Origin and differentiation of lymph node and spleen progenitors during embryonic development.

Secondary lymphoid tissues, including the spleen and lymph nodes, play an essential role in supporting immune responses. These organs are structurally organized into specialized compartments in which the interactions between hematopoietic and stromal cells are crucial for immune cell function. In this Review, we examine the cellular and molecular mechanisms that control spleen and lymph nodes, primarily in mice, with a particular emphasis on the embryonic origins of the different cell types involved. We also discuss the processes that determine the acquisition of lymphoid and mesenchymal cell fate in these tissues, as well as the developmental relationships between embryonic progenitor cells and mature stromal cells. In addition, we address the importance of understanding the sources of progenitor cells in the context of organogenesis and highlight several unresolved questions.