Localization of eosinophils in the thymus by the peroxidase reaction

Thymuses of human fetuses and infants and of young mice were investigated histochemically for peroxidase. Eosinophils were shown to be the only peroxidase-positive cells in the thymus. In human thymuses the eosinophilic cells were predominantly localized in medullar areas, with concentration of cell clusters at the cortico-medullar junction, around or inside Hassall's bodies and occasionally in high numbers in the intraseptal vessels of the cortex. In the normal mouse the eosinophils were evenly distributed throughout the medulla. Treatment with corticosteroids or X-rays produced a severe involution of the thymus with concommitant change in cellular pattern. The central areas of the thymus residue contained lymphocytes while the peripheral regions consisted of reticuloepithelia, macrophages and numerous eosinophils. Azathioprine did not change the morphology of the thymus. The numbers of eosinophils were slightly reduced, the distribution pattern remaining unchanged.