Ultrastructure of Human Fetal Thymus

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Abstract
The ultrastructure of human fetal thymus was examined by electron microscopy. The human fetal thymus is an epithelial network enclosing lymphocytes and segregating them into groups. The epithelium is continuous around blood vessels, along interlobular septa, and beneath the capsule, where it has a basement membrane and separates the lymphocytes from vessels and connective tissue. There is a structural hematothymic barrier at each capillary consisting of endothelium and its basement membrane, a narrow space containing connective tissue fibers, and epithelium with its basement membrane. Many epithelial cells are engorged with phagocytized material, including portions of lymphocytes. Membrane-bound dense bodies, resembling secretory granules or lysosomes, are frequently seen in the epithelial cells and less often in endothelial cells and lymphocytes. Some endothelial cells contain apparatus which appears secretory, while rare cells in the parenchyma seem to be entirely secretory in their morphology. HassaU''s corpuscles consist primarily of epithelial cells undergoing degeneration with formation of granules that resemble keratohyalin of skin. Degenerating lymphocytes are also found in the corpuscles as well as in the cytoplasm of isolated epithelial cells. The biological implications of these findings are discussed.