Laura L. Bruce

Revised Nomenclature for Avian Telencephalon and Some Related Brainstem Nuclei

The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain — in particular the neocortex-like cognitive functions of the avian pallium — requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

The development of vestibulocochlear efferents and cochlear afferents in mice.

We have reinvestigated the embryonic development of the vestibulocochlear system in mice using anterograde and retrograde tracing techniques. Our studies reveal that rhombomeres 4 and 5 include five motor neuron populations. One of these, the abducens nucleus, will not be dealt with here. Rhombomere 4 gives rise to three of the remaining populations: the facial branchial motor neurons; the vestibular efferents; and the cochlear efferents. The migration of the facial branchial motor neurons away from the otic efferents is completed by 13.5 days post coitum (dpc). Subsequently the otic efferents separate into the vestibular and cochlear efferents, and complete their migration by 14.5 dpc. In addition to their common origin, all three populations have perikarya that migrate via translocation through secondary processes, form a continuous column upon completion of their migrations, and form axonal tracts that run in the internal facial germ. Some otic efferent axons travel with the facial branchial motor nerve from the internal facial genu and exit the brain with that nerve. These data suggest that facial branchial motor neurons and otic efferents are derived from a common precursor population and use similar cues for pathway recognition within the brain. In contrast, rhombomere 5 gives rise to the fourth population to be considered here, the superior salivatory nucleus, a visceral motor neuron group. Other differences between this group and those derived from rhombomere 4 include perikaryal migration as a result of translocation first through primary processes and only then through secondary processes, a final location lateral to the branchial motor/otic efferent column, and axonal tracts that are completely segregated from those of the facial branchial and otic efferents throughout their course inside the brain. Analysis of the peripheral distribution of the cochlear efferents and afferents show that efferents reach the spiral ganglion at 12.5 dpc when postmitotic ganglion cells are migrating away from the cochlear anlage. The efferents begin to form the intraganglionic spiral bundle by 14.5 dpc and the inner spiral bundle by 16.5 dpc in the basal turn. They have extensive collaterals among supporting cells of the greater epithelial ridge from 16.5 dpc onwards. Afferents and efferents in the basal turn of the cochlea extend through all three rows of outer hair cells by 18.5 dpc. Selective labeling of afferent fibers at 20.5 dpc (postnatal day 1) shows that although some afferents are still in early developmental stages, some type II spiral ganglion cells already extend for long distances along the outer hair cells, and some type I spiral ganglion cells end on a single inner hair cell. These data support previous evidence that in mice the early outgrowth of afferent and efferent fibers is essentially achieved by birth.

Migratory routes and fates of cells transcribing the Wnt‐1 gene in the murine hindbrain

To investigate the origins, migrations, and fates of Wnt‐1–expressing cells in the murine hindbrain, mice carrying a Wnt‐1 enhancer/lacZ transgene were observed from embryonic day (E) 8 through postnatal day 18. The transgene‐stained ventricular layer waxed and waned prior to and following migrations from it. Stained cells migrated first external to the hindbrain as neural crest and then within it to form typical populations of the rhombic lip, as well as others not recognized as lip derivatives. Migrations originated in a temporally defined sequence, many from discrete rhombomeres. All moved first radially, then rostrally and/or ventrally, ipsi‐, or contralaterally, in the mantle or marginal layers. These movements ultimately formed elements of several nuclei, aligned in four longitudinal bands: dorsal (including the gracile, cuneate, cochlear, and vestibular nuclei, plus cerebellar granular cells), dorsal intermediate (including trigeminal sensory, parvicellular reticular, and deep cerebellar nuclei), ventral intermediate (including lateral and intermediate reticular nuclei), and ventral (including the raphe obscurus and pontine nuclei). Transgene staining often persisted long enough to identify stained cells in their definitive, adult nuclei. However, staining was transient. The strength of the staining, however, was in its ability to reveal origins and migrations in both whole‐mounts and sections, in single cell detail. The present results will permit analyses of the effects of genetic manipulations on Wnt‐1 lineage cells. Developmental Dynamics 235:285–300, 2006.

Comparison of thalamic populations in mammals and birds: Expression of ErbB4 mRNA

The expression of ErbB4 mRNA was examined in dorsal thalamic regions of chicks and rats. In rats, ErbB4 expression was observed in the habenular nuclei, the paraventricular nucleus, intermediodorsal nucleus, the central medial thalamic nucleus, the posterior nucleus, the parafascicular nucleus, the subparafascicular nucleus, the suprageniculate nucleus, the posterior limitans nucleus, the medial part of the medial geniculate nucleus, the peripeduncular nucleus, the posterior intralaminar nucleus, the lateral subparafascicular nucleus, the lateral posterior nucleus, and all ventral thalamic nuclei. In chicks, expression was observed in the subhabenular nucleus, the dorsomedialis posterior nucleus, the dorsointermedius posterior nucleus, the nucleus of the septomesencephalic tract, and areas surrounding the rotundus and ovoidalis nuclei, including the subrotundal and suprarotundal areas, and all ventral thalamic nuclei. Most cells within ovoidalis and rotundus were not labeled. The similar pattern of afferent and efferent projections originating from ErbB4-expressing regions of the mammalian and bird dorsal thalamus suggests that ErbB4 hybridizing cells may be derived from a single anlage that migrates into multiple thalamic regions. Most neurons in the rotundus and ovoidalis nuclei of chick may be homologous to unlabeled clusters of neurons intermingled with ErbB4-expressing cells of the mammalian posterior/intralaminar region.

Effects of microgravity on vestibular development and function in rats: Genetics and environment

Our anatomical and behavioral studies of embryonic rats that developed in microgravity suggest that the vestibular sensory system, like the visual system, has genetically mediated processes of development that establish crude connections between the periphery and the brain. Environmental stimuli also regulate connection formation including terminal branch formation and fine‐tuning of synaptic contacts. Axons of vestibular sensory neurons from gravistatic as well as linear acceleration receptors reach their targets in both microgravity and normal gravity, suggesting that this is a genetically regulated component of development. However, microgravity exposure delays the development of terminal branches and synapses in gravistatic but not linear acceleration‐sensitive neurons and also produces behavioral changes. These latter changes reflect environmentally controlled processes of development.

Electron microscopic differentiation of directly and transneuronally transported DiI and applications for studies of synaptogenesis

The neuronal tracer DiI is a lipophilic dye which diffuses along the lipid bilayer of membranes and sometimes will move transcellularly. We used this tracer to study the development of olivocochlear synapses in the auditory system in rats and chickens by applying DiI directly to severed axons in the olivocochlear bundle. Observations with epi-fluorescent microscopy showed that DiI had labelled efferent axons directly and had labelled spiral ganglion cells and hair cells transneuronally. Ultrastructural analysis of photoconverted DiI tissue in rats revealed that transneuronal diffusion occurred when the plasma membrane of directly labelled axons made contact with the plasma membrane of their target structure. Directly and transneuronally labelled profiles can be distinguished easily at the electron microscopic level. In directly labelled profiles, all plasma, nuclear, endoplasmic reticulum, and outer mitochondrial membranes and cell cytoplasm are labelled leaving only the mitochondrial matrix unstained. However, in transneuronally labelled cells the endoplasmic, nuclear, and immature synaptic membranes are labelled but mitochondrial and non-synaptic plasma membranes are not labelled. This labelling pattern can be explained by diffusion through continuous membranes. These characteristics make DiI diffusion a powerful technique for identifying and studying early events in neuronal development and synapse formation.

Adaptations of the vestibular system to short and long-term exposures to altered gravity

Long-term space flight creates unique environmental conditions to which the vestibular system must adapt for optimal survival of a given organism. The development and maintenance of vestibular connections are controlled by environmental gravitational stimulation as well as genetically controlled molecular interactions. This paper describes the effects of hypergravity on axonal growth and dendritic morphology, respectively. Two aspects of this vestibular adaptation are examined: (1) How does long-term exposure to hypergravity affect the development of vestibular axons? (2) How does short-term exposure to extremely rapid changes in gravity, such as those that occur during shuttle launch and landing, affect dendrites of the vestibulocerebellar system? To study the effects of longterm exposures to altered gravity, embryonic rats that developed in hypergravity were compared to microgravity-exposed and control rats. Examination of the vestibular projections from epithelia devoted to linear and angular acceleration revealed that the terminal fields segregate differently in rat embryos that gestated in each of the gravitational environments.To study the effects of short-term exposures to altered gravity, mice were exposed briefly to strong vestibular stimuli and the vestibulocerebellum was examined for any resulting morphological changes. My data show that these stimuli cause intense vestibular excitation of cerebellar Purkinje cells, which induce up-regulation of clathrin-mediated endocytosis and other morphological changes that are comparable to those seen in long-term depression. This system provides a basis for studying how the vestibular environment can modify cerebellar function, allowing animals to adapt to new environments.

Development of the ear and of connections between the ear and the brain: is there a role for gravity?

This paper outlines the development of the gravistatic sensory system of the ear. First, evidence is presented that a genetic program, for which major transcription factors have already been identified using gene expression studies and targeted mutagenesis, governs the initial development of this system. Second, the formation of sensory neurons and their connections to the brain is described as revealed by tracing studies and genetic manipulations. It is concluded that the initial development of the connections of sensory neurons with mechanosensory transducers of the ear (the hair cells) and the targets in the brainstem (vestibular nuclei) is also dependent on fairly rigid genetic programs. During late embryonic and early postnatal development, however, sensory input appears to be used to fine-tune connections of these sensory neurons with the hair cells in the ear as well as with second order vestibular neurons in the brainstem. This phase is proposed to be critical for a proper calibration of the gravistatic information processing in the brain.

The development of corticocollicular projections in anophthalmic mice

To determine the role of retinal axons in the development of the corticocollicular projection in mice, the lipophilic fluorescent dye, DiI, was used to compare the development of the cortical projections in phenotypically normal (C57BL/6J) mice to that of anophthalmic 129SV/CPorJ mice. Cortical axons in anophthalmic mice found their targets and established a laminar specificity similar to those of cortical axons in normal mice despite the absence of the retinal projection. Cortical axons in normal mice reached the superior colliculus before those in anophthalmic mice and also had a faster rate of growth within the colliculus. Unlike cortical axons in normal mice in early postnatal ages, those in anophthalmic mice formed a disperse bundle in the stratum opticum. Axons labeled by focal applications of DiI into area 17 terminated in a larger and more medial area in anophthalmic mice than in normal mice. Thus, retinal axons are not essential for cortical axons to reach the superior colliculus, but they may have a role in organizing the growth of later-arriving cortical axons. Furthermore, cortical axons can terminate in the superior colliculus with a coarse topography when retinal axons are absent, but they cannot form a topographically refined projection.