A representative phenotype of patients with tricho-rhino-phalangeal syndrome (TRPS) is sparse hair. To understand the developmental defects of these patient's hair follicles, we analyzed the development of hair follicles histologically and biochemically using Trpsl deficient (KO) mice. First, we compared the numbers of primary hair follicles in wild-type (WT) and K embryos at different developmental stages. No differences were observed in the E14.5 skins of WT and KO mice. However, at later time points, KO fetal skin failed to properly develop secondary hair follicles, and the number of secondary hair follicles present in E18.5 KO skin was approximately half compared to that of WT skin. Sonic hedgehog expression was significantly decreased in E17.5 KO skin, whereas no changes were observed in Eda/Edar expression in E14.5 or E17.5 skins. In addition, Noggin expression was significantly decreased in E14.5 and E17.5 KO skin compared to WT skin. In parallel with the suppression of Noggin expression, BMP signaling was promoted in the epidermal cells of KO skins compared to WT skins as determined by immunohistochemistry for phosphorylated Smad1/518. The reduced number of secondary hair follicles was restored in skin graft cultures treated with a Noggin and BMP inhibitor. Furthermore, decreased cell proliferation, and increased apoptosis in KO skin was rescued by Noggin treatment. Taken together, we conclude that hair follicle development in Trpsl KO embryos is impaired directly or indirectly by decreased Noggin expression. (C) 2014 Elsevier Inc. All rights reserved.