THE RELEVANCE OF CELL MICROENVIRONMENTS FOR THE APPEARANCE OF LYMPHO-HEMATOPOIETIC 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 function ing 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 histophysiolo gical results which emphasize the relevance of lympho-haemopoietic str omal elements in the emergence and evolution of vertebrate lymphoid or gans. The most primitive vertebrates, the Agnatha, have no true lympho id organs and, accordingly, their immune responses seem more related t o 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 con tain numerous organs (i.e., gonads, kidney, brain, etc...), the cell m icroenvironments of which support lympho-haemopoiesis mimicking the co ndition of higher vertebrate bone marrow. On the other hand, the lack of germinal centres, another feature of the lymphoid organs of ectothe rmic vertebrates which impedes the selection of B cells raised after s omatic hypermutation, presumably reflects the absence of some of the e lements necessary for this organization.