: The thymus is the central organ of the immune system. It is essential for the development and maintenance of normal immune system, especially cell-mediated immunity. From the morphological point of view, the thymus is divided into two main compartments, cortex and medulla. The thymic microenvironment consists of a network of reticular epithelial cells and other fixed and free cells. The microenvironment of thymus is very important for the selection and maturation of T cells. T cell differentiation occurs via T cell receptors. The major histocompatibility complex participates in interactions between T cells and thymic epithelial cells, in addition to interactions between T cells and dendritic cells, macrophages and myoid cells. The neuroendocrine system regulates early T cell differentiation by the transcription of neuroendocrine genes in the stromal network and expression of cognitive receptors by immature T cells. This work briefly summarizes morphological and ultrastructural characteristics of thymic epithelial cells, dendritic cells, macrophages and myoid cells. It is accompanied by the authors' own photomicrographs and electronmicrograph from a transmission electron microscope. All of these cells play a critical role in the proliferation, differentiation and selection of precursor cells in the T-cell lineage, but the precise mechanisms not well understudood.