Anatomy of the Thymus Gland

In man the thymus is a bilobed gland located predominantly in the ante-rosuperior mediastinum in proximity to the heart (Rosai and Levine, 1976 Kendall, 1981). Its base lies upon the pericardium and great vessels. It is a midline, pyramid-shaped organ (Fig. 2) formed by two fused, identical lobes 

Normal infant thymus. The external appearance reveals that the thymus is formed by two identical lobes that partially fuse in the midline. [From Rosai and Levine (1976). With permission of the Armed Forces Institute of Pathology.] 

No afferent lymphatic vessels have been found in the thymus, although 

Epithelial Cells. The stroma of the thymus is composed of epithelial cells that can be distinguished from mesenchymal cells in that they possess desmosomes and tonofilaments, whereas mesenchymal cells do not. 

Numerous ultrastructural and histochemical studies have now demonstrated conclusively that many of the epithelial cells in the thymus have the characteristics of secretory cells (Singh, 1981). Cortical as well as medullary thymic epithelial cells may exhibit membrane-bound, electron-dense granules, which are also found in a variety of endocrine organs. Epithelial cells containing membrane-bound, electron-dense granules have now been identified in avian (Frazier, 1973 Hakason et al., 1974 Kendall and Frazier, 

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