Earliest lymphoid colonization of neonatal rat lymph nodes: an antigen-specific process?

The present work studied the little known process of lymphoid cell colonization of neonatal lymph nodes, while considering the nodal site of entry of circulating lymphoid cells and the either random or antigen-specific character of the process. Tissue sections of a mesenteric, cervical and popliteal node from each of 57 rats, aged 4 hours to 3 weeks, were analysed. Observations bear on the relative importance of the implication of the subcapsular sinus versus venules of nodes, and the composition of their emerging lymphoid cell population by determining the proportion of lymphocytes and blast-related cells. At 16-20 hours after birth, cell counts yielded a mean proportion of 84% for blast-related cells which decreased to 18% at 3 weeks. These percentages are compatible with values expected for a selective antigen-specific entry of lymphoid cells in nodes, not with values that would result from a random entry of lymphocytes. Moreover, observations revealed that by far most colonizing cells initially enter nodes carried by the afferent lymph, little via their venules.