Thymic epithelial cells constitute the major component of the thymus microenvironment responsible for maturation of immunocompetent T-cells. Differentiation of epithelial cells requires a series of stage- and site-specific cellular interactions. In the early stages of histogenesis of the thymus the interactions between pharyngeal endoderm and mesenchymal cells of neural crest take place. They are controlled by the activity of several genes and its products, transcription factors. Suppression of these genes results in various developmental changes in thymus.