339 Int. J. Morphol., 24(3):339-348, 2006. The Spleen of a Specialy Adapted Mammal: The Little Hairy Armadillo Chaetophractus vellerosus, (Xenarthra, Dasypodidae). A Light and Electron Microscopic Study El Bazo de un Mamífero con Adaptaciones Especiales: el “Peludo Chico” Chaetophractus vellerosus, (Xenarthra, Dasypodidae). Estudio con Microscopía de Luz y Electrónica * Elena Juana Galíndez; * Silvia Estecondo & ** Emma B. Casanave GALÍNDEZ, E. J.; ESTECONDO, S. & CASANAVE, E. B. The spleen of a specialy adapted mammal: The little hairy armadillo Chaetophractus vellerosus, (Xenarthra, Dasypodidae). A Light and electron microscopic study. Int. J. Morphol., 24(3):339-348, 2006. SUMMARY: The spleen is a multifunctional organ. Its microarchitecture reflects so well the phylogeny as the adaptation to niche in each animal group and inclusive in each species. Armadillos, as ancient and isolated mammals, show a mixture of ancestral and modern structures. In this work the microanatomy and cytology of the spleen of the little hairy armadillo Chaetophractus vellerosus was studied. The material was fixed and processed according to the typical methods for light and electron microscopy. Results show a cumulative medium sized spleen with the typical pulps. The white pulp is well developed but less defined than in other armadillos. The presence of follicular-dendritic-like cells and interdigitant dendritic-like cells, as well as clusters of immunocompetent cells agrees with an active immunological activity. The red pulp is a meshwork of circulatory spaces and cells. The presence of splenic sinusoids is recognized for the first time in the genus. Hemopoietic tissue is less developed than in other Euphractini. The microarchitecture and size of the spleen of C. vellerosus is discussed in a phylogenetic and adaptive context. KEY WORDS: Xenarthra; Dasypodidae; Armadillos; Spleen; Chaetophractus vellerosus. INTRODUCTION The spleen is an important organ centrally interposed in the systemic circulation related with the immune, haemocatheretic, flow regulation and myeloid functions. Since their appearance, included in the gut of Agnatha and as an extramural structure from chondrichthyes (Zapata et. al., 1995), it proportionally mixed these functions, according to their architecture and the needs of each animal form. In the beginning, the spleen was a myeloid organ with some immune functions, but at the same time as the different groups appeared and radiated, the microanatomy was remodelled and the immune, myeloid, haemocatheretic and flow regulation functions take different relative importance (Galíndez & Aggio, 1997). Mammals are a good example. In Prototheria, the spleen is mainly a myeloid organ with a scarce immunological importance (Tanaka et al., 1988). In Metatheria (Cisternas & Armati, 1999) and Eutheria (Hartwig & Hartwig, 1985) the splenic structure is similar. The ancestral group Insectivora shows an important splenic myeloid activity (Tanaka, 1994); meanwhile the great diversity of mammal’s orders shows an extensive spectrum in each function, according to phylogeny and/or adaptation (Hartwig & Hartwig). The armadillos are an ancient eutheria group appeared in the limit between the Cretacic and the Tertiary, that has been isolated since their origins to the beginning of the Eocene (Delsuc et al., 2001). They are semifossorial animals with an unusual response to CO 2 increase (Boggs et al., 1998). Moreover, this group has generated interest because their importance as biomedical models in various human and * Lab. Histología Animal, ** Lab. Fisiología Animal, Universidad Nacional del Sur, Dpto. Biología, Bioquímica y Farmacia, Bahía Blanca, Argentina. ** Miembro de la Carrera de Investigador del CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), Argentina. Trabajo subsidiado por SGCyT, UNS 24 B086 y 24B122; y ANPCyT, BID 1728 OC-AR, PICTR 074/03.