The present study was carried out to investigate some histological changes of thymus gland of mature
and senile rabbits. The study was carried out on fifteen apparently healthy mature (six-month-old)
and senile (twelve-month-old) rabbits which were humanly sacrificed, eviscerated and subsequently
tissue specimens from the thymus were taken. It was found that the thymus of mature rabbits was
lobulated organ consisted of stroma and parenchyma, the stroma consisted of connective tissue
capsule and septa. The septal connective tissue rich in collagen fibers and contain low elastic fibers.
Within the septal connective tissue found the muscular artery which characterized by duplicated
internal elastic lamina. The artery branched to give small arteriole. Also, septa contained medium
size vein. Parenchyma consisted of peripheral dark cortex and central light stained medulla. The
cortex heavily populated by small lymphocytes which had dark nucleus and also contained epithelial
reticular cells which had large vesicular nucleus. Medulla contained large lymphocytes, epithelial
reticular cells and Hassall’s corpuscles. The medullary epithelial reticular cells showed positive
reaction for PAS stain. The thymus of senile rabbits showed signs of involution, but it not completely
disappeared. There were some changes, occurred thinning and irregularity of thymic lobule,
progressive cortical thymocyte depletion, thymic cyst formation and lipid infiltration within the
capsule and septa. The present study revealed that thymus was present through all the life, after
maturity occurred involution but complete absence of thymus not occurred.