Main

Pharyngeal arches are bilaterally symmetric ventral structures that develop in a segmental fashion along the anteroposterior axis during embryogenesis (1). The first pharyngeal arch (PA1), which in mammals develops into jaws, lateral skull wall, teeth, middle ear, and part of the tongue and other soft tissue derivatives, is formed as the most rostral and earliest pharyngeal arch beginning at E8.25 in the mouse embryo. This arch rapidly increases in size as it is populated by mesenchyme derived from cranial NCC and develops into the mandibular and maxillary arches at E9.5.

Appropriate epithelial-mesenchymal signaling is essential for proper development of the pharyngeal arches. Genetic analyses in mice provide evidence that numerous homeobox genes, including Msx, Dlx, goosecoid (Gsc), and Prx, and other transcription factors, such as Hand2, are expressed in pharyngeal arch mesenchyme and play essential roles in development of PA1 (1,2).

Members of the Fgf family, particularly Fgf8, are epithelial signals that regulate gene expression during PA1 development (3,4). Inactivation of mouse Fgf8 specifically in PA1 epithelium revealed that Fgf8 promotes mesenchymal cell survival and induces a developmental program required for PA1 morphogenesis (5). Members of the Bmp family also have important roles in outgrowth of PA1 (4). PA1 development appears very sensitive to the level of Bmp signaling during the initial outgrowth phase, and the level of Bmp signaling is tightly regulated by various factors (6).

The secreted protein Shh, a vertebrate ortholog of the Drosophila segment polarity gene, Hedgehog, is essential for normal development of many organs and is implicated as a cause of HPE. Shh is expressed in the pharyngeal arch epithelium and targeted disruption of Shh in mouse leads to near complete absence of craniofacial skeletal elements along with multiple organ defects (7). Recent studies using chick and mouse embryos have suggested that Shh may play a role in NCC development and pharyngeal pouch patterning (810).

Here, we analyzed the developing pharyngeal arches in Shh mutant embryos and found that Shh may be required for outgrowth of PA1 by regulating epithelial-mesenchymal interactions partly via Fgf signaling pathways that ultimately promote cell survival of mesenchymal cells in PA1.

METHODS

Mouse genetic studies.

Mice heterozygous for the Shh mutation or the Pax3-Cre transgene were previously described (7,11). Pax3-Cre transgenic mice were crossed with lacZ reporter mice (Rosa26 reporter: R26R) (12). These mice were crossed and pregnant mothers were killed at E8.5–9.5 to obtain Shh−/−/Pax3-Cre:R26R and Shh−/− embryos. This study has been approved by University of Texas Animal Care and Use Committee.

β-Galactosidase staining.

Embryos were dissected, fixed in 2% paraformaldehyde/PBS with phenol red, and stained in Xgal solution as described previously (13).

Whole-mount and section RNA in situ hybridization.

Whole-mount RNA in situ hybridizations were performed using digoxigenin-labeled antisense riboprobes as described previously (13). Section RNA in situ hybridizations were performed on paraffin-embedded sections of mouse embryos as described previously (14).

TUNEL analysis and cell proliferation assay.

TUNEL analysis was performed on paraffin sections using apoptosis detection kit (ApopTag, Intergen, Purchase, NY) following manufacturer's protocol with the blue fluorescent DAPI nucleic acid stain (Molecular Probes, Eugene, OR). Cell proliferation assay was performed on paraffin section by immunohistochemistry with purified mouse anti-human Ki-67 antibody (2.5 μg/mL) (BD Pharmingen, San Diego, CA) using VECTASTAIN ABC kit (Vector Laboratories, Burlingame, CA) following manufacturer's protocol.

Bead experiments on cultured first pharyngeal arch explants.

The first pharyngeal arches were dissected in DMEM (Invitrogen, Carlsbad, CA) from E9.5 embryos and cultured on membrane filter (Transwell, Corning, Palo Alto, CA). Bead experiments were performed as described previously (15). Explants were washed in PBS and fixed in 4% paraformaldehyde overnight at 4°C, and then subjected to whole-mount in situ hybridization for Fgf8 expression.

RESULTS

Hypoplasia and midline fusion of the first pharyngeal arches in Shh mutant mice.

To examine how Shh signaling might regulate early pharyngeal arch development, we analyzed the developing pharyngeal arches in Shh mutant embryos. At E9.5, Shh mutant embryos were grossly thinner and smaller in the head with hypoplasia of PA1 compared with wild-type littermates, but were similar in length (cranial to caudal) (Fig. 1). Morphometrics of PA1 in wild-type versus Shh mutant embryos demonstrated sizes of 0.405 ± 0.033 mm versus 0.281 ± 0.039 mm along the dorsal-ventral axis, respectively, and 0.313 ± 0.019 mm versus 0.142 ± 0.015 mm along the anteroposterior axis, respectively (p < 0.05 along both axes). In addition to morphologic and histologic indications, hypoplasia of PA1 was demonstrated using a unique property of the Pax3-Cre/R26R transgenic mouse line. In this mouse line, a population of NCC and their progeny can be marked with lacZ by Cre-mediated recombination under control of the Pax3 promoter, which drives transcription specifically in the early migratory NCC that populate the second to fourth pharyngeal arches, but not PA1 (Fig. 1, B and C). We crossed these transgenic mice into the Shh-null background, and stained the obtained embryos with X-gal. The hypoplastic arch in Shh mutants was PA1 as marked by exclusion of lacZ expression (Fig. 1G). In frontal view, PA1 of Shh mutants appeared as a single hypoplastic arch structure with the bilateral PA1s being fused in the midline anterior to the second pharyngeal arches (Fig. 1H). This analysis also demonstrated relatively normal size and patterning of second to fourth pharyngeal arches at E9.5, indicating that NCC migration into second to fourth arches was grossly normal in Shh mutants (Fig. 1, G and H). Histologic analyses suggested a small and fused single arch structure in the midline, absence of the maxillary component and hypoplasia of the mandibular component of PA1 in Shh mutants (Fig. 1I) compared with wild-type embryos (Fig. 1D), although bilaterally symmetric PA1 were initially developed at E8.5 in Shh mutant embryos (Fig. 1J).

Figure 1
figure 1

Hypoplastic PA1 in Shh mutant embryos. Wild-type (A–E) and Shh−/− embryos (F–J) at E9.5 (A–D, F–I) and E8.5 (E, J) are shown. PA1 was hypoplastic in Shh−/− embryos at E9.5 (arrows in F–I) compared with wild-type (arrows in A–D). In Pax3-Cre/R26R transgenic background, the second, third and fourth pharyngeal arches marked by LacZ (arrowheads in A, B, F, G) were comparable between wild-type and Shh mutants. Exclusion of lacZ expression demonstrates that the hypoplastic arch was PA1 in Shh−/− embryos (arrow in G, H). The PA1 of Shh mutants appeared to be a single small structure fused in the midline (arrow in H, I), anterior to the second arches (arrowheads in H, I) compared with wild-type PA1 (arrows in C, D). Hypoplasia of maxillary (*) and mandibular (arrows) of the PA1 was observed in Shh−/− embryos (I) compared with wild-type (D). The bilaterally symmetric PA1 were initially developed at E8.5 in Shh−/− embryos (J), similar to wild-type embryos (E), although they were smaller in size. h, Head; ht, heart; nt, neural tube; ot, otic vesicle. Scale bar, 0.5 mm (A–C, F–H); 0.1 mm (D, E, I, J).

To elucidate a mechanism for the PA1 defect in Shh mutant embryos, we performed in situ hybridization using markers specific for PA1 development. Bmp4 is normally expressed in numerous embryonic domains, including the distal epithelium of the mandibular, maxillary, and frontonasal region, as well as the cardiac outflow tract at E9.5 (Fig. 2, A and M) (16,17). In Shh mutant embryos, the expression of Bmp4 was specifically absent in the maxillary epithelium, whereas its expression in the mandibular and frontonasal epithelium, and the cardiac outflow tract was intact (Fig. 2, D and P). Mhox and Dlx3 are expressed in maxillary and mandibular mesenchyme derived from NCC in wild-type embryos at E9.5 (Fig. 2, B and C). Their expressions were undetectable in the maxillary component, but detectable in the mandibular component of Shh mutant embryos (Fig. 2, E and F). Consistent with these results, the expression of Dlx6 and Hand2, detectable only in the distal mandibular mesenchyme of wild-type embryos at E9.5 (Fig. 2, G and H), were unaltered in Shh mutant embryos (Fig. 2, J and K). In transverse sections of the mandibular arch at E9.5, expression of Bmp4 was detectable only in the distal epithelium of wild-type embryos (Fig. 2M), however, it was detected throughout the PA1 epithelium of Shh mutants (Fig. 2P), suggesting that the proximal mandibular component might be hypoplastic in the Shh mutant PA1. The Bmp4 expression domain in the medial epithelium of PA1 was absent in mutants and the ectodermal epithelium of right and left PA1 appeared to be continuous in Shh mutant embryos (Fig. 2P), probably reflecting a midline defect of the Shh mutant PA1. A defective proximal mandibular component and midline structure of PA1 in Shh mutants was further demonstrated by the expression pattern of Hand genes in transverse sections at E9.5. Hand2 is normally expressed in mesenchyme of the distal, but not proximal, region of the mandibular arch (Fig. 2, H and N). In Shh mutant embryos, the expression of Hand2 was homogenously detectable in PA1 (Fig. 2Q), further suggesting that the proximal region of mandibular arch was severely hypoplastic in these mutants. Hand 1 is normally expressed in the medial mesenchyme of PA1 (Fig. 2O). However, no expression of Hand1 was detectable in Shh mutants (Fig. 2R), consistent with a loss of midline structure in PA1.

Figure 2
figure 2

Severe hypoplasia of the maxillary and the proximal mandibular arches, and defective midline development of PA1 in Shh mutants. Whole-mount in situ hybridization (A–L) and radioactive section in situ hybridization (M–R) of molecular markers at E9.5 are shown. Bmp4 was expressed in the distal epithelium of the mandibular (black arrow), maxillary (white arrow), frontonasal (black arrowhead) region and cardiac outflow tract (white arrowhead) in wild-type embryos (A), whereas the expression of maxillary epithelium was not detectable in Shh−/− embryos (D). Mhox and Dlx3 were expressed in maxillary (white arrow) and mandibular (black arrow) mesenchyme derived from NCC of wild-type embryos (B, C). Their expressions were absent in the maxillary component, but intact in the mandibular component of Shh−/− embryos (E, F). Arrowheads with numbers denote the second to fourth pharyngeal arch mesenchyme derived from NCC. Dlx6 and Hand2 were similarly expressed in the mandibular mesenchyme (arrow) in Shh−/− embryos (J, K) compared with wild-type embryos (G, H). Pax9 was normally detected in the pharyngeal endoderm (pharyngeal pouches) of Shh mutants at E9.5 (L) compared with wild-type embryos (I). In transverse section, Bmp4 was expressed in the distal epithelium of the mandibular component of wild-type embryos (arrows in M). In Shh−/− embryos, Bmp4 was expressed throughout the epithelium of the mandibular arch (arrows in P). Ectodermal epithelium of right and left mandibular arches appeared to be fused in the midline of Shh−/− embryos (arrowhead in P), in contrast to wild-type embryos where they are separate (arrowhead in M). Hand2 is normally expressed in the mesenchyme of the distal (arrows in H, N), but not proximal, region of the mandibular arches (N). In Shh−/− embryos, the expression of Hand2 was homogenously detectable in the fused mandibular arch (arrows in Q). Hand1 is normally expressed in the medial mesenchyme of mandibular arches (O), but it was not detectable in Shh−/− embryos (R). ht, Heart; nt, neural tube; ph, pharynx. Scale bar, 0.3 mm (A–L); 0.1 mm (M–R).

In spite of the PA1 defect, expression of the endodermal marker, Pax9, was normally detected in the pharyngeal endoderm (pharyngeal pouches) of Shh mutants at E9.5 (Fig. 2L) compared with wild type embryos (Fig. 2I), suggesting that initial endodermal development and pharyngeal patterning were unaffected in mouse embryos lacking Shh by E9.5. Significant expressions of the NCC-derived mesenchymal cell markers, Mhox (Fig. 2E), Dlx3 (Fig. 2F), Dlx6 (Fig. 2J) and Hand2 (Fig. 2K) were observed in PA1 as well as other arches of Shh mutant embryos at E9.5, indicating that NCC could, at least in part, migrate and differentiate in PA1. Taken together, Shh signaling may be critical for proper outgrowth of PA1, especially the maxillary arch and the proximal region of mandibular arch, in addition to establishment of the midline structure during PA1 development.

Shh is required for survival of mesenchymal cells in the first pharyngeal arch.

To test whether the failure of PA1 outgrowth in Shh mutant embryos might be the result of a cell survival defect, we performed TUNEL assays on tissue sections to mark apoptotic cells. Little apoptosis was detected in PA1 of wild type at E9.0–9.5 (Fig. 3, A–D). In contrast, a progressive increase in apoptosis was detected in Shh mutants, mainly in the proximal region at E9.0 (Fig. 3, F and G), and throughout PA1 mesenchyme at E9.5 (Fig. 3, H and I). Enhanced apoptotic signals were detectable only in PA1, but not in the second and third arches at E9.5 (Fig. 3, H and I). In contrast, cell proliferation assayed by immunohistochemistry using anti-Ki-67 antibody appeared relatively normal in PA1 of Shh mutants at E9.5 (Fig. 3J) compared with wild-type embryos (Fig. 3E). These results suggest that hypoplasia of PA1 in Shh mutants at E9.5 is mainly due to apoptosis of a substantial proportion of the cells that normally give rise to PA1.

Figure 3
figure 3

Enhanced apoptosis in the PA1 of Shh mutant embryos. Transverse sections of wild-type (A–E) and Shh−/− embryos (F–J) at E9.0 (A, B, F, G) and E9.5 (C–E, H–J) were analyzed by TUNEL assay (B, D, G, I) and counter-stained with DAPI (A, C, F, H), and assayed for cell proliferation by immunohistochemistry using anti-Ki-67 antibody (E, J). Enhanced apoptotic signals were observed in mesenchyme of Shh−/− PA1 (arrows), but not in the second or third pharyngeal arches (arrowheads), beginning from the maxillary and the proximal mandibular region at E9.0 (G) and extending throughout the PA1 at E9.5 (I), compared with wild type (B and D, respectively). Cell proliferation appeared relatively normal in PA1 of Shh mutants (J) compared with wild-type embryos (E). Inset is a higher magnification of the boxed area. nt, Neural tube; ph, pharynx. Scale bar: 0.1 mm

Shh is required for epithelial-mesenchymal signaling in the first pharyngeal arch.

In addition to Bmp4, Fgf8 is a critical signaling molecule that transmits survival signals from the epithelial cells to the adjacent mesenchyme (46). In contrast to Bmp4, which is expressed in the epithelia, albeit in an abnormal pattern (Fig. 2, D and P), Fgf8 was specifically down-regulated in the ectodermal epithelium of PA1 in Shh mutants at E9.5 (Fig. 4, F and G), coinciding with the extensive apoptosis observed. The expression of Fgf8 in the second and third pharyngeal regions was normal at this stage.

Figure 4
figure 4

Requirement of Shh for Fgf signaling during epithelial-mesenchymal interactions in the PA1. Whole-mount and section in situ hybridization of wild-type (A–E) and Shh−/− embryos (F–J) at E9.5 are shown. Fgf8 was expressed in the PA1 ectoderm of wild-type embryos (arrows in A, B) but was down-regulated in Shh mutants (arrows in F, G), with normal expression in the second to fourth pharyngeal arches (arrowheads in A, F). Barx1, Gsc, and Dlx2 were normally expressed in mesenchymal cells in the PA1 (arrows in C–E) and other pharyngeal arches at E9.5 (arrowheads in C–E). In Shh−/− embryos, they were specifically down-regulated in PA1 (arrows in H–J), with normal expression in other arches (arrowheads in H–J). Down-regulation of Barx1, Gsc, and Dlx2 were noted in the maxillary component (*) of mutant PA1 (H–J) compared with wild type (C–E). h, Head; ht, heart; nt, neural tube. Scale bar: 0.3 mm (A, C–F, H–J), 0.1 mm (B, G).

Because Fgf8 was down-regulated in Shh mutants, we examined expression of several transcription factors that are downstream of Fgf8 signaling in the pharyngeal arch mesenchyme (2,5). Genes encoding the homeobox transcription factors Barx1, Gsc, and Dlx2 are expressed in NCC-derived mesenchymal cells in the PA1 and other pharyngeal arches (Fig. 4, C–E), and their expression is dependent upon Fgf8 signaling. Consistent with down-regulation of Fgf8, expression of Barx1, Gsc, and Dlx2 were decreased in the PA1 of Shh mutants at E9.5, whereas expressions in other arches were normal (Fig. 4, H–J). These results indicate that the epithelial-mesenchymal interactions mediated by Fgf8 signaling were affected in PA1 of Shh mutant embryos.

To determine whether Shh can activate Fgf8 expression, we attempted to induce Fgf8 mRNA with Shh-soaked beads in cultured PA1. Compared with the control PA1 cultured with BSA-soaked beads, the expression level of Fgf8 was elevated in PA1 cultured with Shh-soaked beads (Fig. 5A). Despite the limitations of this experiment using cultured tissues, it supports the observation in Shh mutants that Fgf8 may function downstream of Shh signaling during PA1 development.

Figure 5
figure 5

Shh activation of Fgf8 and proposed model for Shh signaling in PA1 development. (A) Representative Fgf8 expression in cultured PA1 explants after application of a Shh-soaked bead or control (BSA-soaked) bead. Higher level of Fgf8 expression was observed in the explants containing Shh-soaked bead. (B) Proposed model for Shh-Fgf signaling in PA1 development. Shh from epithelium may play a role in survival of mesenchymal cells in PA1 partly via Fgf8 signaling. (C) Schematic drawing of the maxillary (pink) and the proximal mandibular (green) components and the midline structure (yellow) are shown in right lateral views and transverse section of embryos at E9.5. Loss of Shh signaling results in severe hypoplasia of the maxillary and the proximal mandibular components, and defective midline development of PA1.

DISCUSSION

Shh signaling plays a primary role for early PA1 development.

In this report, we analyzed pharyngeal arch development in Shh mutant embryos, focusing on PA1, and demonstrated that bilateral PA1 initially form, but they become hypoplastic, resulting in a single fused structure in the midline by E9.5. Exposure to the Shh inhibitor, jervine, at E7.5 frequently leads to only forebrain defects, but no mandibular defects, whereas susceptibility for mandibular defects is highest when pregnant mice are treated around E9.5 (18). A recent study demonstrated that mice lacking Hh-responsiveness specifically in cranial NCC showed a hypoplastic PA1 without mid- or forebrain defects at E11.5 (9). Taken together, the PA1 defect in Shh mutants at E9.5 presented in this study is likely to be a primary defect resulting from lack of Shh signaling.

Our analyses did not address whether NCC migration into PA1 was altered in Shh mutant embryos. Significant expression of the NCC-derived mesenchymal cell markers, Mhox, Dlx3, Dlx6, and Hand2, was observed in PA1 as well as other arches of Shh mutant embryos at E9.5, indicating that NCC could, at least in part, migrate and differentiate in PA1. However, it remains to be elucidated whether appropriate amounts of NCC migrate into PA1 in absence of Shh signaling, inasmuch as Smoak et al. (19) suggested that NCC were migrating along abnormal pathways in Shh mutant embryos based on the expression of NCC markers, CrabP1 and AP2α.

Our analyses revealed the severe hypoplasia of the maxillary component and the proximal mandibular component, and defective midline development of PA1 in Shh mutants (Fig. 5C). The distal mandibular component where Hand2, Mhox, Dlx3, and Dlx6 are highly expressed was less affected. In the current concept of PA1 or jaw development, it has been proposed that “maxillary” and “mandibular” precursors are from distinct origins (20,21), and that identities of “maxillary,” “proximal mandibular,” and “distal mandibular” components are controlled by specific sets of transcription factors and signaling molecules (4). Shh signaling appears to be, directly or indirectly (see discussion below), more critical for development of maxillary and proximal mandibular region, and establishment of midline structure than identities of the distal mandibular region.

Enhanced apoptosis and altered epithelial-mesenchymal interactions lead to severe hypoplasia of PA1 in Shh mutants.

Our data suggest that the hypoplastic PA1 in Shh mutant embryos is mainly as a result of enhanced apoptosis in PA1 mesenchyme, indicating that Shh signaling functions in survival of PA1 mesenchymal cells. This is consistent with the observation that blocking Shh signaling in chick embryos by anti-Shh antibody results in significant increase in apoptosis in NCC-derived mesenchyme in pharyngeal arches (8), and recent experiments using lysotracker red (molecular probes) (10,19). Enhanced apoptosis of mesenchymal cells was also documented in PA1 of Wnt1-Cre; Smon/c mouse embryos that were created by crossing mice harboring the Wnt1-Cre transgene with those that contain loxP sites around the Shh receptor Smoothened (Smo) to remove Hh-signaling specifically in the cranial NCC lineage (9).

Whether Shh directly acts as a survival factor for mesenchymal cells in PA1 remains to be studied. Mesenchymal cells in PA1 can directly respond to Shh signaling, as they express the Shh receptors, Smo and Patced (Ptc), and the Gli family of transcription factors that transduce Shh signaling (4,22). Jeong et al. (17) have proposed a model in which Shh signaling directly regulates growth of pharyngeal arches via combinatorial expression of several forkhead (Fox) transcription factors, and we have previously reported that Foxc2 can mediate Shh signaling in craniofacial and cardiovascular development (23). These observations suggest that Shh signaling may directly play a role in survival of mesenchymal cells and growth of PA1 through epithelial-mesenchymal interaction.

Alternatively, it is also possible that Shh signaling may promote mesenchymal cell survival in PA1 by regulating the expression of other growth factors. Our molecular analysis revealed that Fgf8 expression was down-regulated in PA1 ectoderm of Shh mutants at E9.5. Tissue-specific inactivation of Fgf8 by Cre-mediated recombination in the ectoderm of the PA1 leads to mesenchymal cell death around E9.5 and impairs development of PA1 (5), suggesting that Fgf8 is required, directly or indirectly, for survival of neural crest–derived mesenchyme in PA1. We also observed down-regulation of pharyngeal mesenchyme markers, Barx1, Gsc, and Dlx2, that are putative targets of Fgf8 signaling in PA1 of Shh mutant embryos. Fgf8 is normally able to induce expression of these genes, whereas Shh alone is not sufficient (35). In addition, other mesenchymal markers, including Hand2, Mhox, and Dlx3 were not down-regulated in Shh mutants, suggesting that there was not a generalized defect of the pharyngeal arch mesenchyme. Consistent with the idea that Shh signaling functions upstream of Fgf8 in PA1 development, Shh mutants have more severe PA1 defects than those that result from tissue-specific loss of Fgf8 function in PA1, where only the proximal region of PA1 is most severely affected (5). Our result that Fgf8 is activated by Shh-soaked beads in PA culture supports this idea. We therefore favor the interpretation that the PA1 phenotype of Shh mutants, at least in part, reflects a lack of Fgf8 signaling during critical epithelial-mesenchymal interactions (Fig. 5B), although we still cannot rule out the possibility that Fgf8 expression is decreased secondary to the morphologic defect.

Implication in human diseases.

Mild midline defects of PA1 may lead to SMMCI. Interestingly, SMMCI is suggested as a characteristic finding in HPE patients with SHH mutations (24). In addition, SHH mutations have been identified not only in SMMCI with HPE, but also SMMCI without HPE (24,25). These findings are consistent with a primary role of Shh in PA1 as suggested in our study.

Severe hypoplasia of PA1 in humans, on the other hand, leads to agnathia or micrognathia. Agnathia alone occurs very rarely, and is often associated with HPE and sometimes with situs inversus totalis (25,26), all of which occur in the setting of SHH disruption in mouse and human. Moreover, Shh lies upstream of Tbx1, a major genetic determinant of DiGeorge/22q11.2 deletion syndrome that is often associated with micrognathia (15,23). It is intriguing to speculate that Shh is not only a disease gene for HPE but also a genetic modifier of other human syndromes associated with agnathia/micrognathia.