Ultrastructure of Different Types of Thymic Epithelial Cells in the Rat Thymus

Kim, Bong Seon; Lee, Kyeong Hee; Lee, Hee Woo; Kim, Jae Bong; Kim, Sung Il; Yoon, Sik

Abstract

Thymic epithelial cells constitute a major component of the thymic microenvironment. The thymus is involved in the regulation of the proliferation, maturation and differentiation of thymocytes. There is some controversy about the classification of thymic epithelial cell types. Traditionally, thymic epithelial cells have been divided into cortical and medullary epithelial cell types. In general, the thymic epithelium can be broadly subdivided into subcapsular, cortical and medullary epithelial cells, and Hassall's corpuscles by immunocytochemical methods. Although a few studies were performed on the ultrastructural characteristics of the different types of thymic epithelial cells, there is still some controversy about the classification of thymic epithelial cell subsets. Thus, the present study was performed to investigate the ultrastructural features of thymic epithelial cell subsets in adult male Sprague-Dawley rats, which are the most commonly used species of rat for biological researches, using transmission electron microscopy to shed more light on the heterogeneity of thymic epithelial cells. On the basis of ultrastructural features, we could identify and classify eight subsets of epithelial cells in normal rat thymus. In particular, this study provided a clear and easy way to identify the type 3 epithelial cells by their characteristic 'perinuclear arrangement pattern of relatively short bundles of tonofilaments'. This is an important finding since the type 3 epithelial cells has been considered to be the most difficult type to identify among various thymic epithelial cell types. The results of the present ultrastructural study of thymic epithelial cells provided more insight into the heterogeneity of thymic epithelial cells, and can contribute to the understanding of roles played by different types of thymic epithelial cells.