Background: The thymus is the location of T cell development, where lymphoid precursors mature and are exported to the periphery through a sequence of interactions with the thymic epithelium.
Objectives: To assess the anatomical and histological study on the development of the thymus in the embryo and adult mice.
Materials and methods: This study was developmental; fifty mice were divided randomly into 5 groups; group A: Pregnant female mice were dissected at the end of the 1^st week of gestation to get their embryos. Group B: Pregnant female mice were dissected at the end of the 2^nd week of gestation to get their embryos.  Group C: Pregnant female mice were left to deliver their fetuses. Group D: Fetuses was left to complete their 1^st week. Group E was designed as the control group in which the adult male and female mice were dissected and prepared for histological examination to compare their thymus with the embryonic and fetal thymus.
Results:  All animals of the group A had no thymus gland while all the other mice had thymus gland. Thymus appeared lobulated with no differentiation between the cortex and the medulla in the newborn age. Hassle's corpuscles could be detected at the 6^th postnatal age.
Conclusion: Age-related differences in the morphology and histology of TH reflects the development of T cell and immunity and its crucial role in immune disorders. Mice TH gain full maturity by the end of 1^st postnatal week and involution appears in the 6^th postnatal week of age. The distinction between the medulla and cortex gradually disappears as people age.