LM of part of the cortex of a child?s thymus at high magnification. Many closely packed lymphocytes (Ly) with small, round, densely stained nuclei predominate. Epithelial reticular cells (ERC) have large, round euchromatic nuclei, with prominent nucleoli. Their cytoplasm is eosinophilic. Processes of these cells (arrows) appear to invest capillaries (Cap), many of which are seen in transverse section. This pattern in the cortex constitutes the blood-thymus barrier, which limits access of blood-borne antigens to immature lymphocytes. 820×. H&E.