Figure 1. General scheme of secondary lymphoid organ development.

As noted in the text of the review, the details in the developmental pathways of Peyer’s patches and individual lymph nodes are often slightly different. Therefore, this scheme incorporates elements of both Peyer’s patch and lymph node development and outlines major steps that are likely to be in common in these pathways. Importantly, this scheme implies several events that are purely speculative. For example, it is unknown whether LTo cells are the direct precursors of mature stromal cells and follicular dendritic cells (FDCs). It is also unclear whether LTi cells and LTin cells are maintained in mature secondary lymphoid organs in adults.