Alkaline phosphatase in differentiating guinea pid thymocytes: an ultracytochemical study.

Alkaline phosphatase (AP) was demonstrated ultracytochemically on the cell membrane of guinea pig thymocytes. AP usually covers the whole cell surface. In a small subpopulation of thymocytes the cell membrane is totally AP-negative. These cells are located by light microscopy in the thymic medulla and have been shown previously to possess the migratory properties of mature thymocytes. This AP-negative subpopulation most probably represents the T cell population in the thymic medulla. No distinct morphologic differences could be observed between these AP-positive and AP-negative thymocytes. Additionally, a small AP-negative cell population with characteristics of immature lymphoid cells was noted. These lymphoid cells, which are seen more numerously in the fetal thymus, might represent the differentiating stem cells in the guinea pig thymus.