Lateral congenital anomalies of the pharyngeal apparatus: part III. cadaveric representation of the course of second and third cleft and pouch fistulas.

"Stepladder" surgery for fistula from second or third pharyngeal cleft and pouch is "blind." Neither intraoperative methylene blue injection and probing nor preoperative imaging (fistulogram ultrasound, computed tomography, magnetic resonance imaging) reveal three-dimensional anatomic relations of fistulas. This article describes the most common second and third fistula courses and demonstrates representation of their tracts with wires in human cadavers. A second cleft and pouch fistula, at its external opening, pierces superficial cervical fascia (and platysma), then investing cervical fascia, and travels under the sternocleidomastoid muscle, superficial to the sternohyoid and anterior belly of omohyoid. It ascends along the carotid sheath, and at the upper border of the thyroid cartilage it pierces the pretracheal fascia. Characteristically, it courses between the carotid bifurcation and over the hypoglossal nerve. After passing beneath the posterior belly of the digastric muscle and the stylohyoid, it hooks around both glossopharyngeal nerve and stylopharyngeus muscle. The fistula reaches the pharynx below the superior constrictor muscle. The course of a third cleft and pouch fistula is similar until it has pierced pretracheal fascia; then it passes over the hypoglossal nerve and behind the internal carotid, finally descending parallel to the superior laryngeal nerve, reaching the thyrohyoid membrane cranial to the nerve.

Sprouty genes are essential for the normal development of epibranchial ganglia in the mouse embryo.

Fibroblast growth factor (FGF) signalling has important roles in the development of the embryonic pharyngeal (branchial) arches, but its effects on innervation of the arches and associated structures have not been studied extensively. We investigated the consequences of deleting two receptor tyrosine kinase (RTK) antagonists of the Sprouty (Spry) gene family on the early development of the branchial nerves. The morphology of the facial, glossopharyngeal and vagus nerves are abnormal in Spry1-/-;Spry2-/- embryos. We identify specific defects in the epibranchial placodes and neural crest, which contribute sensory neurons and glia to these nerves. A dissection of the tissue-specific roles of these genes in branchial nerve development shows that Sprouty gene deletion in the pharyngeal epithelia can affect both placode formation and neural crest fate. However, epithelial-specific gene deletion only results in defects in the facial nerve and not the glossopharyngeal and vagus nerves, suggesting that the facial nerve is most sensitive to perturbations in RTK signalling. Reducing the Fgf8 gene dosage only partially rescued defects in the glossopharyngeal nerve and was not sufficient to rescue facial nerve defects, suggesting that FGF8 is functionally redundant with other RTK ligands during facial nerve development.

Absence of the vagus nerve in the stomach of Tbx1-/- mutant mice.

BACKGROUND: Tbx1 is a member of the Tbox family of binding domain transcription factors. TBX1 maps within the region of chromosome 22q11 deleted in humans with DiGeorge syndrome (DGS), a common genetic disorder characterized by numerous physical manifestations including craniofacial and cardiac anomalies. Mice with homozygous null mutations in Tbx1 phenocopy this disorder and have defects including abnormal cranial ganglia formation and cardiac neural crest cell migration. These defects prompted us to investigate whether extrinsic vagus nerve or intrinsic enteric nervous system abnormalities are prevalent in the gastrointestinal tract of Tbx1 mutant mice. METHODS: We used in situ hybridization for Ret, and immunohistochemical staining for neurofilament, HuC/D and ßIII-tubulin to study cranial ganglia, vagus nerve, and enteric nervous system development in Tbx1 mutant and control mice. KEY RESULTS: In Tbx1(-/-) embryos, cranial ganglia of the glossopharyngeal (IXth) and vagus (Xth) nerves were malformed and abnormally fused. In the gastrointestinal tract, the vagus nerves adjacent to the esophagus were severely hypoplastic and they did not extend beyond the gastro-esophageal junction nor project branches within the stomach wall, as was observed in Tbx1(+/+) mice. CONCLUSIONS & INFERENCES: Although cranial ganglia morphology appeared normal in Tbx1(+/-) mice, these animals had a spectrum of stomach vagus innervation defects ranging from mild to severe. In all Tbx1 genotypes, the intrinsic enteric nervous system developed normally. The deficit in vagal innervation of the stomach in mice mutant for a gene implicated in DGS raises the possibility that similar defects may underlie a number of as yet unidentified/unreported congenital disorders affecting gastrointestinal function.

The survival of vagal and glossopharyngeal sensory neurons is dependent upon dystonin.

The vagal and glossopharyngeal sensory ganglia and their peripheral tissues were examined in wild type and dystonia musculorum mice to assess the effect of dystonin loss of function on chemoreceptive neurons. In the mutant mouse, the number of vagal and glossopharyngeal sensory neurons was severely decreased (70% reduction) when compared with wild type littermates. The mutation also reduced the size of the circumvallate papilla (45% reduction) and the number of taste buds (89% reduction). In addition, immunohistochemical analysis demonstrated that the dystonin mutation reduced the number of PGP 9.5-, calcitonin gene-related peptide-, P2X3 receptor- and tyrosine hydroxylase-containing neurons. Their peripheral endings also decreased in the taste bud and epithelium of circumvallate papillae. These data together suggest that the survival of vagal and glossopharyngeal sensory neurons is dependent upon dystonin.

Disruption of semaphorin III/D gene causes severe abnormality in peripheral nerve projection.

The molecules of the collapsin/semaphorin gene family have been thought to play an essential role in axon guidance during development. Semaphorin III/D is a member of this family, has been shown to repel dorsal root ganglion (DRG) axons in vitro, and has been implicated in the patterning of sensory afferents in the spinal cord. Although semaphorin III/D mRNA is expressed in a wide variety of neural and nonneural tissues in vivo, the role played by semaphorin III/D in regions other than the spinal cord is not known. Here, we show that mice homozygous for a targeted mutation in semaphorin III/D show severe abnormality in peripheral nerve projection. This abnormality is seen in the trigeminal, facial, vagus, accessory, and glossopharyngeal nerves but not in the oculomotor nerve. These results suggest that semaphorin III/D functions as a selective repellent in vivo.

Perinatal lethality and defects in hindbrain development in mice homozygous for a targeted mutation of the zinc finger gene Krox20.

Krox20 is a zinc finger gene expressed in rhombomeres 3 and 5 during hindbrain development in vertebrates. Mice homozygous for a targeted mutation that deletes the majority of the Krox20 genes, including the zinc finger DNA-binding domain, died shortly after birth. The primary phenotype of the homozygous mutant animals was the loss of rhombomeres 3 and 5. This resulted in fusions of the trigeminal ganglion with the facial and vestibular ganglia, and of the superior ganglia of the glossopharyngeal and vagus nerves. These fusions resulted in a disorganization of the nerve roots of these ganglia as they entered the brain stem. These data demonstrate that Krox20 plays an essential role during development of the hindbrain and associated cranial sensory ganglia in mice.

Cot death and the third branchial arch.

Congenital anomalies of the parathyroids in cases of cot death have been reported by several workers. Carotid-body hypoplasia also has been noted in typical cases of cot death. Neurally conditioned airway occlusion at the oropharyngeal level has been postulated as a precipitating factor of cot death. All these lines of evidence involve structures derived from the third branchial arch, and it is suggested that developmental arrest of this arch in early gestation may be the pathandemical basis of cot death.