Development of the embryonic chicken thymus: I. Characteristic synchronous morphogenesis of lymphocytes accompanied by the appearance of an embryonic thymus-specific antigen

Abstract

Differentiation of the lymphoid cells of embryonic chicken thymus was studied morphologically and serologically from the tenth day of incubation to hatching. Lymphoid cells became markedly smaller in size at two stages, from the eleventh to the thirteenth and from the fifteenth to the sixteenth days of incubation. During the first stage, lymphoid cells changed notably in other criteria as well. The mean generation time was prolonged from approximately 12 to 36 hr on Day 13. The proportion of those cells that possessed embryonic thymus-specific antigen synchronously increased from 4–7 to 60–75% during the 24-hr period between the twelfth and the thirteenth days and reached a plateau level on Day 14. Morphologically, cytoplasmic processes became conspicuous in lymphoid cells on Days 11 and 12. These processes probably separated from the main body of the actively transforming lymphoid cells. In this phenomenon, a septum composed of microfilaments and small vesicles was formed at the neck of the process. Between the twelfth and the thirteenth days, the majority of lymphoid cells took the features typical of differentiated lymphocytes, i.e., small round shapes with scanty cytoplasm and distinct heterochromatin in their nuclei.