Development of Thy 1 positive mouse thymocytes. Analysis of early fetal thymocytes by flow microfluorometry

Early mouse fetal thymocytes were analyzed for cell size and amount of Thy 1 by flow microfluorometry. There were very few Thy 1 positive cells in the embryonic thymus of the 13th day; in the 14th day fetal thymus, the number of Thy 1 positive cells increased, though the Thy 1 content remained at relatively low levels. Both the number of Thy 1 positive cells and the content of Thy 1 greatly increased by the 15th day, but the Thy 1 content was still definitely lower than that of adult thymocytes. These results indicate that Thy 1 negative thymocyte precursors increase in Thy 1 content gradually when they differentiate into thymocytes. The present study also suggests that immature thymocytes like the 15th day fetal thymocytes, even though they are definitely Thy 1 positive, bear Thy 1 at much lower levels than most of the adult thymocytes, indicating a distinct difference in Thy 1 content between immature thymocytes and most of the adult thymocytes.