Localization of terminal deoxynucleotidyl transferase (TdT) in rat thymus using immunoperoxidase methods

Indirect and peroxidase anti-peroxidase (PAP) immunoenzymatic methods were used to detect terminal deoxynucleotidyl transferase (TdT) in imprints and formalin-fixed paraffin sections of normal rat thymus. TdT is found in the nuclei of small lymphocytes in imprint samples from neonatal and adult rat thymus, showing granular or circular patterns of peroxidase reaction products. Diffuse brown reaction products of peroxidase are located in both the nuclei and cytoplasm of medium and large lymphocytes. Indirect measurements show that, as age progresses, the percentage of peroxidase-positive cells decreases in all types of lymphocytes, from 72.4% on the 11th day to 54.8% in the 5th month, whereas that of negative cells increases from 14.4% to 39.4%. In formalin-fixed paraffin sections, peroxidase-positive lymphocytes are found mainly in the cortex and cortico-medullary boundary, and only rarely in the medulla.