Fig. 7. Timeline of lingual development. Comparison of expression patterns of Shh, Ptc, and Gli1 with lingual innervation and morphology. At E12, a tongue bud is visible with broad epithelial Shh expression and broad epithelial and mesenchymal Ptc expression. The tongue is a distinct structure by E12.5. At E12.5, expression of all three genes has cleared from the midline and exists in parasagittal rows along the tongue as well as in the location of the future circumvallate papilla. Prior to and at E13, nerves have not reached the lingual epithelium. From E13-E16, the tongue elongates and takes on adult shape. At E13.5, Shh is found only in developing papillary epithelium. Ptc is expressed in papillary precursor epithelium and mesenchyme. Expression of Shh and Ptc remains restricted to taste papillae from this time onward. At E14, nerve fibers have reached the basement membrane below developing papillae. At E15, papillae are visible on the tongue and nerve fibers begin to penetrate the papillary epithelium. Papillae are densely innervated by E16. At E16 and beyond, Shh is expressed only in the central epithelial core of fungiform papillae, while Ptc is found in a wider region surrounding the center of each papilla.
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