Aim: In order to identify subtle changes in thymus cell morphology during development was histologically examined thymus glands at different stages.

Material and Method: Thymus tissues of 14, 15, 16, 17, 18, 19 daily fetus, 6 montly adult, 1 years old senil rats the materials of this study have been investigated. Tissues fixed with 10 % formaldehyde. 5m m thicked paraffin sections were stained by Heamatoxilen and Eosine.

Results: 14 day old fetuses, trabecular structures which extend from the capsule had distincted into the lobulles to thymus. The connective tissue septa of the thymus had developed. In the 14 th -15. days, cortico-medullar junction was discriminated. In that days, the vascularization of thymus had been beginned. In the16-18. days, were seen well organized to structuraly discriminated of the medulla from cortex . In the 19. day, the Hassall corpusculles had been beginned to develop in the medulla. In adults, Hassall corpuscules observed large number in the medulla. Thymocytes into structure of cortex were more basophilic and more density which they were done essential structure of cortex. In the thymus of rat in the one year age, was dramatically exhibited tissue changes. In particularly, were observed reduction in the number of the thymocytes and in the areas interlobulles and intralobulles have been increased adipose tissue (fatty infiltration).

Conclusion: In conclusion, thymus is remodelling, with time. Thymus involution is not only a simple shrinkage of the organ, is result of a series of the mechanismal process among different cells.