Figure 1326

Electron micrograph of a thymic (Hassall's) corpuscle. This relatively low magnification electron micrograph shows some of the nuclei (N) and cytoplasm of the concentrically arranged epithelioreticular cells of a thymic (Hassall's) corpuscle. Bundles of intermediate filaments, keratohyalin granules, and lipid droplets are also evident within the cytoplasm of the epithelioreticular cells. Fully keratinized cells (black layer) are present in the center of the thymic corpuscle, x 5,000. (Courtesy of Dr. Johannes A. C. Rhodin.)

constitute the blood-thymus barrier between the T cells and the lumen of cortical blood vessels from the lumen outward, are

• Lining endothelium of the capillary wall. The endothelium is of the continuous type with occluding junctions. It is highly impermeable to macromolecules and is considered a major structural component of the barrier within the cortical parenchyma. The underlying basal lamina of endothelial cells and occasional pericytes are also part of the capillary wall.

• Macrophages in the surrounding perivascular connective tissue. Antigenic molecules that escape from the capillary lumen into the cortical parenchyma may be phagocytosed by macrophages residing in this tissue.

macrophage developing T cells

blood-- thymus barrier type I

, ... epithelioreticular basal lamina r n

@ endothelium pericyte

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