Histochemical localization of adenosine triphosphatase activity in thymus: A light microscopical and ultrastructural study

Summary Ultrastructural localization of ATPase at high pH in the presence of Ca²⁺ showed that activity in thymocyte precursors was stronger than in mature thymocytes. The activity was localized in the nuclear envelope, rough endoplasmic reticulum, Golgi apparatus and mitochondria. The difference in activity was attributed to a marked decrease in ATPase-containing organelles, mainly the endoplasmic reticulum in the mature thymocytes. This appears to be related to the proliferative activity of the cells rather than to the immunological maturity of the thymocytes. A very strong activity, also localized in the same organelles, was present in the macrophages and interdigitating cells which might have a secretory function and possibly contribute to thymocyte maturation. The Ca²⁺—ATPase activity in the nuclear envelope—endoplasmic reticulum system suggests that these may be the sites for storage and regulation of cytoplasmic calcium.