Authors' contributions
J.W. did most of the mouse dissections and analyses, A.N. did some of the histological analyses, J.L. generated the Alk5FXFXmice, M.D. did some of the expression analyses, H.M.S. participated in design and provided the Tgfbr2 mutant embryos and V.K. generated the Alk2FXFX mice, designed and supervised the experiments and wrote the manuscript. All authors have read and approved the final manuscript.