The human thymus. II. Partial characterization of thymocytes separated on bovine serum albumin gradients

Thymocyte populations from 9 postnatal normal human thymuses (9 mth to 14 yr) were partially characterized after separation from structural elements by mechanical dissociation or spontaneous release from short-term explant cultures. Populations were fractionated on a 17 to 35% discontinuous bovine serum albumin (BSA) gradient and fractions characterized by morphology, pulse labeling index, spontaneous incorporation of labeled thymidine and uridine, E-rosetting ability (4 degrees C) and terminal deoxynucleotidyl transferase (TdT) activity. The following observations were made: (1) the gradient distribution of lymphocytes followed a bell-shaped curve, but with a greater percentage of less dense cells in older thymuses; (2) 60 to 80% of lymphoblasts (3-7% of total thymocytes) were in the 21-23% BSA fractions; (3) spontaneous incorporation of labeled thymidine and uridine was highest in 21-23% fractions; (4) 40 to 55% of lymphoblasts were in active DNA synthesis; (5) less than 50% of lymphocytes from the 17 to 21% fractions formed E rosettes compared to 95% of unfractioned cells; (6) one or two distinct peaks of TdT activity were identified; four thymuses had one peak in the small/medium lymphocyte fractions, whereas in two specimens a second peak was seen. Enrichment of lymphocyte subpopulations of the human thymus for immature cells may aid in further delineation of thymic function in developing man.