Normal mature thymocytes proliferate before emigrating to the periphery, an
d continuous bromodeoxyuridine labeling showed that more than 30% of fully
mature thymic emigrants have replicated DNA in the 24 h before exit. The pe
rcentage of DNA-synthetizing single-positive (SP) thymocytes is transiently
augmented during the postnatal period, with peaks on days 2 and 4 for CD4
and CD8 cells, respectively. Similar kinetics were observed in mouse chimer
as made by transfer of normal bone marrow cells into RAG-2-deficient mice.
These data show that proliferation of mature thymocytes is developmentally
regulated. The proliferation peaks (on days 16 and 18 post transfer) observ
ed in simple bone marrow chimeras were abolished when lymph node T cells we
re mixed with the bone marrow cell inoculum, suggesting that the peripheral
pool controls the late thymic expansion. The phenotype of cycling SP thymo
cytes is atypical. they do not regulate activation and adhesion surface mol
ecules like peripheral activated T cells.