Composition and Function of Haemolymphatic Tissues in the European Common Shrew

Bray, Daniel P.; Bennett, Malcolm; Stockley, Paula; Hurst, Jane L.; Kipar, Anja

Our study is the first to examine haemolymphatic tissue structure, compoosition and function in shrews using modern techniques, and one of only a small number to explore immune responses of wild animals in their natural environment. With regards to both morphology and composition, the spleen, lymph nodes, thymus and bone marrow in S. araneus were found to be very similar to their equivalents in other mammalian species [39, 42] . In common with a number of species, including the musk shrew Suncus murinus, both bone marrow and spleen were identified as sites of haematopoiesis in S. araneus [43] . The results of our study also confirm that the pancreas of Aselli, which is specific to shrews, can be considered as a large, specialised lymph node [33] . The presence of a cortex with both follicles and paracortical T cell zones renders previous controversial assumptions regarding the organ's function incorrect: the pancreas of Aselli is neither a specific site of exclusive B cell production nor a functional analogue of the bursa of Fabricius in birds [35] . However, it differs from normal lymph nodes in that the centre (medulla) contains a very high proportion of plasma cells. In adulthood, the number of plasma cells and the relative size of the medulla seem to increase, until almost the entire organ is composed of plasma cells. Such a feature has not been described under physiological circumstances in any other species, and suggests that the pancreas of Aselli in S. araneus functions as a storage site for plasma cells, particularly in older animals. Lymph nodes in S. araneus were also found to contain a higher number of plasma cells than normally observed in other species [39, 42] , which may emphasise a general tendency towards progressive plasma cell storage in common shrews.