Regional-specific endodermal signals direct neural crest cells to form the three middle ear ossicles

Abstract

Defects in the middle ear ossicles - malleus, incus, and stapes - can lead to conductive hearing loss. During development, neural crest cells (NCCs) migrate from the dorsal hindbrain to specific locations in pharyngeal arch (PA) 1 and 2, to form the malleus-incus and stapes, respectively. It is unclear how migratory NCCs reach their proper destination in PA and initiate mesenchymal condensation to form specific ossicles. We show that secreted molecules sonic hedgehog (SHH) and bone morphogenetic protein 4 (BMP4) emanating from the pharyngeal endoderm are important in instructing regional-specific NCC condensation to form malleus-incus and stapes, respectively. Tissue-specific knockout of Shh in the pharyngeal endoderm or Smoothened (a transducer of SHH signaling) in NCCs causes the loss of malleus-incus condensation in PA1 but only affects the maintenance of stapes condensation in PA2. By contrast, knockout of Bmp4 in the pharyngeal endoderm or Smad4 (a transducer of TGF-β/BMP signaling) in the NCCs disrupts NCC migration into the stapes region in PA2, affecting the stapes formation. These results indicate that regional-specific endodermal signals direct formation of specific middle ear ossicles.