Figure 2.

Genetic and molecular networks regulating the thymus organogenesis. The thymus is a definitive endoderm derivative. The main steps of organogenesis of the pharyngeal apparatus and thymus formation are described, with time scales in mouse and human, key markers genes and signalling molecules. First, gradients of cytokines and small molecules pattern the endoderm, resulting in an anterior domain between D24-D26 in human. Further patterning results in formation of the pharyngeal domain and its segmentation in pharyngeal arches at 6 weeks. The 3PP forms under singling involving retinoic acid (RA), Hedgehog (SHH), NOTCH and fibroblast growth factors (FGF). 3PP gives rise to the thymic primordium expressing FOXN1 by week 7. By week 8, the thymic primordium migrates under TGFβ signaling to its final position and TEC mature in cTEC and mTEC.