Thymocyte subpopulations in young and adult mice. I. Separation by density gradient and steroid treatment.

Thymocyte subpopulations obtained after centrifugation on Ficoll density gradients have been studied in young and 6-week-old mice. Whereas the totality of the thymocytes appeared theta-positive, gradient fractionation showed the existence of a subpopulation of immunoglobulin and theta-negative lymphocytes among the lightest cells. Similar separation was obtained in both age groups in terms of the distribution of such theta-negative cells as well as of phytomitogen responses. However, cell pools with characteristics of cortical cells were less well represented in 2 to 10-day-old mice than in adult mice, indicating that post-natal thymic development is essentially associated with the appearance of cortical cells. The gradient did not however, provide a clear separation assessed on other parameters such as theta antigen, concanavalin A and phytohemagglutinin responses and steroid sensitivity. Cellular heterogeneity for a given cell density is discussed in relation to the hypothesis of two separate maturation pathways of thymocytes.