Histological changes and macrophage activity in the adult guinea-pig thymus.

Guinea-pigs were injected intracardially with Indian ink at the age of 2 months and killed at intervals ranging from 2-8 months later. As animals matured, shrinking thymic lobules were replaced by fat revealing a capsule and interlobular blood vessels. Fat droplets were also evident in some Hassall's corpuscles. In contrast with the spleen and liver, only a small amount of carbon was present in the thymus, but the quantity of particulate material remained approximately the same in each organ at varying periods after injection. In the thymus, carbon-bearing macrophages were concentrated at the cortico-medullary junction, perivascularly in the cortex, and to a lesser extent in the medulla. Carbon was also present in a few Hassall's corpuscles, in the capsule and among the interlobular fat. The prediction that carbon would progressively accumulate in Hassall's corpuscles was not confirmed. The observations indicate that macrophages bring foreign particulate material to the normal thymus, that this material is stored in greater amounts in macrophages than in Hassall's corpuscles, and suggest a continual exchange of macrophages between the adult thymus and the circulation.