Disruption of sonic hedgehog signaling alters growth and patterning of lingual taste papillae

Sonic hedgehog signaling plays an essential role during embryonic salivary gland epithelial branching morphogenesis

Gene targeting studies indicate that sonic hedgehog (Shh) signaling plays an essential role during craniofacial development. Because numerous mandibular derivatives (e.g., teeth, tongue, Meckel's cartilage) are absent in Shh null mice and the embryonic submandibular salivary gland (SMG) develops from the mandibular arch, we postulated that Shh signaling is important for embryonic SMG development. To address this question, we first determined the spatiotemporal distribution of Shh; two transmembrane proteins, patched 1 (Ptc) and Smoothened (Smo), which act as a negative or a positive regulator of the Shh signal, respectively; and the Gli 3 transcription factor, which is downstream of the Shh signal. The epithelial localization of Shh, Ptc, Smo, and Gli 3 suggests that Shh signaling may act within the epithelium in a juxtacrine manner. The SMG phenotype in our embryonic day (E) 18.5 Shh null mice can be characterized as "paedomorphic," that is, it fails to progress to ontogenic stages beyond the Early Pseudoglandular ( approximately E14). In a complementary set of experiments, we used organ culture to evaluate the effect of enhanced or abrogated Shh signaling on embryonic SMG development in vitro. Paired E13 (Late Initial Bud stage) or E14 (Pseudoglandular stage) SMGs were cultured in the presence or absence of exogenous Shh peptide supplementation; Shh-supplemented explants exhibit a significant stage-dependent increase in branching morphogenesis compared with control explants. Furthermore, by using cyclopamine, a steroidal alkaloid that specifically disrupts the Shh pathway, to abrogate endogenous Shh signaling in vitro, we found a significant decrease in branching in cyclopamine-treated explants compared with controls, as well as a significant decrease in epithelial cell proliferation. Our results indicate that Shh signaling plays an essential role during embryonic SMG branching morphogenesis. Exogenous FGF8 peptide supplementation in vitro rescues the abnormal SMG phenotype seen in cyclopamine-treated explants, demonstrating that overexpression of a parallel, but related, downstream signaling pathway can compensate for diminished Shh signaling and restore embryonic SMG branching morphogenesis.

Cell contact-dependent mechanisms specify taste bud pattern during a critical period early in embryonic development

After gastrulation, the pharyngeal endoderm is specified to give rise to taste receptor organs without further signaling from other embryonic tissues. We hypothesized that intercellular signaling might be responsible for the specification of taste buds. To test if and when this signaling was occurring, intercellular contacts were transiently disrupted in cultures of pharyngeal endoderm from axolotl embryos, and the number, size, and distribution of taste buds analyzed. Disruption of cell contacts at progressive time points, from neurula to late tail bud stages, revealed a critical period, during mid-tail bud stages, when disruption of cell contacts resulted in a significant increase in taste bud number and size. The spatial distribution of taste buds was also altered; taste buds were more clustered in explants disrupted during the critical period. These effects were not due to general alterations in mitosis and apoptosis. Rather, at least three aspects of taste bud patterning, i.e., number, size, and distribution, are governed by mechanisms dependent on normal cell contacts during a concise time window. Furthermore, our findings are consistent with specification of taste buds by means of lateral inhibitory signaling, which we hypothesize results from cell contact-dependent or short-range diffusible signals.

Morphological evidence of the importance of epithelial tissue during mouse tongue development

The morphogenesis of fungiform papillae occurs in a stereotyped pattern on the dorsal surface of the tongue in mice from embryonic day 12 (E12) to E17. The histological results and ultrastructural observations showed the development of specific structures in the epithelium into fungiform papillae. Prior to the morphological changes, the Bmp-4 and Shh transcripts are expressed in a restricted area on the dorsal surface. These results suggest that the development of fungiform papillae requires an epithelium and mesenchyme interaction during morphogenesis. In order to obtain direct evidence of the epithelium and mesenchyme interaction during tongue papillae morphogenesis, the formation of fungiform papillae was examined after a recombination assay. In order to confirm the epithelium and mesenchyme interactions during the early development of the mouse tongue, a recombination assay was conducted after the recombination assay at E12.5 and E13.5 for 2 days using an in vitro organ culture. From the recombination assay results, the E13.5 epithelial portion of the fungiform papillae could determine the position of the newly formed fungiform papillae with the epithelial signaling molecules. E13.5 was a critical stage for fungiform papillae morphogenesis. Fungiform papillae can be considered to be small epithelial appendages, which are formed via the epithelium and mesenchyme interactions.

Notch-associated gene expression in embryonic and adult taste papillae and taste buds suggests a role in taste cell lineage decisions

The Notch signaling pathway is involved in cell fate decisions during development. To explore the role of this signaling cascade in the taste system, we investigated the expression patterns of Notch signaling genes in fetal and adult mouse tongues using in situ hybridization. Three of the four murine Notch receptors, their ligands, Delta-like 1 (Dll-1), Jagged1, and Jagged2, as well as three transcription factors, Hes1, Hes6, and Mash1, are expressed in the embryonic taste epithelium. Expression is first detected in the circumvallate papilla at embryonic day E14.5, when Notch1, Jagged1, and Jagged2 are expressed broadly in the papilla and general lingual epithelium. In contrast, Mash1 and Hes6 are restricted to only a few epithelial cells in the apical region of the developing papilla. By E18.5, many of the genes now exhibit a bimodal expression pattern in the papillary epithelium: apically and dorsally they are expressed in sparse clusters of cells, while more ventrally expression typically occurs throughout the lower regions of the trenches. The extent of papilla innervation was compared with Mash1 and Hes6 expression. At E14.5, when Hes6 and Mash1 are already expressed in small numbers of epithelial cells, PGP9.5 immunoreactive fibers have not yet invaded the epithelium, consistent with the specification of taste bud primordia prior to nerve contact. All of the genes examined (except Notch2) are also expressed in subsets of cells within circumvallate taste buds in adult mice, although Notch1 is restricted to basal cells adjacent to taste buds. The onset of embryonic Notch associated gene expression after the morphological differentiation of the circumvallate papilla argues that this signaling cascade may specify taste receptor cell lineages within an already specified taste papilla. Similarly, Notch gene expression in adult taste buds suggests continued roles in cell lineage determination and cell turnover.