Placodal sensory ganglia coordinate the formation of the cranial visceral motor pathway

Making developmental sense of the senses, their origin and function

The primary senses-touch, taste, sight, smell, and hearing-connect animals with their environments and with one another. Aside from the eyes, the primary sense organs of vertebrates and the peripheral sensory pathways that relay their inputs arise from two transient stem cell populations: the neural crest and the cranial placodes. In this chapter we consider the senses from historical and cultural perspectives, and discuss the senses as biological faculties. We begin with the embryonic origin of the neural crest and cranial placodes from within the neural plate border of the ectodermal germ layer. Then, we describe the major chemical (i.e. olfactory and gustatory) and mechanical (i.e. vestibulo-auditory and somatosensory) senses, with an emphasis on the developmental interactions between neural crest and cranial placodes that shape their structures and functions.

Defective Vagal Innervation in Murine Tbx1 Mutant Hearts

Haploinsufficiency of the T-box transcription factor TBX1 is responsible for many features of 22q11.2 deletion syndrome. Tbx1 is expressed dynamically in the pharyngeal apparatus during mouse development and Tbx1 homozygous mutants display numerous severe defects including abnormal cranial ganglion formation and neural crest cell defects. These abnormalities prompted us to investigate whether parasympathetic (vagal) innervation of the heart was affected in Tbx1 mutant embryos. In this report, we used an allelic series of Tbx1 mouse mutants, embryo tissue explants and cardiac electrophysiology to characterise, in detail, the function of Tbx1 in vagal innervation of the heart. We found that total nerve branch length was significantly reduced in Tbx1^+/- and Tbx1^neo2/- mutant hearts expressing 50% and 15% levels of Tbx1. We also found that neural crest cells migrated normally to the heart of Tbx1^+/-, but not in Tbx1^neo2 mutant embryos. In addition, we showed that cranial ganglia IX^th and X^th were fused in Tbx1^neo2/- but neuronal differentiation appeared intact. Finally, we used telemetry to monitor heart response to carbachol, a cholinergic receptor agonist, and found that heart rate recovered more quickly in Tbx1^+/- animals versus controls. We speculate that this condition of decreased parasympathetic drive could result in a pro-arrhythmic substrate in some 22q11.2DS patients.

Neural crest and placode interaction during the development of the cranial sensory system

In the vertebrate head, the peripheral components of the sensory nervous system are derived from two embryonic cell populations, the neural crest and cranial sensory placodes. Both arise in close proximity to each other at the border of the neural plate: neural crest precursors abut the future central nervous system, while placodes originate in a common preplacodal region slightly more lateral. During head morphogenesis, complex events organise these precursors into functional sensory structures, raising the question of how their development is coordinated. Here we review the evidence that neural crest and placode cells remain in close proximity throughout their development and interact repeatedly in a reciprocal manner. We also review recent controversies about the relative contribution of the neural crest and placodes to the otic and olfactory systems. We propose that a sequence of mutual interactions between the neural crest and placodes drives the coordinated morphogenesis that generates functional sensory systems within the head.

Transplantation of Xenopus laevis tissues to determine the ability of motor neurons to acquire a novel target

The evolutionary origin of novelties is a central problem in biology. At a cellular level this requires, for example, molecularly resolving how brainstem motor neurons change their innervation target from muscle fibers (branchial motor neurons) to neural crest-derived ganglia (visceral motor neurons) or ear-derived hair cells (inner ear and lateral line efferent neurons). Transplantation of various tissues into the path of motor neuron axons could determine the ability of any motor neuron to innervate a novel target. Several tissues that receive direct, indirect, or no motor innervation were transplanted into the path of different motor neuron populations in Xenopus laevis embryos. Ears, somites, hearts, and lungs were transplanted to the orbit, replacing the eye. Jaw and eye muscle were transplanted to the trunk, replacing a somite. Applications of lipophilic dyes and immunohistochemistry to reveal motor neuron axon terminals were used. The ear, but not somite-derived muscle, heart, or liver, received motor neuron axons via the oculomotor or trochlear nerves. Somite-derived muscle tissue was innervated, likely by the hypoglossal nerve, when replacing the ear. In contrast to our previous report on ear innervation by spinal motor neurons, none of the tissues (eye or jaw muscle) was innervated when transplanted to the trunk. Taken together, these results suggest that there is some plasticity inherent to motor innervation, but not every motor neuron can become an efferent to any target that normally receives motor input. The only tissue among our samples that can be innervated by all motor neurons tested is the ear. We suggest some possible, testable molecular suggestions for this apparent uniqueness.

Lhx2-dependent specification of olfactory sensory neurons is required for successful integration of olfactory, vomeronasal, and GnRH neurons

Inactivation of the LIM-homeodomain 2 gene (Lhx2) results in a severe defect in specification of olfactory sensory neurons (OSNs). However, the ramifications of lack of Lhx2-dependent OSN specification for formation of the primary olfactory pathway have not been addressed, since mutant mice die in utero. We have analyzed prenatal and postnatal consequences of conditionally inactivating Lhx2 selectively in OSNs. A cell-autonomous effect is that OSN axons cannot innervate their target, the olfactory bulb. Moreover, the lack of Lhx2 in OSNs causes unpredicted, non-cell-autonomous phenotypes. First, the olfactory bulb shows pronounced hypoplasia in adults, and the data suggest that innervation by correctly specified OSNs is necessary for adult bulb size and organization. Second, absence of an olfactory nerve in the conditional mutant reveals that the vomeronasal nerve is dependent on olfactory nerve formation. Third, the lack of a proper vomeronasal nerve prevents migration of gonadotropin-releasing hormone (GnRH) cells the whole distance to their final positions in the hypothalamus during embryo development. As adults, the conditional mutants do not pass puberty, and these findings support the view of an exclusive nasal origin of GnRH neurons in the mouse. Thus, Lhx2 in OSNs is required for functional development of three separate systems.

Differential contribution of Neurog1 and Neurog2 on the formation of cranial ganglia along the anterior-posterior axis

BACKGROUND

The neural crest (NC) and placode are transient neurogenic cell populations that give rise to cranial ganglia of the vertebrate head. The formation of the anterior NC- and placode-derived ganglia has been shown to depend on the single activity of either Neurog1 or Neurog2. The requirement of the more posterior cranial ganglia on Neurog1 and Neurog2 is unknown.

RESULTS

Here we show that the formation of the NC-derived parasympathetic otic ganglia and placode-derived visceral sensory petrosal and nodose ganglia are dependent on the redundant activities of Neurog1 and Neurog2. Tamoxifen-inducible Cre lineage labeling of Neurog1 and Neurog2 show a dynamic spatiotemporal expression profile in both NC and epibranchial placode that correlates with the phenotypes of the Neurog-mutant embryos.

CONCLUSION

Our data, together with previous studies, suggest that the formation of cranial ganglia along the anterior-posterior axis is dependent on the dynamic spatiotemporal activities of Neurog1 and/or Neurog2 in both NC and epibranchial placode.

Influence of mesodermal Fgf8 on the differentiation of neural crest-derived postganglionic neurons

The interaction between the cranial neural crest (NC) and the epibranchial placode is critical for the formation of parasympathetic and visceral sensory ganglia, respectively. However, the molecular mechanism that controls this intercellular interaction is unknown. Here we show that the spatiotemporal expression of Fibroblast growth factor 8 (Fgf8) is strategically poised to control this cellular relationship. A global reduction of Fgf8 in hypomorph embryos leads to an early loss of placode-derived sensory ganglia and reduced number of NC-derived postganglionic (PG) neurons. The latter finding is associated with the early loss of NC cells by apoptosis. This loss occurs concurrent with the interaction between the NC and placode-derived ganglia. Conditional knockout of Fgf8 in the anterior mesoderm shows that this tissue source of Fgf8 has a specific influence on the formation of PG neurons. Unlike the global reduction of Fgf8, mesodermal loss of Fgf8 leads to a deficiency in PG neurons that is independent of NC apoptosis or defects in placode-derived ganglia. We further examined the differentiation of PG precursors by using a quantitative approach to measure the intensity of Phox2b, a PG neuronal determinant. We found reduced numbers and immature state of PG precursors emerging from the placode-derived ganglia en route to their terminal target areas. Our findings support the view that global expression of Fgf8 is required for early NC survival and differentiation of placode-derived sensory neurons, and reveal a novel role for mesodermal Fgf8 on the early differentiation of the NC along the parasympathetic PG lineage.

Plasticity of neural crest-placode interaction in the developing visceral nervous system

The reciprocal relationship between rhombomere (r)-derived cranial neural crest (NC) and epibranchial placodal cells derived from the adjacent branchial arch is critical for visceral motor and sensory gangliogenesis, respectively. However, it is unknown whether the positional match between these neurogenic precursors is hard-wired along the anterior-posterior (A/P) axis. Here, we use the interaction between r4-derived NC and epibranchial placode-derived geniculate ganglion as a model to address this issue. In Hoxa1(-/-) b1(-/-) embryos, r2 NC compensates for the loss of r4 NC. Specifically, a population of r2 NC cells is redirected toward the geniculate ganglion, where they differentiate into postganglionic (motor) neurons. Reciprocally, the inward migration of the geniculate ganglion is associated with r2 NC. The ability of NC and placodal cells to, respectively, differentiate and migrate despite a positional mismatch along the A/P axis reflects the plasticity in the relationship between the two neurogenic precursors of the vertebrate head.

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 βIII-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.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.