Although the thyroid parenchyma is characterized by high proliferative ability, mitoses in it are seldom observed. Earlier it was considered that the main type of thyrocyte reproduction was amitotic division. However, biochemical investigations of late showed that the thyroid parenchyma during rapid growth in tissue culture intensively incorporated thymidine, this being an evidence of DNA replication indicating mitoses. To overcome this contradiction we staged experiments on adult male rats. First, their thyroids were slightly activated with thyrotropin at small doses, then in 6-8 h (when the number of amitosis-like thyrocyte nuclei increased markedly) the thyroids were excised and after proper histological treatment their sections were subjected to hydrolysis in IN HCI for the releasing of thymidine from DNA. Then thymidine was stained with Schiff reagent and its amount determined by cytophotometry. All the experimental animals displayed thymidine level redoubling, which demonstrated an authentic DNA replication despite the failure of mitoses. Thyrocyte nuclei suffered narrowing or constriction in their middle or were separated into two daughter nuclei. Thus in these cases DNA replication coincided not with mitoses but with endomitosis, as far as the nuclear membrane in thyrocyte nuclei was preserved and chromosome spiralization and formation of achromatic spindle failed. The physiological role of endomitoses in the proliferation of the thyroid parenchyma was that they ensured its more rapid growth than that resulting from karyokinesis.
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