Figure 2.

The thymus organogenesis. The thymus organogenesis is characterized by two genetic phases. The first stage is independent from the Foxn1 expression (1) and consists in the induction and outgrowth of the thymic epithelial anlage from the third pharyngeal pouch, through the expression of genes including the Eya1, Six, Hoxa3, and Tbx1. Hoxa3 and Eya1 are also required in neural crest cells (NCCs). In the phase 2, Foxn1 regulates the expression of CCL25, Dll4, and Hoxa3, necessary for the thymocytes and TECs differentiation. During this step, cTECs (expressing K8 and K18 keratin type) and mTECs (expressing K5 and K14 keratin type) originate from the same bi-potential TEC progenitor. The crosstalk between TECs and developing thymocytes is required to generate fully mature TECs and functional T cells.