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In a recent cytuphotometric study of the effects of growth hormone on rat liver protein and nucleic acids, Di Stefano, et al. ( 1 ) reported that, within a given class of ploidy. the deoxyribose nucleic acid ( DK44) content of liver nuclei was not affected either by hypophysectomy or by the administration of a purified grolwth hormone preparation. However, differences were noted in ratios of diploid and tetraploid nuclei in livers of hypophysectomized rats as compared with hypophysectomized rats treated with growth hormone and intact control animals. The diploid, tetraploid ratios observed in the hypophysectomized animals were found to be consistent with the pattern seen in younger animals. while the pattern in hypophysectomized animals treated with growth hormone was characteristic of that found in older intact animals.In an attempt to arrive at a better understanding of the mechanisms responsible for these differences in ploidy patterns, it seemed pertinent to investigate the ploidy ratios in livers of hypophysectomized r'ats during the first 8 days postoperatively, and to correlate these with some index of mitotic activity in the samle livers.Methods. Hypophysectomized and intact male Sprague-Dawley rats were purchased from the Hormone Assay Laboratory. Chicago, Ill., separated into groups containing 10 animals per group and maintained on a diet of canned dog food which was fed ad Libitum. These groups were treated as follows: 1. One group of intact control animals (average body weight on arrival, approximately 75 g) was sacrificed when their average body weight reached that of the hypophysectomized animals. This group of animals (average body weight, 93.7 & 1.2 g at the time of sacrifice) -___-* Supported by a grant-in-aid from Amer. Cancer SOC. upon recom~mendation of $Corn*mittee on Growth of National Research Counoil.was comparable to the hypophysectomized groups on the day of the uperation and therefore constitutes what might be called the initial intact control group. 2. Five groups of hypophysectomized animalls were sacrificed 1, 2,4, 6 and 8 dmays postoperatively. The average weights of animals in these groups were 93. 2 t 2.2, 93.0 5 1.6, 91.1 k 1.6, 91.0 t 2.4, and 95.9 t 2.5 g respectively. 3. One group of hypophysectomized rats (8 days postoperatively) weighing approximately 92.7 t 1.6 g, were injected with 0.2 5 mg of a purified growth hormone preparation (Armour & Co., Lot. No. R491017)t twice daily by intramuscular injection for 7 days and sacrificed on the following day. At the time of sacrifice the average body weight of animals in this group was 137.0 t 2.3 g. 4. One group of intact control animals weighing approxitmately 91.6 t 0.6 g, were given 0.25 ml of 0.85% saline as before twice daily for 7 days and sacrificed on the following day. The average body weight of the animals in this group at the time of sacrifice was 142 t 1.6 g. All animals were sacrificed by decapitation, and the livers removed rapidly and weighed. Small pieces were fixed immediately in 2 5% neutral formalin, washed in r...

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