Ultrastructural studies of thymic reticulum: I. Epithelial [corrected] component.

In studying the epithelial component of the mouse thymic reticulum, we identified by their ultrastructural aspect three types of epithelial cells: type I, present in the cortical and medullary zones; and type II and type III, present only in the medulla. These cells form a network entrapping lymphocytes. In addition, some cells are regrouped and form two types of associations that are found in the medulla: cystic cavities and Hassall's corpuscles [corrected]. Comparisons are made between our observations and those described in older publications. The morphologies and roles of these cells as well as the terminologies used to describe them are discussed in light of the new knowledge acquired concerning the function of the thymus.