Populations of lymphocytes separated from human thymocytes

Thymocytes from twenty-two human foetal and post-natal thymuses were separated according to their buoyant density. Thymocytes from eight were separated into multiple fractions by means of continuous gradients of bovine serum albumin (BSA), pH 5.1 and iso-osmolar with human cells, and thymocytes from fourteen were separated into two fractions of density less than and greater than 1.068 g/cm3. Fractions were tested for antigen-binding lymphocytes (ABL) to 125I-labelled human thyroglobulin, for response to phytohaemagglutinin (PHA) and for rosette-forming cells (RFC) using sheep red blood cells. For subjects of all ages there was a pronounced enrichment of both ABL and thymocytes responsive to PHA amoung low or 1.064-1.065 g/cm3 density thymocytes. In older subjects there was a second enrichment of ABL among high or 1.072-1.073 g/cm3 density thymocytes. RFC were distributed over a wider range of densities, and although they did not form a discrete subpopulation they predominated among high density thymocytes.