The morphogenesis of the neonatal spleen, mesenteric node, and Peyer's patches has been characterized in terms of the distribution of T antigen-positive and immunoglobulin-positive cells by using immunofluorescent markers. Newborn BALB/c mouse nodes have many T, but few Ig-bearing cells; conversely, the spleen at this stage contains only rare T cells, while prominent accumulations of Ig-positive cells occur surrounding conspicuous vessels. At about 3 days of age, T cells appear in increasing numbers in a perivascular position, displacing the cuffs of Ig-positive cells, and thus the basic pattern of organization charateristic of the adult splenic white pulp is established. The mesenteric node and Peyer's patches, in the early neonatal period, are both characterized by the presence of scattered Ig-bearing cells, in contrast to the highly organized distribution in the spleen. By the 3rd or 4th day, however, the Ig-positive cells of the nodes and patches form accumulations distributed in roughly the adult pattern. The paucity of T cells in the spleen and of Ig-bearing cells in the mesenteric node of the newborn has been shown not to be due to their failure specifically to trap infused adult T and B cells.

Kinetic analysis of Ig-positive cell accumulation reveals that such cells in the neonatal mesenteric node are probably not generated in situ. Moreover, the level of proliferation in the Peyer's patches is insufficient to account for the increase in even the splenic Ig-bearing cells, and it is thus highly unlikely, at least in the neonatal mouse, that the Peyer's patches are analogs of the avian Bursa of Fabricius.