Light micrograph of a human thymus gland stained with haematoxylin and eosin. The thymic parenchyma is composed of small lobules separated by adipose (fat) tissue showing a dark peripheral cortex and a central clear medulla. The peripheral cortex appears more stained, due to the high density of T-lymphocyte precursor cells. In the paler centre of each lobule there are many Hassall's corpuscles.