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DEVELOPMENTAL ABNORMALITIES
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The function of the thymus is to produce T lymphocytes, which are crucial components of cell-mediated immunity. The primordia of the thymus develop from the third pharyngeal pouches during the fifth to sixth gestational weeks. The paired primordia then migrate medially and caudally where they meet at the midline by the eighth embryonic week. Connective tissue extends between the lobes, but the thymic tissue itself is not completely fused. Each lobe of the thymus maintains a thin fibrous connection to the adjacent inferior lobe of the thyroid gland. Incomplete or excessive migration of the thymic primordia can lead to ectopic or aberrant locations of thymic tissue.
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Prior to puberty, the predominant constituents of the thymic cortex are lymphocytes. The central medulla is predominantly composed of epithelial cells (especially Hassall corpuscles). Other tissue components include plasma cells, eosinophils, histiocytes, and chromaffin cells. Although the thymus is at its greatest size relative to overall body mass during infancy, there is continued growth of the gland during childhood such that its maximum size occurs around the age of puberty. Soon after the onset of puberty, growth slows, and the interlobular septa of the thymus become thicker. Around the age of 15 years, gradual involution begins, with infiltration of the gland by fatty tissue and progressive decrease in the number of cortical lymphocytes.
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In young children, the thymus has a quadrilateral shape and convex lateral margins. There is a triangular or a bilobed configuration in older children, with the left lobe more prominent than the right. There is considerable variability in position, size, and configuration of the normal thymus. In children, the thymus usually abuts the sternum, and there is no plane of delineation between the thymus and the sternum on lateral radiographs. The thymus may obscure the heart on frontal radiographs of infants, and correlation with a lateral view is essential for accurately assessing cardiac size (Figure 7-1). Occasionally, a normal thymus mimics an anterior mediastinal mass or lung consolidation. On standard chest radiographs, the sail sign and thymic wave sign suggest that a mediastinal soft-tissue density is actually caused by the thymus (Figure 7-2). Frontal oblique radiographs are useful for selected patients to confirm that a density projecting over the lung actually represents the thymus rather than lung pathology.
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