PATHOLOGY

Detailed explanation-1: -THE THYMUS IS A SPECIALIZED ORGAN THAT DIRECTS THE DEVELOPMENT AND SELECTION OF T CELLS WHICH DIRECT ADAPTIVE IMMUNITY. THYMIC FUNCTION IS SPATIALLY AND TEMPORALLY REGULATED AND WANES WITH AGE. THYMIC OUTPUT IS ESSENTIAL DURING EARLY LIFE TO ESTABLISH IMMUNE COMPETENCE AND HOMEOSTASIS BUT IS DISPENSABLE THEREAFTER.

Detailed explanation-2: -The thymus gland has dual embryonic origin. Thymic epithelium develops during the sixth week of gestation, from the ventral diverticular epithelium of the third pharyngeal pouch along with the thyroid and parathyroid gland. It extends posterolaterally into the surrounding mesoderm as two flask like structures.

Detailed explanation-3: -The NK/T precursors migrate to the thymus, where thymocyte development proceeds in three multipartite phases: the DN phase (neither CD4 nor CD8 appears on the thymocyte surface), the DP phase (both CD4 and CD8 are present), and the SP phase (either CD4 or CD8 is present on the surface of a mature T cell).