The thymic epithelial reticulum and interdigitating cells in SIV-induced thymus atrophy and its comparison with other forms of thymus involution.

Alterations in the thymus were investigated in the early course of SIV infection of rhesus monkeys and compared with age-related and acute accidental thymus atrophy. The SIV-induced pathology was characterized by shrinkage of the thymic parenchyma and capsule, whereas in age-related thymus atrophy, the size of the capsule remained unaltered and the emerging space was filled by fatty tissue. Acute accidental thymus involution is characterized by massive cell death of the thymocytes, but there was no increase in pycnotic thymocytes either in SIV-induced or in age-related thymus atrophy. Ultrastructural analysis revealed no major differences between the juvenile control and the aged thymus. In contrast, SIV-induced thymus atrophy exhibited severe alterations of the epithelial cells of the cortex and the interdigitating dendritic cells, which were not found in the aged thymus nor in the juvenile control cortex.