Thymus histology

The thymus is the organ where T lymphocytes, the defense cells that later recognize invaders, mature. Under the microscope, each thymic lobule looks split in two: a cortex on the outside, which stains dark, and a medulla on the inside, which stains lighter.

That difference in color has a simple explanation, and it comes down to density. The cortex is packed with thymocytes, which are T lymphocytes that are still immature; so many nuclei sit together that the stain builds up and the area looks darker, like a crowd seen from far away.

The cortex is also protected. There is a blood-thymus barrier, that is, a barrier between the blood and the thymus, made of several layers of cells that keep antigens circulating in the blood from reaching it. The trainees are schooled without distractions.

That barrier does not cover the whole organ: it exists only in the cortex and is absent from the medulla. That gap is thought to be the way out, the point where T lymphocytes that have finished their training leave the thymus.

And there is something found in no other organ: Hassall corpuscles, whorls of epithelial cells that take part in the maturation of those same thymocytes. When a pathologist sees them on a slide, they know they are looking at thymus.