Fragments and suspensions of human fetal thymus were incubated in the presence of3H-TdR to permit study of the distribution and morphology of DNA-synthesizing cells. Results of light and EM autoradiography showed that 1. although DNA-synthesizing cells were present in the medulla, the vast majority of these cells were localized in the thymic cortex, 2. cells with the typical EM appearance of small lymphocytes and lymphoid blast cells both synthesized DNA, and 3. cells in S-phase were predominantly 8 to 12 μm in size
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