AbstractThe thymic nurse cell (TNC) consists of an epithelial cell enclosing lymphoid elements and is found in enzymic digests of the thymus. Although these structures have been implicated in the normal intrathymic development of T lymphocytes, little is known about thein situstructure of this unusual cell complex. In this study, various dyes were introduced into the intact thymus and their differential permeability was used to demonstrate that the TNC exists as a sealed structurein situ.The lymphocytes within the TNC were shown to be isolated from the general thymic environment. Prelimi‐ nary studies on these lymphocytes and the physiology of their active release from individual, micromanipulated TNC in microcultures are reporte