Histopathology of Thymus.

<p>(A) S1PL^+/+ thymus: A broad thymic cortex is densely packed with thymic lymphocytes (lower panel) (B) S1PL^−/− thymus: The hypoplastic cortex is very thin and hypocellular due to severe cortical lymphoid depletion and apoptosis. At higher magnification (lower panel) the remaining cells in the thymic cortex include vacuolated stromal/epithelial cells and smaller numbers of granulocytes, macrophages, lymphocytes, and apoptotic cells. In contrast, the markedly expanded and hypercellular thymic medulla of S1PL^−/− mice contains numerous mature lymphocytes as well as vacuolated stromal/epithelial cells, macrophages, and histiocytes (lower panel). (C) S1PL^H/H thymus: The cortex contains minimally decreased numbers of lymphocytes while there is a minimal expansion of the medulla by increased numbers of lymphocytes (D) S1PL^H/− thymus: Thinning of the thymic cortex is accompanied by mild expansion of the medulla due primarily to increased numbers of lymphocytes (lower panel). H&E stain.