The relevance of cell microenvironments for the appearance of lympho-haemopoietic tissues in primitive vertebrates.

In higher vertebrates, mainly in mammals, a role for the non-lymphoid components of lymphoid organs in governing the maturation and functioning of immune system has been largely demonstrated. In contrast, such a role in the evolution of the vertebrate immune system has only been evidenced indirectly. In the present review we summarize histophysiological results which emphasize the relevance of lympho-haemopoietic stromal elements in the emergence and evolution of vertebrate lymphoid organs. The most primitive vertebrates, the Agnatha, have no true lymphoid organs and, accordingly, their immune responses seem more related to the non-anticipatory defence mechanisms of invertebrates than to the immune responses of vertebrates. So, the appearance and evolution of vertebrate lymphoid organs seems closely related with the emergence of immune capacities. Thymus, spleen and gut-associated lymphoid organs appear early in phylogeny whereas lymph nodes and bone marrow are late phylogenetical adquisitions. However, bone marrowless vertebrates contain numerous organs (i.e., gonads, kidney, brain, etc...), the cell microenvironments of which support lympho-haemopoiesis mimicking the condition of higher vertebrate bone marrow. On the other hand, the lack of germinal centres, another feature of the lymphoid organs of ectothermic vertebrates which impedes the selection of B cells raised after somatic hypermutation, presumably reflects the absence of some of the elements necessary for this organization.