Hassall's corpuscle: a regressive formation.

Alpha naphthylesterase and leucineaminopeptidase activities in Hassall's corpuscles in the Gallus dom, and Cavia cobaya were found to be similar to those in man and in other animals. Histochemical findings of the thymus during the course of the growing phase have shown that these two enzymes are closely connected with the initial formation of a Hassall's corpuscle. Some medullary parenchymal epitheliocytes react strongly at this time. At first present in small clusters, they gradually become more numerous and finally escape from the network. Next they tend to cluster together, enclosing other cells with them. This change is connected with a new structure acquired by the cells. Enzymatic activity, antigenic properties and immunological reactions are only present in the early growth stages of a Hassall's corpuscle, when the cells are often hypertrophic. The component parts of the lamellated corpuscle are subject to involutive phenomena; they cease to function and to supply structural material until they finally disappear. The thymus is able to provide formative material without having recourse to Hassall's corpuscle. However, we are inclined to consider the progressive phase of Hassall's corpuscle to be important, as then elements in growth are clearly differentiated.