Bernd Fritzsch

Many of the mechanisms underlying neural development are basically similar in vertebrates and invertebrates. Among vertebrates, popular species for experimental studies are zebrafish, the South African clawed toad, the chick embryo and mice. In mice, many spontaneously occurring mutations affecting the cerebral cortex and the cerebellum have been described. Their molecular analysis, combined with transgenic technology to achieve ectopic gene expression and targeted gene ablation, has made the mouse the mammal of choice for molecular genetic studies of early development.

In this chapter, mechanisms of development will be discussed with emphasis on neural induction (► Sect. 2.2), cell lineage studies and fate mapping (► Sect. 2.3), pattern formation of the forebrain and the hindbrain (► Sect. 2.4), specification of cell fate from the spinal cord to the telencephalon (► Sect. 2.5), neurogenesis, gliogenesis and migration, of the cerebral cortex in particular (► Sect. 2.6), axon outgrowth and guidance, focussing on the corpus callosum, the pyramidal tract, thalamocortical projections and the formation of topographic maps (► Sect. 2.7) and programmed cell death (► Sect. 2.8).

Many of the mechanisms underlying neural development are basically similar in vertebrates and invertebrates. Among vertebrates, popular species for experimental studies are zebrafish, the South African clawed toad, the chick embryo and mice. In mice, many spontaneously occurring mutations affecting the cerebral cortex and the cerebellum have been described. Their molecular analysis, combined with transgenic technology to achieve ectopic gene expression and targeted gene ablation, has made the mouse the mammal of choice for molecular genetic studies of early development.

The brain stem is composed of the midbrain (the mesencephalon) and the hindbrain (the rhombencephalon), and is, at least during development, segmentally organized. The midbrain is composed of two temporarily present segments known as mesomeres, whereas the hindbrain is composed of eight, and more recently of 12, rhombomeres (r0–r12), counting the isthmic rhombomere as r0. The cerebellum arises from the first and second rhombomere (r0 and r1). The brain stem also contributes 10 of the 12 cranial nerves, III–XII. A great number of genes are involved in the proper development of the brain stem. The isthmus organizer regulates the early development of the mesencephalon and of the rostral part of the rhombencephalon. Each rhombomere is characterized by a unique combination of Hox genes, its Hox code. In mice, spontaneous and targeted (knockout) mutations in these genes result in specific, rhombomere-restricted disruptions in the development of motor nuclei of cranial nerves. Such a “rhombomeropathy” has been described for the HOXA1 gene.
In this chapter, patterning of the brain stem and its segmentation are discussed in ► Sect. 7.2, followed by an overview of the development and developmental disorders of the cranial nerves (► Sect. 7.3). In ► Sect. 7.4, the development of the auditory system, its molecular basis, some of its disorders, and genes associated with deafness are discussed. Clinical cases illustrate some major malformations.

The molecular origin of taste buds depends on Eya1, Sox2, and Shh, among other genetic factors. Similarly, taste sensory neuron development depends on a set of genes required to form three epibranchial placodes, the geniculate, petrosal, and nodose. Placodal neuron differentiation depends on Neurog2 in mammals. Additional genes are required for these ganglia to develop the peripheral axons that innervate the three regions of the tongue containing taste buds, fungiform (geniculate, VII), foliate (petrosal, IX), and circumvallate (nodose, X). Taste buds sense at least five distinct taste modalities: sweet, bitter, umami, sour, and salt, and possibly more (e.g., fat). G-protein-coupled receptors on Type II cells are required for sweet, umami, and bitter transduction and Type III transduce sour. The cellular location of salt transduction remains unclear. Two additional cell types are common in the taste bud: the Type I cell, which functions as glia, and a differentiating Type IV cell type. Axons of the three peripheral ganglia reach the hindbrain to provide the solitary tract. The nucleus of the solitary tract (nST) has partially distinct and partially overlapping sensory input. Output of the nST stays ipsilateral to innervate the thalamus before it innervates the cortex, adjacent to the rhinal fissure that will in humans to form the insula cortex.

The trigeminal sensory system, the largest sensory input of all sensory projections, extends from the rhombomere 2 to the spinal cord. Sensory input provides a unique distribution to several sensory receptors that extend through the lemniscal to the cortex. Development starts with a set of genes that drive three branches of the trigeminal ganglion reaching the ophthalmic, maxillary, and mandibular neurons, which depend on the bHLH gene, Neurog1. The diversity of glabrous and hairy sensory inputs is unique to each fiber but have a common innervation, the Merkel cells, that depends on the bHLH gene, Atoh1. The central trigeminal neurons depend, among several genes, on the bHLH genes Ascl1, Olig3, and Ptf1a, that initiate the neuronal differentiation of several genes (Phox2b, Lmx1b, Lbx1), to develop excitatory and inhibitory second order neurons. Hoxa2 is essential for the normal expression of rhombomere 2 that counteracts the expression of Fgf8 in r0. The mesencephalic trigeminal projection extends from the midbrain to the cortex; it bifurcates to innervate trigeminal sensory neurons with peripheral fibers, in particular the mandibular branch. The lemniscal system is continuing sensory information to reach the thalamus, the VPM. From the somatosensory cortex, it will provide three primary and secondary innervations that present a ‘homunculus’ in mammals. Two genes, Fgf8 and Wnt1, have been identified for the normal somatosenation. The sensory innervation can be adjusted by addition or deletion of sensory input, generate both manipulations and genetic alterations to drive the unique whiskers of rodents to reach the barrel field.

Vision starts with opsins, under control of Pax6 and related genes. Chordate vision hinges on a genome duplication early in vertebrate evolution, which allowed a single neurosensory cell type (as seen, for example, in tunicates and ascidians) to split into two cell types in vertebrates: These are a ciliated sensory cell (photoreceptor) that terminates in a ribbon, and parallel populations of retinal ganglion cells (RGCs). The RGCs have diversified via Atoh7 regulatory pathways into 30 distinct cell types. Recent data suggest that certain ascidians have three distinct eye fields, which may be the basis of retinas, pineal, and parapineal in lampreys and other gnathostomes. Building on the role of melanopsin is its unique input to the suprachiasmatic nucleus as the basis of the circadian rhythms (Per/Cry) that cooperates with retinal and pineal to drive melatonin. Topographic input is distinct to each of the retina's targets, with a bilateral suprachiasmatic nucleus input, partial segregation of the ipsilateral and contralateral lateral geniculate nucleus, broad inputs of the accessory nucleus that connects to oculomotor system and a large input to superior colliculus. Multiple gene pathways controlling these properties are now clarified through interaction of distinct genetic analysis and methods. While no such central cortical projection is obvious among tunicates, all vertebrates have unique central projections to reach the cortex.

The dorsal spinal cord and brain receive sensory input from peripheral sensory neurons and sensory receptor cells. In chordates, the brain and spinal cord develop from the dorsal ectoderm that eventually forms the neural tube, whereas the various sensory neurons and receptor cells develop from distinct placodes and neural crest cells for the vertebrate species that have them. Antero-posterior patterning of the hindbrain and spinal cord is driven largely by Hox genes, along with other genes. In addition, there is a shared general dorso-ventral sequence of gene expression, Atoh1, Neurog1/2, Ascl1, and Ptf1a, among others, to regulate and pattern the dorsal aspects of the hindbrain and spinal cord. The development of sensory neurons and receptor cells is primarily driven by early expression of Eya1/Six1, followed by Sox2. Later expression of bHLH genes in placode and neural crest cells further differentiate sensory neurons, specifically Neurog1/2 (most sensory neurons) and Atoh7 (retinal ganglion neurons). Mechanosensory cells, such as hair cells, critically depend on Atoh1, whereas the sensory cells in the visual system, the rods and cones, depend on Neurod1. Moreover, taste receptors depend on Sox2 and Neurog2. Olfactory receptors are unique as they are located on olfactory neurons that have a direct input into the olfactory bulb. This chapter will provide a brief overview of brain and spinal cord development along with the development of sensory neurons and receptor cells in each of the sensory systems.

Electroreception, a sense that was first described by Lissmann, has gained tremendous knowledge in the past 70 years and is now considered one of the earliest inventions of vertebrates. However, the electrosensory division was lost multiple times (hagfish, frogs, amniotes, most teleosts) and independently evolved twice in derived teleosts. Electroreceptor neurons branch exclusively from the ventral branch of the anterior lateral line fibers mostly to the head in a common unique pattern in most non-teleost vertebrates, whereas in teleost vertebrates electroreceptor organs are found on both head and trunk and afferents project via the anterior and posterior lateral line nerve. Most electroreceptors’ hair cells have a primary cilium (kinocilium), but it differs with respect to its length and the presence (lamprey, salamanders, derived teleosts) or absence of actin-rich microvilli (stereovilli; cartilaginous fish, lungfish, caecilians). Hair cells that are dominated by a kinocilium are usually innervated by a single afferent fiber. Innervation terminates in contact with variously sized and shaped synaptic ribbons. Central projections reach the dorsal nucleus where afferents overlap in many vertebrates. Multiple and serial topological innervations are known in derived teleosts, notably in gymnotids and mormyrids. Second-order projections reach the torus semicircularis as a main hub. From here, fibers extend to the thalamus and the telencephalon in derived teleosts.