Two subpopulations of guinea-pig thymocytes with different maturation stages.

Two subpopulations (F4 and F6) of guinea-pig thymocytes were separated by using bovine serum albumin gradient centrifugation. The majority of F4 thymocytes were weakly alkaline phosphatase (AP) positive cells, while most of F6 thymocytes were strongly AP positive. The significant difference between their AP activities was confirmed biochemically. In the ultracytochemical study the majority of unfractionated and F6 thymocytes were light and AP positive, whereas F4 thymocytes were mostly dark, weakly AP positive cells. F4 thymocytes responded well to PHA and Con A, while F6 thymocytes failed to respond to these mitogens. The subpopulations did not differ in their homing properties. These findings indicate that strongly AP-positive cells are immature thymocytes and weakly AP-positive cells represent a more mature subpopulation of thymocytes. A hypothetical scheme for differentiation of guinea-pig thymocytes is presented.