**Abstract**

Thymus is a ductless, highly organized, bilobed encapsulated gland of the lymphoid organs that contributes in thymopoiesis. Thymus plays an important function in the assortment, progress and profusion of T cells. The mature subsets of thymus dependent lymphocytes linked with the thymic epithelial and other cells developed the microstructure that protect the body from the harmful foreign micro-organism. Most of the thymic lobular areas experienced the parenchymal cells hypoplasia, undergone infiltration of stromal FCT and experienced thymic atrophy with age progression. As the host gets adult, the regression of the thymus and the thymopoiesis occurs, which ultimately boost the vulnerable situations of the host and open a gateway to autoimmune diseases. Since past decades, scientists are intensely investigated to develop some tactics for the improvements of the thymus performance including T-cell regeneration and maturation with age progression. This unique organ is continuously altered morphologically with age and disease; however, this microarchitectural alteration and its possible modulations is not yet clear. Therefore, the main purpose of this chapter is to highlight the microstructural compartments and physiological modification of the thymus with age. Also, the chapter is suggesting the possible alternative ways to improve its durable physio-morphology in vertebrates.

**Keywords:** thymus lobules, histophysiology, lymphocytes, atrophy
