Hassall s corpuscles are concentric clusters of keratinized epithelial cells located within the thymic medulla of humans and guinea pigs but are scant in mouse and rat. They are considered to be the terminally differentiated stages of medullary thymic epithelial cells (mTECs) but the mechanisms of their origin are unclear. We have previously deleted the TGF-beta type II receptor (TGFbetaRII) specifically in mouse TECs and reported that these mice have mitigated thymic involution and exhibit earlier reconstitution post-irradiation. In this study, we analyzed the differentiation of mTECs in the TGFbetaRII-knockout mice. Interestingly, the TGFbetaRII-knockout mice display enhanced development of Hassall s corpuscles. The expression of Aire, stromal-cell-derived factor 1 and thymic stromal lymphopoietin in the thymi of the TGFbetaRII-knockout mice was similar to that previously reported for the human thymus. In addition, the putative epithelial progenitor markers MTS20 and MTS24 labeled Hassall s corpuscles in normal mice, but the extent and intensity of this staining were greatly enhanced in Hassall s corpuscles of the TGFbetaRII-knockout mice. The phosphorylated forms of ERK and JNK were also found in Hassall s corpuscles of the TGFbetaRII-knockout mice. Taken together, we suggest that TGFbetaRII-mediated signaling in TECs inhibits their development into Hassall s corpuscles in mice.