Electron microscopic study of involuting thymus of “lethargic” mutant mice

The thymus, which undergoes spontaneous involution three weeks after birth in “lethargic” mutant mice, was studied by electron microscopy. Ultrastructural alterations observed in the involuting thymus of “lethargic” mice resemble those of acute thymic involution induced by the administration of adrenal corticosteroids. The responsive cells in thymic involution of “lethargic” mice were cortical thymocytes, macrophages, and epithelial cells. The first indication of the involution was the appearance of a large number of degenerating thymocytes in the cortex. Pyknotic nuclei of degenerating thymocytes were observed within the macrophages. Macrophages of the involuting thymus were characterized by their content of thymocyte pyknotic nuclei as well as a variety of cytoplasmic inclusions. Cytoplasmic inclusions were also found in the epithelial cells. The inclusions were of two different types: (a) tonofibrils in markedly increased numbers in relation to a non‐involuted thymus, and (b) large vacuoles with dense bodies and/or myelin figures, as found in a normal cell, but several times the number one would expect to find. A large number of lipid‐laden cells were found in the involuted thymus. This type of cell was not seen in the normal thymus. Numerous Hassall's corpuscles were also found in the involuted thymus of “lethargic” mice.