Development of T and B cell areas in peripheral lymphoid organs of the rat.

In the present study the early development of peripheral lymphoid organs (spleen, popliteal lymph node, mesenteric lymph node and Peyer's patches) is described in terms of homing patterns of T and B cells, demonstrated with immunohistoperoxidatic detection of characteristic membrane antigen in normal rats and with routine histology in neonatally thymectomized rats. In the first days after birth the peripheral lymphoid organs are almost exclusively populated by T cells. After neonatal thymectomy lymphocytes appear in the dome areas of Peyer's patches from four to six days after birth, in mesenteric and popliteal lymph nodes lymphocytes are found in the outer cortex from day 6 and day 8 respectively and in the marginal zone of the spleen from eight days onwards. These lymphocytes showed no membrane staining when reacted for T antigen with immunohistoperoxidatic techniques. The morphological evidence for considering Peyer's patches of rats as central inductive sites for the generation of B cells is poor. The discrepancy in the order of appearance of T and B cell (sub)populations in spleen compartments in normal ontogenetic development and lethally irradiated, stem cell reconstituted animals is discussed.