Flow microfluorometric analysis of mouse thymus development in vivo and in vitro

The phenotypic properties of lymphoid cells in the developing embryonic thymus were characterized using monoclonal antibodies and flow microfluorometry. CBA/J‐T6/T6 thymocytes stained with antibodies directed against Thy‐1.2, Lyt‐1, Lyt‐2 or H‐2K^k were simultaneously analyzed for fluorescence intensity and forward light scatter (FLS), a cell size‐related parameter. Whereas Thy‐1 and Lyt‐1 antigens were already present on 15‐day fetal thymocytes, Lyt‐2 expression was first detectable on day 16 and increased rapidly thereafter to reach adult levels by day 19. Concomitant with these phenotypic changes, rapid changes in FLS occurred during this time period. The FLS distribution of Lyt‐2⁺ cells was initially homogeneously high (day 16) but became biphasic at days 17–18. Thereafter, the lower FLS subpopulation predominated. FLS changes in Lyt‐2⁻ cells could be dissociated kinetically from changes in the Lyt‐2⁺ subpopulation. Thus high FLS Lyt‐2⁻ cells were the predominant subpopulation throughout the entire fetal period and could still be detected after birth, when a population with lower FLS first appeared. The embryonic thymus developing in vivo was then compared with the 13‐day embryonic thymus maintained for 14 days in an in vitro organ culture system. Based on a combination of fluorescence and FLS analysis, the organ‐cultured thymus appeared to share certain phenotypic properties with the 18–19 day in vivo developing thymus.