125I-DU (5-IODO-2′-deoxyuridine) in autoradiographic studies of cell proliferation

Abstract

5- 125I-2′-deoxyuridine ( 125I-DU) was used as thymidine analogue for autoradiography of bone marrow cells of the rat. It was shown that 125I (gamma, 35 keV) gives an autoradiographic resolution sufficient for determining the relative labeling intensities of single cells. 125I-DU is incorporated, following a single injection, into the same type of bone marrow cells and yield labeling indices nearly identical to those observed with 3H-thymidine. A comparison of the relative labeling intensity of myelocytes and segmented neutrophiles at 1 hr 1 day and 3 days after injection of 125I-DU indicates that IDU is not lost from the DNA in these cells during division and subsequent maturation. The data obtained with 3H-thymidine is comparable to that obtained with 125I-DU. Since IDU is reutilized inefficiently or not at all on DNA catabolism in contrast to thymidine, single cell autoradiography can be used to study the effect of thymidine reutilization on measurements of cell proliferation and DNA renewal made with labeled thymidine. An investigation of the distribution of labeled cells of the megacaryocytic series at 1 hr and 3 days after a single injection of 125I-DU or of 3H-thymidine shows the effect of thymidine reutilization likely responsible for a near 100 per cent labeling of all megacaryocytic forms at day 3 after injection of 3H-thymidine. The data also indicate that a “quiescent stem cell” with regard to DNA synthesis cannot be assumed to be the only immediate precursor for megacaryoblasts. These precursor cells must synthesize DNA at least once within 3 days prior to differentiating into recognizable megacaryocytic elements. The overall mitotic index in the bone marrow did not change over a period of 3 days attesting the lack of recognizable radiation effects from 125I and of metabolic toxicity of IDU at the dose level used in the experiments. The autoradiographic efficiency of 125I in comparison with 3H under the conditions of the experiment is discussed.