Thymic nurse cells: their functional ultrastructure.

Thymic nurse cells are defined in vitro as multicellular complexes of epithelial cells and thymocytes. Although these structures have been implicated in the intrathymic differentiation of thymocytes, little is known about the biology of this cell complex and about the occurrence of the cells in the thymus in situ. Therefore, to clarify the matter, in this review we have presented characteristics of epithelial cells capable of forming complexes with thymocytes, in light of the literature data and the experience of the authors. The structure of cells within the complexes allowed us to distinguish three types of thymic nurse cells. Three-dimensional reconstruction of the thymus and observations employing TEM and SEM demonstrated the presence of distinct types of complexes in various topographic regions of the thymus. Where possible, the functional relevance of the morphological data was analyzed.