AbstractThymus of guinea pigs and C3H mice was fixed in 1% osmium tetroxide, embedded in Araldite or Maraglas, and studied under the electron microscope.Hassall's corpuscles showing the typical concentric, lamellated pattern are observed. Reticular cells form layers around central elements of variable appearance. One or more reticular cells or cystic structures often constitute the centers of corpuscles. Nuclear degeneration, swollen cytoplasm, and intracellular fibrils are characteristic findings. Desmosomes may be quite numerous and may cover a large portion of a cell's surface. Intracellular fibrils are particularly dense at membranous sites and often are apparently unrelated to desmosomes. Reticular cells at the periphery of thymic corpuscles may be contiguous with cells of the cytoreticulum. Many reticular cells contain organelles generally associated with protein synthesis or active transport.Intercellular and intracellular cysts are regularly seen and are frequently related to thymic corpuscles. Some cysts may be manifestations of cell synthesis and storage.The epithelial nature of thymic corpuscles and cysts is evident. Thymic corpuscles may arise by localized collapse of the cytoreticulum.