Opposing effects between retinoic acid and Fibroblast growth factor in patterning the common crus and semicircular canals of the inner ear

The three orthogonally arranged semicircular canals and their sensory organs, cristae, of the inner ear are responsible for detecting angular head movements. A main functional component of this vestibular apparatus is the evolutionarily conserved unit of the anterior and posterior canals joined by a central stalk, the common crus. A resorption process (removal of epithelial cells) carves out these three connecting structures from an epithelial outpocket of the developing otic vesicle known as the vertical canal pouch. While molecules such as Fibroblast growth factors (FGF) and Bone morphogenetic proteins (BMP) emanating from the prospective sensory cristae and the rim of the canal pouch, counteract with the resorption process to form the canals, it is not known if there is an independent mechanism that mediates common crus formation. Here, we show that genes encoding retinoic acid (RA) synthesizing enzymes, Aldh1a2 and Aldh1a3, are expressed in the presumptive common crus region of the canal pouch. Blocking endogenous RA activity abolishes common crus formation. Endogenous RA may mediate common crus formation by limiting proliferation of the peri-otic mesenchyme at the prospective common crus region and thus counteracting the resorption process. Additionally, RA and FGF antagonize each other in patterning the common crus and canals, respectively. Ectopic FGF2 downregulates Aldh1a2 expression in the common crus, whereas Bmp2, required for canal formation, may be an intermediate which is co-regulated by RA and FGF.