FIGURE 8.

Mammalian tongue (A) and FP formation from the lingual epithelia layer (B). A: development of the mammalian tongue: fourth postovulatory week of human development is shown on the left and the 5-month stage is shown on the right-hand side. The anterior two-thirds of the tongue (shown in red) emerges from the lateral lingual swellings and tuberculum impar while the posterior third (shown in green) emerges from the copula. I–IV: position of the 5 pharyngeal arches. The posterior third of the tongue is derived from endoderm, whereas the anterior two-thirds of the tongue is derived from ectoderm, a fact that resonates through the many differences observed in innervation, taste bud composition, nature of taste receptor cells (TRC), and TRC renewal. B: schema showing early morphogenesis of fungiform papillae (FP) from taste placodes during mouse tongue development. 1: Morphogenesis of the taste placode begins at E12.5 through the epithelial-mesenchymal interaction signals [fibroblast growth factor 8 (Fgf8) and sonic hedgehog (shh) and its ligands Gli1 and Patched (Ptc) and bone morphogenic protein 2 and 4 (Bmp2 and 4)] from the epithelium are shown above in blue and those from the mesenchyme are shown below in red. 2: At E13, epithelial thickening occurs with the epithelial cells at the basal epithelium on the anterior part of the tongue increased in height. 3: At embryonic day (E) 13.5, the columnar epithelial cells begin to form an arch to which expression of the epithelial signaling molecules is restricted. 4: At E14, the FP commit to the next developmental phase with nerve innervations shown in green: neuronal innervation is required for taste bud development within the FP.