Reappraisal of age-related morphological changes in the rat thymus--morphometric study.

A quantitative morphometric study, using a routine histological technique, was carried out with 72 rats aged 7 and 15 days, 1, 4 and 9 months and 1 1/2 years. A micrometric eyepiece, calibrated by means of a stage micrometer, was employed for measurement. Change due to aging included loss of septa, increased fatty infiltration, diminished demarcation of the cortex from the medulla and a decrease in the number of Hassall's corpuscles. The thymus of rats aged 1-4 months was considered to be active; it had well-developed septa, the largest lobules, with complete demarcation of the cortex and medulla, and the maximum number of Hassall's corpuscles. Differences between some of these parameters in males and females were observed.