Study of the cytoplasmic lipids in thymic involution

In the rat, some reticular cells of the thymus contain sudanophilic and paraldehyde fuchsinophil granules. They appear in 1 mth old rats in the cortico medullary junction. Their number increases in older animals. These colourless granules are acido resistant and display a yellowish autofluorescence. A large number of lipid and small paraldehyde fuchsinophilic granules are found in the reticular cells of the thymus cortex after the involution following a single injection of hydrocortisone. However, after regeneration nearly all these granules disappear. The ultrastructure of the reticular cells indicates their mesenchymal nature. They contain granular, vesicular and lamellar inclusions showing acid phosphatase activity. In the human species, 'inverted thymuses' contain a considerable amount of lipids in reticular cells that have phagocytized debris of thymocytes destroyed during involution. These cells also contain small paraldehyde fuchsinophil granules. In the atrophic thymuses, in infants, these are coarse, acido resistant and autofluorescent. Their ultrastructure is similar to that observed in rats. The granules are rare in the atrophic thymuses of children more than 1 yr old, or in the thymic remnants of adults. This material does not imply a possible secretory activity; it probably indicates a transient storage of metabolites resulting from the lysis of thymocytes most of which seem to be reutilized during the regeneration of the thymic cortex.SCOPUS: ar.j