscopid slide so that both tissues were exposed to exactly the same steps of preparation afd staining circumstances. Measurements were made under oil emersion magnification. With the use of a calibrated diaphram it was possible to measure the transmission qf l=ght through a piug of nucleus which was slightly less than the miclear diametqr. The _ value for absorbance in the plug was then converted to the sphere, which mepresentec the volume of the micleus. The mean diameter of each nucleus was determirpd DSy direct micrometer measurements. DNA was recorded in arbitrary units. Tha vate for a normal resting micleus was determined and standardized against rat li erp ceiis. A value approximately equal to the diploid value in the rat cell was obtaing#i /&=<m measurement on the liver. When thiouracil which is a powerful stimulus to hyperplasia of the thyrcid ws given to an animal, the diameters of the nuclei of normal thyroid cells :npre=sed. The DNA also increased toward values which represented two times the dipidid w=lue, 2t which point the cell normally is expected to divide. Rats were given 10, 30, or 100 pic of radioiodine after a temporary ppriod cf codideficiency to inhance the uptake of the 131], Thereafter a diet containing a moral amount of iodine was resumed. One half of each group of animals was then Bivem thicrec in their drinking water. Same of each group of animals were sacrificed atj16 weexs, the remaining at 26 weeks. None of the doses of 1311 used in this series of experiments was sufficr=ntly great to cause architectural distortion of the thyroid. However, the largRst Sioses were sufficient to interfere with the expected increase in thyroid weight usually produced in normal thyroids by thiouracil. The lesser doses of fhat is ragiat=on