Identification and characterization of a cell surface marker for embryonic rat spinal accessory motor neurons

A detailed review of the spinal accessory nerve and its anatomical variations with cadaveric illustration

The spinal accessory nerve, considered part of the eleventh cranial nerve, provides motor innervation to sternocleidomastoid and trapezius. A comprehensive literature review and two cadaveric dissections were undertaken. The spinal accessory nerve originates from the spinal accessory nucleus. Its rootlets unite and ascend between the denticulate ligament and dorsal spinal rootlets. Thereafter, it can anastomose with spinal roots, such as the McKenzie branch, and/or cranial roots. The spinal accessory nerve courses intracranially via foramen magnum and exits via jugular foramen, within which it usually lies anteriorly. Extracranially, it usually crosses anterior to the internal jugular vein and lies lateral to internal jugular vein deep to posterior belly of digastric. The spinal accessory nerve innervates sternocleidomastoid, receives numerous contributions in the posterior triangle and terminates within trapezius. Its posterior triangle course approximates a perpendicular bisection of the mastoid-mandibular angle line. The spinal accessory nerve contains sensory nociceptive fibres. Its cranial nerve classification is debated due to occasional non-fusion with the cranial root. Surgeons should familiarize themselves with the variable course of the spinal accessory nerve to minimize risk of injury. Patients with spinal accessory nerve injuries might require specialist pain management.

Transcription factor support for the dual embryological origin of the sternocleidomastoid and trapezius muscles

The embryological origin of the trapezius and sternocleidomastoid muscles has been debated for over a century. To shed light on this issue, the present anatomical study was performed. Five fresh frozen human cadavers, three males and two females, were used for this study. Samples from each specimen's trapezius and sternocleidomastoid were fixed in 10% formalin and placed in paraffin blocks. As Paired like homeodomain 2 (Pitx2) and T-box factor 1(Tbx1) have been implicated in the region and muscle type regulation, we performed Tbx1 and Pitx2 Immunohistochemistry (IHC) on these muscle tissue samples to identify the origin of the trapezius and sternocleidomastoid muscles. We have used the latest version of QuPath, v0.4.3, software to quantify the Tbx and Pitx2 staining. For the sternocleidomastoid muscle, for evaluated samples, the average amount of positively stained Tbx1 and Pitx2 was 25% (range 16%-30%) and 18% (range 12%-23%), respectively. For the trapezius muscles, for evaluated samples, the average amount of positively stained Tbx1 and Pitx2 parts of the samples was 17% (range 15%-20%) and 15% (14%-17%), respectively. Our anatomical findings suggest dual origins of both the trapezius and sternocleidomastoid muscles. Additionally, as neither Pitx2 nor Tbx1 made up all the staining observed for each muscle, other contributions to these structures are likely. Future studies with larger samples are now necessary to confirm these findings.

Axonal Growth of Midbrain Dopamine Neurons is Modulated by the Cell Adhesion Molecule ALCAM Through Trans-Heterophilic Interactions with L1cam, Chl1, and Semaphorins

The growth of axons corresponding to different neuronal subtypes is governed by unique expression profiles of molecules on the growth cone. These molecules respond to extracellular cues either locally though cell adhesion interactions or over long distances through diffusible gradients. Here, we report that that the cell adhesion molecule ALCAM (CD166) can act as an extracellular substrate to selectively promote the growth of murine midbrain dopamine (mDA) neuron axons through a trans-heterophilic interaction with mDA-bound adhesion molecules. In mixed-sex primary midbrain cultures, the growth-promoting effect of ALCAM was abolished by neutralizing antibodies for components of the Semaphorin receptor complex Nrp1, Chl1, or L1cam. The ALCAM substrate was also found to modulate the response of mDA neurites to soluble semaphorins in a context-specific manner by abolishing the growth-promoting effect of Sema3A but inducing a branching response in the presence of Sema3C. These findings identify a previously unrecognized guidance mechanism whereby cell adhesion molecules act in trans to modulate the response of axonal growth cones to soluble gradients to selectively orchestrate the growth and guidance of mDA neurons.SIGNIFICANCE STATEMENT The mechanisms governing the axonal connectivity of midbrain dopamine (mDA) neurons during neural development have remained rather poorly understood relative to other model systems for axonal growth and guidance. Here, we report a series of novel interactions between proteins previously not identified in the context of mDA neuronal growth. Significantly, the results suggest a previously unrecognized mechanism involving the convergence in signaling between local, adhesion and long-distance, soluble cues. A better understanding of the molecules and mechanisms involved in establishment of the mDA system is important as a part of ongoing efforts to understand the consequence of conditions that may result from aberrant connectivity and also for cell replacement strategies for Parkinson's disease.

Motor neurons and the generation of spinal motor neuron diversity

Motor neurons (MNs) are neuronal cells located in the central nervous system (CNS) controlling a variety of downstream targets. This function infers the existence of MN subtypes matching the identity of the targets they innervate. To illustrate the mechanism involved in the generation of cellular diversity and the acquisition of specific identity, this review will focus on spinal MNs (SpMNs) that have been the core of significant work and discoveries during the last decades. SpMNs are responsible for the contraction of effector muscles in the periphery. Humans possess more than 500 different skeletal muscles capable to work in a precise time and space coordination to generate complex movements such as walking or grasping. To ensure such refined coordination, SpMNs must retain the identity of the muscle they innervate. Within the last two decades, scientists around the world have produced considerable efforts to elucidate several critical steps of SpMNs differentiation. During development, SpMNs emerge from dividing progenitor cells located in the medial portion of the ventral neural tube. MN identities are established by patterning cues working in cooperation with intrinsic sets of transcription factors. As the embryo develop, MNs further differentiate in a stepwise manner to form compact anatomical groups termed pools connecting to a unique muscle target. MN pools are not homogeneous and comprise subtypes according to the muscle fibers they innervate. This article aims to provide a global view of MN classification as well as an up-to-date review of the molecular mechanisms involved in the generation of SpMN diversity. Remaining conundrums will be discussed since a complete understanding of those mechanisms constitutes the foundation required for the elaboration of prospective MN regeneration therapies.

Arx together with FoxA2, regulates Shh floor plate expression

Mutations in the Aristaless related homeodomain transcription factor (ARX) are associated with a diverse set of X-linked mental retardation and epilepsy syndromes in humans. Although most studies have been focused on its function in the forebrain, ARX is also expressed in other regions of the developing nervous system including the floor plate (FP) of the spinal cord where its function is incompletely understood. To investigate the role of Arx in the FP, we performed gain-of-function studies in the chick using in ovo electroporation, and loss-of-function studies in Arx-deficient mice. We have found that Arx, in conjunction with FoxA2, directly induces Sonic hedgehog (Shh) expression through binding to a Shh floor plate enhancer (SFPE2). We also observed that FoxA2 induces Arx through its transcriptional activation domain whereas Nkx2.2, induced by Shh, abolishes this induction. Our data support a feedback loop model for Arx function; through interactions with FoxA2, Arx positively regulates Shh expression in the FP, and Shh signaling in turn activates Nkx2.2, which suppresses Arx expression. Furthermore, our data are evidence that Arx plays a role as a context dependent transcriptional activator, rather than a primary inducer of Shh expression, potentially explaining how mutations in ARX are associated with diverse, and often subtle, defects.

Occipital somites guide motor axons of the accessory nerve in the avian embryo

The accessory nerve (nervus accessorius) displays a unique organization in that its axons ascend along the rostrocaudal axis after exiting the cervical spinal cord and medulla oblongata and thereafter project ventrally into the periphery at the first somite level. Little is known about how this organization is achieved. We have investigated the role of somites in the guidance of motor axons of the accessory nerve using heterotopic transplantations of somites in avian embryos. The formation of not only accessory nerve but also the vagal nerve was affected, when a more caudal occipital somite (somites 2-4) was grafted to the position of the first occipital somite. Our study reveals that only the first occipital somite permits the development of ventral projection of accessory axons, a process that is inhibited by more caudal occipital somites.

ALCAM regulates motility, invasiveness, and adherens junction formation in uveal melanoma cells

ALCAM, a member of the immunoglobulin superfamily, has been implicated in numerous developmental events and has been repeatedly identified as a marker for cancer metastasis. Previous studies addressing ALCAM's role in cancer have, however, yielded conflicting results. Depending on the tumor cell type, ALCAM expression has been reported to be both positively and negatively correlated with cancer progression and metastasis in the literature. To better understand how ALCAM might regulate cancer cell behavior, we utilized a panel of defined uveal melanoma cell lines with high or low ALCAM levels, and directly tested the effects of manipulating these levels on cell motility, invasiveness, and adhesion using multiple assays. ALCAM expression was stably silenced by shRNA knockdown in a high-ALCAM cell line (MUM-2B); the resulting cells displayed reduced motility in gap-closure assays and a reduction in invasiveness as measured by a transwell migration assay. Immunostaining revealed that the silenced cells were defective in the formation of adherens junctions, at which ALCAM colocalizes with N-cadherin and ß-catenin in native cells. Additionally, we stably overexpressed ALCAM in a low-ALCAM cell line (MUM-2C); intriguingly, these cells did not exhibit any increase in motility or invasiveness, indicating that ALCAM is necessary but not sufficient to promote metastasis-associated cell behaviors. In these ALCAM-overexpressing cells, however, recruitment of ß-catenin and N-cadherin to adherens junctions was enhanced. These data confirm a previously suggested role for ALCAM in the regulation of adherens junctions, and also suggest a mechanism by which ALCAM might differentially enhance or decrease invasiveness, depending on the type of cadherin adhesion complexes present in tissues surrounding the primary tumor, and on the cadherin status of the tumor cells themselves.

Motor axon exit from the mammalian spinal cord is controlled by the homeodomain protein Nkx2.9 via Robo-Slit signaling

Mammalian motor circuits control voluntary movements by transmitting signals from the central nervous system (CNS) to muscle targets. To form these circuits, motor neurons (MNs) must extend their axons out of the CNS. Although exit from the CNS is an indispensable phase of motor axon pathfinding, the underlying molecular mechanisms remain obscure. Here, we present the first identification of a genetic pathway that regulates motor axon exit from the vertebrate spinal cord, utilizing spinal accessory motor neurons (SACMNs) as a model system. SACMNs are a homogeneous population of spinal MNs with axons that leave the CNS through a discrete lateral exit point (LEP) and can be visualized by the expression of the cell surface protein BEN. We show that the homeodomain transcription factor Nkx2.9 is selectively required for SACMN axon exit and identify the Robo2 guidance receptor as a likely downstream effector of Nkx2.9; loss of Nkx2.9 leads to a reduction in Robo2 mRNA and protein within SACMNs and SACMN axons fail to exit the spinal cord in Robo2-deficient mice. Consistent with short-range interactions between Robo2 and Slit ligands regulating SACMN axon exit, Robo2-expressing SACMN axons normally navigate through LEP-associated Slits as they emerge from the spinal cord, and fail to exit in Slit-deficient mice. Our studies support the view that Nkx2.9 controls SACMN axon exit from the mammalian spinal cord by regulating Robo-Slit signaling.

Crossing the border: molecular control of motor axon exit

Living organisms heavily rely on the function of motor circuits for their survival and for adapting to ever-changing environments. Unique among central nervous system (CNS) neurons, motor neurons (MNs) project their axons out of the CNS. Once in the periphery, motor axons navigate along highly stereotyped trajectories, often at considerable distances from their cell bodies, to innervate appropriate muscle targets. A key decision made by pathfinding motor axons is whether to exit the CNS through dorsal or ventral motor exit points (MEPs). In contrast to the major advances made in understanding the mechanisms that regulate the specification of MN subtypes and the innervation of limb muscles, remarkably little is known about how MN axons project out of the CNS. Nevertheless, a limited number of studies, mainly in Drosophila, have identified transcription factors, and in some cases candidate downstream effector molecules, that are required for motor axons to exit the spinal cord. Notably, specialized neural crest cell derivatives, referred to as Boundary Cap (BC) cells, pre-figure and demarcate MEPs in vertebrates. Surprisingly, however, BC cells are not required for MN axon exit, but rather restrict MN cell bodies from ectopically migrating along their axons out of the CNS. Here, we describe the small set of studies that have addressed motor axon exit in Drosophila and vertebrates, and discuss our fragmentary knowledge of the mechanisms, which guide motor axons out of the CNS.

Transitional Nerve: A New and Original Classification of a Peripheral Nerve Supported by the Nature of the Accessory Nerve (CN XI)

Classically, the accessory nerve is described as having a cranial and a spinal root. Textbooks are inconsistent with regard to the modality of the spinal root of the accessory nerve. Some authors report the spinal root as general somatic efferent (GSE), while others list a special visceral efferent (SVE) modality. We investigated the comparative, anatomical, embryological, and molecular literature to determine which modality of the accessory nerve was accurate and why a discrepancy exists. We traced the origin of the incongruity to the writings of early comparative anatomists who believed the accessory nerve was either branchial or somatic depending on the origin of its target musculature. Both theories were supported entirely by empirical observations of anatomical and embryological dissections. We find ample evidence including very recent molecular experiments to show the cranial and spinal root are separate entities. Furthermore, we determined the modality of the spinal root is neither GSE or SVE, but a unique peripheral nerve with a distinct modality. We propose a new classification of the accessory nerve as a transitional nerve, which demonstrates characteristics of both spinal and cranial nerves.

Mis-expression of L1 on pre-crossing spinal commissural axons disrupts pathfinding at the ventral midline

In vertebrates, spinal commissural axons project along a transverse path toward and across the floor plate (FP). Post-crossing commissural axons alter their responsiveness to FP-associated guidance cues and turn to project longitudinally in a fasciculated manner prior to extending away from the midline. The upregulation of the neural cell adhesion molecule L1 on crossed commissural axon segments has been proposed to facilitate pathfinding on the contralateral side of the FP. To explore this possibility in vivo, we used Math1 regulatory sequences to target L1 to commissural axons before they cross the ventral midline. L1 mis-expression did not alter the distribution of commissural axon-associated markers or the ventral extension of commissural axons toward the midline. However, commissural axons often stalled or inappropriately projected into the longitudinal plane at the ipsilateral FP margin. These observations suggest that L1-mediated pathfinding decisions are normally delayed until axons have crossed the ventral midline (VM).

UNC5C is required for spinal accessory motor neuron development

In both invertebrates and vertebrates, UNC5 receptors facilitate chemorepulsion away from a Netrin source. Unlike most motor neurons in the embryonic vertebrate spinal cord, spinal accessory motor neuron (SACMN) cell bodies and their axons translocate along a dorsally directed trajectory away from the floor plate/ventral midline and toward the lateral exit point (LEP). We have recently shown that Netrin-1 and DCC are required for the migration of SACMN cell bodies, in vivo. These observations raised the possibility that vertebrate UNC5 proteins mediate the presumed repulsion of SACMN away from the Netrin-rich ventral midline. Here, we show that SACMN are likely to express UNC5A and UNC5C. Whereas SACMN development proceeds normally in UNC5A null mice, many SACMN cell bodies fail to migrate away from the ventral midline and inappropriately cluster in the ventrolateral spinal cord of mouse embryos lacking UNC5C. These results support an important role for UNC5C in SACMN development.

Developmental potential of radial glia investigated by transplantation into the developing rat ventricular system in utero

During development there is a clear correlation between position of dividing progenitor cells, mode of division and developmental potential, suggesting that the local environment of progenitor cells may influence their cell fate [ 17 (6), 639-647]. The contribution of these conditions was investigated here by transplantation of radial glial progenitor cells into isotopic, isochronic, heterotopic and heterochronic environment conditions. Neuronal cells were removed from E14 spinal cords using negative immunoselection. The remaining radial glia were transplanted into the ventricular system of host embryos and pups. Distance of migration as well as morphological and antigenic phenotype of transplanted radial glia was examined after various survival times post transplantation. Host age clearly influenced migration and differentiation of transplant cells, with transplant cells migrating further in younger hosts and differentiating earlier in older aged host environments. Evidence is presented showing that most transplanted spinal cord radial glia give rise to astrocytes. In addition some transplanted radial glia were shown to give rise to neurons in spinal cord regions. Radial glia did not appear to generate neurons in the brains of host animals until postnatal ages, perhaps because transplanted radial glia were isolated from spinal cord and thus may not have been influenced to behave as endogenous radial glia in the brain which commonly produce neurons.

Distribution of EphB receptors and ephrin-B1 in the developing vertebrate spinal cord

Contact-dependent interactions between EphB receptors and ephrin-B ligands mediate a variety of cell-cell communication events in the developing and mature central nervous system (CNS). These predominantly repulsive interactions occur at the interface between what are considered to be mutually exclusive EphB and ephrin-B expression domains. We previously used receptor and ligand affinity probes to show that ephrin-B ligands are expressed in the floor plate and within a dorsal region of the embryonic mouse spinal cord, while EphB receptors are present on decussated segments of commissural axons that navigate between these ephrin-B domains. Here we present the generation and characterization of two new monoclonal antibodies, mAb EfB1-3, which recognizes EphB1, EphB2, and EphB3, and mAb efrnB1, which is specific for ephrin-B1. We use these reagents and polyclonal antibodies specific for EphB1, EphB2, EphB3, or ephrin-B1 to describe the spatiotemporal expression patterns of EphB receptors and ephrin-B1 in the vertebrate spinal cord. Consistent with affinity probe binding, we show that EphB1, EphB2, and EphB3 are each preferentially expressed on decussated segments of commissural axons in vivo and in vitro, and that ephrin-B1 is expressed in a dorsal domain of the spinal cord that includes the roof plate. In contrast to affinity probe binding profiles, we show here that EphB1, EphB2, and EphB3 are present on the ventral commissure, and that EphB1 and EphB3 are expressed on axons that compose the dorsal funiculus. In addition, we unexpectedly find that mesenchymal cells, which surround the spinal cord and dorsal root ganglion, express ephrin-B1.

The role of floor plate contact in the elaboration of contralateral commissural projections within the embryonic mouse spinal cord

In vertebrate embryos, commissural axons extend toward and across the floor plate (FP), an intermediate target at the ventral midline (VM) of the spinal cord. After decussating, many commissural axons turn into the longitudinal plane and elaborate diverse projections. FP contact is thought to alter the responsiveness of these axons so that they can exit the FP and adopt new trajectories. However, a requirement for the FP in shaping contralateral commissural projections has not been established in higher vertebrates. Here we further analyze to what extent FP contact is necessary for the elaboration of decussated commissural projections both in cultured, FP-excised spinal cord preparations and in gli2-deficient mice, which lack a FP. In FP-lacking spinal cords, we observe a large number of appropriately projecting contralateral commissural projections in vivo and in vitro. Surprisingly, even though gli2 mutants lack a FP, slit1-3 mRNA and their receptors (Robo1/2) are expressed in a wild-type-like manner. In addition, blocking Robo-Slit interactions in FP-lacking spinal cord explants prevents commissural axons from leaving the VM and turning longitudinally. Thus, compared to FP contact, Slit-Robo interactions are more critical for driving commissural axons out of the VM and facilitating the elaboration of a subset of contralateral commissural projections.

A comparison of cell transplantation and retroviral gene transfection as tools to study lineage and differentiation in the rat spinal cord

Establishing the cell lineage relationships of cells during development allows insight into when and where developmental decisions are made. In the developing spinal cord, the origin and fate of radial glial has yet to be determined. One way in which to address this question is to transplant enriched populations of radial glia into the ventricular zone (VZ) region of host embryos to examine the lineage and differentiation pattern of these cells. An indirect selection procedure using immunomagnetic beads (Dynabeads; Dynal Biotech) was used here to isolate spinal cord radial glia. This negative immunoselection procedure resulted in a high yield of radial glia. A fluorescent cytoplasmic dye (Cell Tracker Green CMFDA) was used to label radial glia before transplantation. The role of radial glia as progenitor cells can also be examined using a green fluorescent protein (GFP)-expressing retroviral vector. The retroviral vector allows dividing cells in the VZ region of the spinal cord to be tracked by labelling them with GFP. Both techniques were utilised here to successfully label and examine embryonic spinal cord radial glia in vivo after a microinjection of either fluorescently labelled radial glia or replication-incompetent GFP-expressing retrovirus in utero.

Molecular control of spinal accessory motor neuron/axon development in the mouse spinal cord

Within the developing vertebrate spinal cord, motor neuron subtypes are distinguished by the settling positions of their cell bodies, patterns of gene expression, and the paths their axons follow to exit the CNS. The inclusive set of cues required to guide a given motor axon subtype from cell body to target has yet to be identified, in any species. This is attributable, in part, to the unavailability of markers that demarcate the complete trajectory followed by a specific class of spinal motor axons. Most spinal motor neurons extend axons out of the CNS through ventral exit points. In contrast, spinal accessory motor neurons (SACMNs) project dorsally directed axons through lateral exit points (LEPs), and these axons assemble into the spinal accessory nerve (SAN). Here we show that an antibody against BEN/ALCAM/SC1/DM-GRASP/MuSC selectively labels mouse SACMNs and can be used to trace the pathfinding of SACMN axons. We use this marker, together with a battery of transcription factor-deficient or guidance cue/receptor-deficient mice to identify molecules required for distinct stages of SACMN development. Specifically, we find that Gli2 is required for the initial extension of axons from SACMN cell bodies, and that netrin-1 and its receptor Dcc are required for the proper dorsal migration of these cells and the dorsally directed extension of SACMN axons toward the LEPs. Furthermore, in the absence of the transcription factor Nkx2.9, SACMN axons fail to exit the CNS. Together, these findings suggest molecular mechanisms that are likely to regulate key steps in SACMN development.

Preservation of segmental hindbrain organization in adult frogs

To test for possible retention of early segmental patterning throughout development, the cranial nerve efferent nuclei in adult ranid frogs were quantitatively mapped and compared with the segmental organization of these nuclei in larvae. Cranial nerve roots IV-X were labeled in larvae with fluorescent dextran amines. Each cranial nerve efferent nucleus resided in a characteristic segmental position within the clearly visible larval hindbrain rhombomeres (r). Trochlear motoneurons were located in r0, trigeminal motoneurons in r2-r3, facial branchiomotor and vestibuloacoustic efferent neurons in r4, abducens and facial parasympathetic neurons in r5, glossopharyngeal motoneurons in r6, and vagal efferent neurons in r7-r8 and rostral spinal cord. In adult frogs, biocytin labeling of cranial nerve roots IV-XII and spinal ventral root 2 in various combinations on both sides of the brain revealed precisely the same rostrocaudal sequence of efferent nuclei relative to each other as observed in larvae. This indicates that no longitudinal migratory rearrangement of hindbrain efferent neurons occurs. Although rhombomeres are not visible in adults, a segmental map of adult cranial nerve efferent nuclei can be inferred from the strict retention of the larval hindbrain pattern. Precise measurements of the borders of adjacent efferent nuclei within a coordinate system based on external landmarks were used to create a quantitative adult segmental map that mirrors the organization of the larval rhombomeric framework. Plotting morphologically and physiologically identified hindbrain neurons onto this map allows the physiological properties of adult hindbrain neurons to be linked with the underlying genetically specified segmental framework.

Localization of motoneurons that extend axons through the ventral rami of cervical nerves to innervate the trapezius muscle: A study using fluorescent dyes and 3D reconstruction method

It has been suggested that in addition to motor axons, which extend directly into the spinal accessory nerve (SAN), ventral rami-associated motor fibers of cervical nerves also innervate the trapezius muscle. Using fluorescent dye labeling and 3D reconstruction in adult rats, this study clarifies the localization of motoneurons, which extend axons either directly through the SAN or through the ventral rami of cervical nerves to innervate the trapezius. DiI or DiI and DiO were used to label the ventral rami of cervical nerves entering the SAN, as well as branches of the SAN. We show that motoneurons whose axons pass through the ventral rami of cervical nerves and then enter the SAN, and those extending axons directly through the SAN are distributed within the same area. The neurons that extend axons through the SAN had a greater diameter than those axons that pass through the cervical nerves en route to the trapezius muscle. In addition, the axons that ultimately extend through the SAN exit the spinal cord dorsolaterally, while those that pass through the cervical nerves extend out the spinal cord through the ventral roots. We presume that the neurons that extend axons through the SAN are mainly alpha-motoneurons and that those projecting axons through the cervical nerves to the trapezius are mainly gamma-motoneurons. Taken together, these results could explain why patients in whom the SAN was used to treat brachial plexus injury retain some control of the trapezius muscle.

Evolutionary Patterns of Cranial Nerve Efferent Nuclei in Vertebrates

All vertebrates have a similar series of rhombomeric hindbrain segments within which cranial nerve efferent nuclei are distributed in a similar rostrocaudal sequence. The registration between these two morphological patterns is reviewed here to highlight the conserved vs. variable aspects of hindbrain organization contributing to diversification of efferent sub-nuclei. Recent studies of segmental origins and migrations of branchiomotor, visceromotor and octavolateral efferent neurons revealed more segmental similarities than differences among vertebrates. Nonetheless, discrete variations exist in the origins of trigeminal, abducens and glossopharyngeal efferent nuclei. Segmental variation of the abducens nucleus remains the sole example of efferent neuronal homeosis during vertebrate hindbrain evolution. Comparison of cranial efferent segmental variations with surrounding intrinsic neurons will distinguish evolutionary changes in segment identity from lesser transformations in expression of unique neuronal types. The diversification of motoneuronal subgroups serving new muscles and functions appears to occur primarily by elaboration within and migration from already established segmental efferent pools rather than by de novo specification in different segmental locations. Identifying subtle variations in segment-specific neuronal phenotypes requires studies of cranial efferent organization within highly diverse groups such as teleosts and mammals.

Axon fasciculation defects and retinal dysplasias in mice lacking the immunoglobulin superfamily adhesion molecule BEN/ALCAM/SC1

The immunoglobulin superfamily adhesion molecule BEN (other names include ALCAM, SC1, DM-GRASP, neurolin, and CD166) has been implicated in the control of numerous developmental and pathological processes, including the guidance of retinal and motor axons to their targets. To test hypotheses about BEN function, we disrupted its gene via homologous recombination and analyzed the resulting mutant mice. Mice lacking BEN are viable and fertile, and display no external morphological defects. Despite grossly normal trajectories, both motor and retinal ganglion cell axons fasciculated poorly and were occasionally misdirected. In addition, BEN mutant retinae exhibited evaginated or invaginated regions with photoreceptor ectopias that resembled the "retinal folds" observed in some human retinopathies. Together, these results demonstrate that BEN promotes fasciculation of multiple axonal populations and uncover an unexpected function for BEN in retinal histogenesis.

Spatiotemporal distribution of neuronal calcium sensor-1 in the developing rat spinal cord

The present study revealed the localization of neuronal calcium sensor (NCS)-1 immunoreactivity (IR) in the developing rat spinal cord. The NCS-1 IR first appeared at embryonic day 12 in the peripheral nerves and their somata. Intense NCS-1 IR was expressed in ascending and descending tracts in the white matter during the late prenatal period, which gradually decreased to the faint level during postnatal development. Intense NCS-1 IR was colocalized with growth associated protein (GAP)-43 IR in the marginal zone and with the glutamate-aspartate transporter (GLAST) IR in the radial processes traversing the marginal zone. In the adult rat white matter, radially oriented astrocytes and astrocytes in the glia limitans were double-labeled for NCS-1 and glial fibrillary acidic protein (GFAP), whereas small dots on finger-like dendritic projections were double-labeled for NCS-1 and synaptophysin. In the developing gray matter, the NCS-1 IR appeared at embryonic day 12 and gradually increased in the neuronal somata and neuropil, reaching a plateau after the end of the 4th postnatal week. The small dots in neuropil were colabeled for NCS-1 and GFAP or NCS-1 and synaptophysin in the adult rat gray matter. These results strongly suggest that NCS-1 is involved in axogenesis and synaptogenesis in the developing rat spinal cord. NCS-1 can serve as a Ca(2+)-sensor not only in neurons but also in radial glial cells or even in radially oriented astrocytes in the adult rat spinal cord.