Ultrastructural localization of Terminal deoxynucleotidyl Transferase (TdT) in rat thymocytes.

TdT positive cells in rat thymus belong to distinct subsets as shown by light and electron microscopic immunocytochemistry. Using polyclonal antibodies to calf TdT and peroxidase labeled goat anti-rabbit IgG it has been possible to identify several subpopulations of TdT positive thymocytes. Large blasts corresponding to thymocytes at early maturational stages, are strongly positive for TdT which is diffusely distributed in both the nucleus and the cytoplasm. Smaller cells which correspond presumably to more advanced stages of maturation display nuclear TdT only, or are negative. Ultrastructural analyses of TdT indicate that the localization of the enzyme is related to the morphological features of the cells and TdT expression corresponds to maturational stages of T-cells.