Effect of Hydrocortisone on the Utilization of Tritiated Thymidine for Skeletal Growth in the Rat.

Thymidine is a specific precursor of deoxyribonucleic acid (DNA) and is utilized for chromosomal replication by the nuclei of cells when they double their DNA content in interphase before cell division (Reichard and Estborn, 1951; Friedkin, Tilson, and Roberts, 1956; Gall and Johnson, 1960; Mirsky and Osawa, 1961). Autoradiography after the administration of thymidine labelled with tritium (3H-thymidine) to experimental animals is now established as a valuable tracer of DNA synthesis in actively dividing cells, for the energy of the beta radiation emitted by tritium (0 018 MeV) is sufficiently low to allow precise localization of the label in single nuclei that are preparing to divide. Furthermore, the time taken for DNA synthesis is considerably longer than that required for actual nuclear division, so that a 3H-thymidine labelling index becomes a more sensitive indicator of cell renewal and proliferation than conventional assessments of the mitotic index (Cronkite, Bond, Fliedner, and Rubini, 1959; Leblond, Messier, and Kopriwa, 1959; Crane and Dutta, 1963). In the experiments described in this paper, we have used this technique to identify the various cells in normal rat skeletal tissues that utilize labelled thymidine and have measured the depression in the labelling index that results from treatment with hydrocortisone.