From Gene Expression to Neural Networks

In combination, transcription factors encoded by these and other gene families are expressed in a kind of checkerboard pattern of intersecting rhombomeres and longitudinal bands. How does this relate to the regional pattern of neuronal differentiation? The spatial and temporal correlation of hindbrain neuron types to gene expression patterns is far from complete, but several conclusions can be made.

First, certain neuron groups are neatly delimited longitudinally by rhombomeric domains, and certain groups are neatly delimited along the floor plate-roof

Figure 8 The relationship of hindbrain nuclei to rhombomeric domains in the chicken embryo. Nuclei are ordered roughly along the floor plate-roof plate axis from bottom to top. Different shades of grey indicate classes of related nuclei. Note that the rhombomeric pattern exhibits phylogenetic variants. cn, cerebellar nuclei; a, angularis; la, laminaris; mc, magnocellularis; Dd, Deiters dorsalis; s, superior vestibular; Dv, Deiters ventralis; tg, tangentialis; d, descending vestibular; A, vestibular cell group A; B, vestibular cell group B; m, medial vestibular; p, principal trigeminal; t, descending trigeminal; sol, nucleus of solitary tract; dV, dorsal trigeminal (r, rostral part); V, trigeminal motor; VI, abducens; aVI, accessory abducens; VII, facial; IX, glossopharyngeal; X, vagus; XII, hypoglossal; lld, nucleus of lateral lemniscus, (d, dorsal; i, intermediate; v, ventralis); os, superior olive (r, rostral part); rIX, retrofacial glossopharyngeal; amb, ambiguus; sVII, superficial facial; pl/pm, lateral and medial pontine nuclei; oi, inferior olive (modified from Marin and Puelles, 1995).

Figure 8 The relationship of hindbrain nuclei to rhombomeric domains in the chicken embryo. Nuclei are ordered roughly along the floor plate-roof plate axis from bottom to top. Different shades of grey indicate classes of related nuclei. Note that the rhombomeric pattern exhibits phylogenetic variants. cn, cerebellar nuclei; a, angularis; la, laminaris; mc, magnocellularis; Dd, Deiters dorsalis; s, superior vestibular; Dv, Deiters ventralis; tg, tangentialis; d, descending vestibular; A, vestibular cell group A; B, vestibular cell group B; m, medial vestibular; p, principal trigeminal; t, descending trigeminal; sol, nucleus of solitary tract; dV, dorsal trigeminal (r, rostral part); V, trigeminal motor; VI, abducens; aVI, accessory abducens; VII, facial; IX, glossopharyngeal; X, vagus; XII, hypoglossal; lld, nucleus of lateral lemniscus, (d, dorsal; i, intermediate; v, ventralis); os, superior olive (r, rostral part); rIX, retrofacial glossopharyngeal; amb, ambiguus; sVII, superficial facial; pl/pm, lateral and medial pontine nuclei; oi, inferior olive (modified from Marin and Puelles, 1995).

plate axis by longitudinal gene expression domains, whereas others are not (Fig. 8). There are several potential explanations for examples of noncongruity between neuron groups and these domains. For example, the pattern as described to date may be incomplete. Additional genes could subdivide the currently known expression pattern into additional domains; similarly, additional information about the phenotypic diversity of neuron groups may introduce novel group subdivisions. Alternatively, the noncongruity might result from the dynamic features of both gene expression and neuron group formation. As noted previously, gene expression patterns can change over time, and neurons can migrate, so correspondences may be evident only within very particular time windows and then disappear.

Second, the relationship between gene expression patterns and neuron groups is not necessarily applicable only to the classically defined cytoarchitectonic nuclei of the hindbrain. Rather, correlations may be stronger to neuron groupings defined by specific phenotypic characters, such as neurotransmitter profile or axon projection pattern. For example, within such populations as the reticulospinal neurons and the vestibular nuclear complex, subdivisions on the basis of axon projection pathway are more readily correlated to gene expression domains than are the classical nuclear divisions.

Third, it appears that the relationship between gene expression patterns and neuron groups can be extended to the connectivity patterns of hindbrain neurons. This feature has only been examined in a few instances, but there are compelling examples of neuron groups whose subdivision according to termination patterns onto synaptic targets can be correlated to gene expression domains. The vestibular nuclear complex provides an example. Here, the different subgroups connect in stereotyped patterns to target motoneur-ons, creating highly specific reflex pathways for eye and body movements. Although the action of particular genes in establishing this pattern of connectivity has not been experimentally tested, the striking correlation of the vestibular subgroups to gene expression domains suggests a direct link. Other functional systems within the hindbrain similarly appear to be constructed through the action of regional patterns of gene expression. For example, primordial respiratory activity is generated by a neural network with definable components localized to specific rhombo-meres. Genetic manipulations that perturb the rhom-bomeric pattern lead to specific functional deficits in the network.

To summarize, hindbrain neurons are organized into distinct cranial nerve and other nuclei, with the reticular formation as a central core. The pattern of specific functional subdivisions within these nuclei and within the reticular formation is likely to be directly linked to the highly mosaic pattern of gene expression seen at early stages of hindbrain development. Moreover, this relationship likely contributes to establishing the basic pattern of synaptic connectivity within hindbrain networks. Identifying the gene combinations responsible for specifying the various neuron types and their synaptic connections is one of the major challenges of future research on the hindbrain.

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