produced proportionately less impairment to this capacity for glandulap hypertrophy. (An example of the effect on the gland weight under the influence of the stimulus is shown from previous experiments in Figure 5.) Even though there was no obvious evidence of architectural damagp to the thyroid, it was believed that these glands had sustained some intrinsi physiological impairment because there was an increase in cell height. The histoche method showed that in the nonstimulated nonirradiated thyroid the DNA of ea J nucleus was quite uniform from cell to cell. See Figure 8A. The stimulus of thiouf aione resulted in sane miclei containing more DNA than the average resting c bart in none did the amount exceed two times the diploid value. Some of these cellB were presimec to be preparing to divide. After 10 we of 1311 the DNA content and showed slightly more variation. nublear size The variation in DNA among individuallcells was expressed as a standard deviation of the mean. Some nuclei of mdiatefi thyroids contained far more DNA than two times the diploid value, at which poing the cells should have divided. The administration of thiouracil to the animals irnadiated glands caused an i content. The effects of 30 pe of ith slightly variation in nuclear size and injthe DNA I was even more striking. The nuclei with excessive DNA appeared even without the superimposed greatly exaggerated when the stimulus of thiouracil was applied. lafge stimulus. irregular This was Further experiments, lasting over several additional years, mcorejanimals and a greater range of dosage of I were used (0, 5, 10, 20 and 40 pix; plowed te intervals up to ls years before sacrifice). After doses of as little ab 5 pe, there was little if any evidence of nuclear change; but when thiouracil was buperimposed, the variation in DNA content in nuclei again appeared. At levels of 2) and 30 pe there was spontaneous variation in DNA content. Examples of occasional. nuclei _ containing multiples of approximately 4, 8, and 16 times the diploid falues of DMA were found in same of these large bizarre nuclear forms. It was postugated from these experiments that the cells had the capacity to build up DNA when] for some reason cell division was thwarted. stimulated but This histochemical method of measuring DNA in nuclei may be quesfioned as to preciseness by some workers. However, if the staining procedure is ve strictly controlled when comparisons are made with nonirradiated animals and abgforbance is measured in a plug of nuclei that closely approached the size of the tole “nucleus, t method has proved in our hands to be quite reproducible and very use in interpreti the nature of the large bizarre nuclear forms that had previously beenjfobserved by ordinary histological techniques in irradiated thyroids. The results $f these experiments carried cut over a number of years were published in consiferable detail. Dobyns, B.M., and Didtschenko, I.: Radiation from Radioiodine. Nuclear Changes in Thyroidal Epith@lium Following J. Clin. Endocrinol. & Metab. 21: The The Large Bizarre Nuclear Forms in Human Thyroid Tissue and the Them in 699-728, 1961. Measurgment cr DNA From time to time there have been opportunities to procure supgical means samples of thyroid tissue from patients previously treated with 1S I. Having firnly established the method of Feulgen staining and quantitative microspectfcchotcmetry