To investigate the development of the reticular network of the thymus with aging and under pathologic conditions, we performed reticulin stains on the following samples; 5 fetal thymi (22 to 33 weeks of gestational age) and 35 postnatal thymi (less than 1 month to 33 years of age). The latter included 1 hyperplastic thymus, 4 pathologically involuted thymi and 1 physiologically involuted thymus as well as 29 normal thymi. Reticulin fibers were invariably seen along the capsule and interlobular septae of all the thymi. In fetal thymi, reticulin fibers circumscribed only cortical blood vessels and Hassall's corpuscles. Postnatal thymi from the children aged less than 1 month showed discontinuous reticulin fibers along the blood vessels of the corticomedullary junction. With aging, the amount of reticulin fibers increased and formed a "fibroreticular network(FRN)" from the branching point of the interlobular septae along the corticomedullary junction. It completely circumscribed the outer medulla in fully developed thymi. In the hyperplastic thymus, the reticular network retained its original structure. Both pathologically and physiologically involuted thymi revealed irregularly collapsed reticulin fibers. These findings suggest that the reticular network of the thymus consists of FRNs as well as capsule and interlobular septae and matures with aging before involution.
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