Thymic nurse cell complexes (TNC-c), isolated from mouse thymuses at 1 and 2 h after i.v. injection of 6-(3H)thymidine, were analyzed in autoradiographs of semithin serial sections with regard to their size and the distribution of labeled thymocytes in individual types of complexes. The total number of thymocytes per complex reflects the type of complex. In a parallel study, localization of labeled thymocytes within individual zones of thymic cortex was examined. Thymocyte division within complexes may yield sequential complex generations differing in number per complex. However, thymocytes within complexes differ from each other in division kinetics. Half of the thymocytes that had been labeled 1 h after injection divided within 2 h. The rapidly dividing fraction of thymocytes were distributed within small complexes containing 2–8 cells and corresponded to the distribution of labeled cells in the outer thymic cortex. The proportion of labeled cells within large complexes resembled the distribution of labeled cells in the deep cortex. The data support the view that microenvironmental factors within TNC-c are responsible for both inducing thymocytes to enter the cell cycle and the negative selection (cell death) of some thymocyte
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