GUT EPITHELIUM - A FIRST LEVEL LYMPHOID ORGAN

A new theory implies that the epithelium of the gut in e.g. fishes is a first level lymphoid organ with the same influence on lymphocytes and lymphoid tissue as the bursa Fabricii in birds. The mammals may be in the process of developing a special bursa equivalent-the epithelium covering the gut-associated lymphoid tissue. But even in mammals the epithelium of the whole gut may function as a bursa Fabricii. In this investigation a series of questions about the kinetics of the lymphocytes within the gut epithelium in germfree and conventional rats has been asked and partly answered. At least 2-3 per cent of the lymphocytes within the gut epithelium, here called theliolymphocytes, synthesize DNA in situ. As many as 12 per cent of the theliolymphocytes at the tip of the villi of the small intestine are less than three days old. These data, and other data on the theliolymphocytes given in this paper, can be best explained by the assumption that theliolymphocytes are to a large extent immigrants, and that these immigrants constitute a selection of young lymphocytes compared to the ordinary mixture of mostly old blood lymphocytes. There is some weak evidence that the theliolymphocytes do not leave the body together with the epithelial cells at the tip of the villi, but reenter the lamina propria. The absence of clearcut differences between germfree and conventional animals regarding the kinetics of the theliolymphocytes supports the original theory. The theory discussed in this article was partly based on the assumption that fishes, amphibians and reptiles did not show demonstrable gut-associated lympho-epithelial organs, to which a bursa function could be ascribed. However, such microscopical lympho-epithelial organs have recently been demonstrated in the gut of fishes and reptiles (Fichtelius, Finstad, Good, to be published). The structure of all these organs is characterized by a close spatial relationship between epithelium and collections of lymphocytes within the lamina propria of the gut. The kinetics of theliolymphocytes and proprialymphocytes is still very interesting, even if the diffuse bursa equivalent of vertebrates very probably includes both the epithelium and part of the proprialymphocytes. © 1968.