Normal thymuses of young-to-adult healthy dogs have been studied with light and electron microscopy. Two well-defined zones are observed both in the medulla and in the cortex. In the outer cortex including the subcapsular region, reticular cells and mitotic lymphoblasts are more numerous than in the inner cortex, whereas in the central portion of the medulla the small proportion of free lymphoid cells is more evident than in the outer medulla. Reticular cells can be classified into two main groups according to some morphologic features: epithelial and mesenchymal. Epithelial reticular cells seem to undergo a gradual transformation from the subcapsular region to the medulla, and finally they may become heterogeneous Hassall corpuscles. Mesenchymal reticular cells are represented by macrophages and dendritic-like cells. The latter cells are located in the inner medulla and fill the intercellular spaces with their long cytoplasmic processes. In some occasions these processes appear degenerated and plasma cells and eosinophilic leucocytes are found. This fact suggests different functional stages and a possible participation of the normal thymic medulla of the adult dog as a peripheral lymphoid organ.
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