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PubMed2006

Direct visualization of nucleogenesis by precerebellar neurons: involvement of ventricle-directed, radial fibre-associated migration.

Nuclei are aggregates of neurons distributed in the central nervous system and are fundamental functional units that share anatomical and physiological features. Despite their importance, the cellular basis that leads to nucleogenesis is only poorly understood. Using exo utero electroporation with an enhanced yellow fluorescent protein (EYFP) gene, we show that the precerebellar neurons derived from the lower rhombic lip (lRL) undergo multiple migration steps to form nuclei. After the unilateral transfer of EYFP to the lRL of embryonic day 12.5 mice, EYFP-labelled neurons migrate tangentially from the lRL in two distinct streams, one towards the ventral metencephalon and the other towards the ventral myelencephalon. These neurons cross the ventral midline and then become radially directed. Labelled neurons in the tangential migratory streams form contralateral clusters in the external cuneate nucleus (ECN) and lateral reticular nucleus (LRN) in the myelencephalon, and bilateral clusters in the pontine grey nucleus (PGN) and reticulotegmental nucleus (RTN) in the metencephalon. Before forming the clusters, EYFP-labelled neurons begin to migrate radially towards the ventricle in close apposition to nestin-positive radial fibres, and then they aggregate as they detach from the fibres. Inhibition of cadherin function in ECN and LRN progenitors caused ipsilateral formation of the ECN and LRN, implying that the transition of their migration from tangential to radial involves a cell-intrinsic mechanism. These observations suggest that nucleogenesis of precerebellar neurons is a result of multi-phasic migration, and that ventricle-directed radial glia-guided migration is a key step for nucleogenesis.

Animals↗

PubMed1992

Local and transient expression of E-cadherin involved in mouse embryonic brain morphogenesis.

We found that E-cadherin (uvomorulin) is transiently expressed in restricted regions of the metencephalon, mesencephalon and diencephalon of mouse embryonic brain. This expression first occurred in parts of the mesencephalon and diencephalon at around E9.5, and subsequently extended to the primordia of cerebellum, the dorsal midline of mesencephalon and some other regions of the embryonic brain. These E-cadherin expressions ceased by E15 except at the dorsal midline. Immunohistological analyses showed that E-cadherin-positive cells are radially arranged in the neural tube and the E-cadherin-positive regions are sharply demarcated from E-cadherin-negative regions. Axons extending from some of the E-cadherin-positive regions also expressed this molecule. When embryonic brains were dissociated into single cells and cultured as monolayers, E-cadherin-positive cells formed clusters that were segregated from E-cadherin-negative cells. E9.5 brain fragments containing metencephalon and mesencephalon were isolated, explanted on Nucleopore filters and cultured in the absence or presence of antibodies to E-cadherin. This antibody treatment removed most of the E-cadherin molecules from the explants and consequently affected their growth pattern. To analyze cellular events induced by the antibody treatment, we stained these explants with an antiserum to En whose distribution was found to overlap in part with that of E-cadherin and found that the pattern of En staining was altered by the anti-E-cadherin antibody treatment. These results suggest that the local and transient expression of E-cadherin in embryonic brain is involved in regional pattern formation in this organ.

Animals↗

PubMed1997

Evidence that FGF8 signalling from the midbrain-hindbrain junction regulates growth and polarity in the developing midbrain.

The developing vertebrate mesencephalon shows a rostrocaudal gradient in the expression of a number of molecular markers and in the cytoarchitectonic differentiation of the tectum, where cells cease proliferating and differentiate in a rostral to caudal progression. Tissue grafting experiments have implicated cell signalling by the mesencephalic-metencephalic (mid-hindbrain) junction (or isthmus) in orchestrating these events. We have explored the role of Wnt-1 and FGF8 signalling in the regulation of mesencephalic polarity. Wnt-1 is expressed in the caudal mesencephalon and Fgf8 in the most rostral metencephalon. Wnt-1 regulates Fgf8 expression in the adjacent metencephalon, most likely via a secondary mesencephalic signal. Ectopic expression of Fgf8 in the mesencephalon is sufficient to activate expression of Engrailed-2 (En-2) and ELF-1, two genes normally expressed in a decreasing caudal to rostral gradient in the posterior mesencephalon. Ectopic expression of Engrailed-1 (En-1), a functionally equivalent homologue of En-2 is sufficient to activate ELF-1 expression by itself. These results indicate the existence of a molecular hierarchy in which FGF8 signalling establishes the graded expression of En-2 within the tectum. This in turn may act to specify other aspects of A-P polarity such as graded ELF-1 expression. Our studies also reveal that FGF8 is a potent mitogen within the mesencephalon: when ectopically expressed, neural precursors continue to proliferate and neurogenesis is prevented. Taken together our results suggest that FGF8 signalling from the isthmus has a key role in coordinately regulating growth and polarity in the developing mesencephalon.

Animals↗

PubMed1997

Comparative localization of fibroblast growth factor receptor-1, -2, and -3 mRNAs in the rat brain: in situ hybridization analysis.

The present study provides a detailed comparative description in the adult rat brain of areas that express mRNAs coding for the fibroblast growth factor subtype receptors 1-3 (FGFR1-3). One observation in this analysis was a widespread expression in the brain of all three FGFR mRNAs, according to the following rank order: FGFR1, diencephalon < telencephalon < mesencephalon and metencephalon < myelencephalon; FGFR2 and FGFR3, telencephalon < diencephalon < mesencephalon and metencephalon < myelencephalon. Another observation was an apparent cellular specificity in their basal expression. Thus, the FGFR1 mRNA was expressed mainly in large and weakly stained cells, whereas FGFR2 transcripts were expressed primarily in small and strongly stained cells and in cells of brain regions devoid of neuronal cells, such as the white matter. FGFR3 mRNA was always detected in small and strongly stained cells with scattered distribution and was not expressed in the white matter. However, FGFR2 mRNA was weakly expressed also in large cells localized in some nuclei of the lower brainstem, in the diagonal band, and in the septum. Furthermore, in the medial habenula and in the nuclei of the pons, there exists a high density of cells expressing both FGFR1 and FGFR2 (60-100%). With neurotoxic lesions involving 6-hydroxydopamine microinjections in the substantia nigra, reactive glial cells in the lesioned area and surrounding the cannula tract showed an increase in the expression of both FGFR1 and FGFR2 mRNAs, whereas no increased expression was found for FGFR3 mRNA. Taken together, these findings showed that these three FGF receptors exist in all subtypes of cells of each brain region. Their apparent cellular specificity suggests that these receptor subtypes can have a differential trophic role in the brain, reflecting the various biological activities shown by the ligands of the FGF family.

Animals↗

PubMed1999

Distribution of parvalbumin immunoreactivity in the brain of the tench (Tinca tinca L., 1758).

The distribution of parvalbumin (PV) immunoreactivity in the tench brain was examined by using the avidin-biotin-peroxidase immunocytochemical method. This protein was detected in neuronal populations throughout all main divisions of the tench brain. In the telencephalic hemispheres, PV-immunopositive neurons were distributed in both the dorsal and ventral areas, being more abundant in the area ventralis telencephali, nucleus ventralis. In the diencephalon, the scarce distribution of PV-containing cells followed a rostrocaudal gradient, and the most evident staining was observed in the nucleus periventricularis tuberculi posterioris and in a few nuclei of the area praetectalis. In the mesencephalon, abundant PV-immunoreactive elements were found in the tectum opticum, torus semicircularis, and tegmentum. In the tectum opticum, PV-immunoreactivity presented a laminar distribution. Three PV-containing neuronal populations were described in the torus semicircularis, whereas in the tegmentum, the PV staining was mainly located in the nucleus tegmentalis rostralis and in the nucleus nervi oculomotorii. In the metencephalon, Purkinje cells were PV-immunopositive in the valvula cerebelli, lobus caudalis cerebelli, and in the corpus cerebelli. In the myelencephalon, PV immunoreactivity was abundant in the nucleus lateralis valvulae, in the nucleus nervi trochlearis, nucleus nervi trigemini, nucleus nervi abducentis, nucleus nervi glossopharyngei, and in the formatio reticularis. Mauthner cells were also PV immunostained. By contrast to other vertebrate groups, only a restricted population of PV-containing neurons was GABA-immunoreactive in the tench, demonstrating that this calcium-binding protein cannot be considered a marker for GABAergic elements in the teleost brain. This study demonstrates a low phylogenetic conservation of the distribution of PV comparing teleosts and tetrapods.

Animals↗

PubMed2000

The metencephalic floor plate of chick embryos expresses two secretory glycoproteins homologous with the two glycoproteins secreted by the subcommissural organ.

The nature and the function of the compounds secreted by the floor plate (FP) of the metencephalon are little known. The FP cells of the hindbrain react with antibodies (AFRU) against the glycoproteins secreted by the subcommissural organ (SCO). One of the these proteins, RF-Gly I, is a 540-kDa core glycosylated protein. The aims of the present investigation were to identify by immunoblot the AFRU-immunoreactive compound secreted by the FP of chick embryos, to establish temporal and regional patterns of this secretory activity, and to obtain information about the fate of these compounds. It was established that the SCO and FP of chick embryos secrete two AFRU-immunoreactive compounds of 540 and 230 kDa. The two compounds secreted by the FP have been designated as FP-Gly I and FP-Gly II. The expression of these proteins was circumscribed to the metencephalic FP, and occurred from HH 29 to HH 36. Within the FP cells, FP-Gly I and FP-Gly II were confined to the supranuclear and apical regions, which under the electron microscope displayed numerous cisternae of the rough endoplasmic reticulum and granules. Aggregates of AFRU-immunoreactive material appeared on the free surface of the FP. The possibility that FP-Gly I and FP-Gly II are released into the ventricle to reach distant targets is discussed.

Animals↗

PubMed2001

Regionalization of the optic tectum: combinations of gene expression that define the tectum.

The optic tectum differentiates from the alar plate of the mesencephalon. Here, the molecular mechanisms for differentiation of the tectum are reviewed. Mis-expression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to become the tectum. En, Fgf8, Pax2 and Pax5, exist in a positive feedback loop for their expression so that mis-expression of any of these genes acts on the feedback loop resulting in induction of the optic tectum in the diencephalon. Otx2 and Gbx2 can repress the expression of each other and contribute to the formation of the posterior border of the tectum. Mis-expression of Otx2 in the metencephalon changed the fate of its alar plate to the tectum. The anterior border of the tectum might be determined as a result of repressive interaction of Pax6 with En1 and Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to the ventralization of the tissue, that is, the area affected by Shh differentiates into the tegmentum. It is proposed that the brain vesicle that expresses Otx2, Pax2 and En1 might differentiate into the tectum.

Animals↗

PubMed2003

Differential age-related changes in the regional metencephalic volumes in humans: a 5-year follow-up.

We examined changes in the metencephalon of healthy adults who underwent magnetic resonance imaging twice, 5 years apart. The volumes of the cerebellar hemispheres, anterior and posterior vermis, declive-folium-tuber of the vermis, and the ventral pons were measured by operators unaware of subjects' characteristics and the time of scan. The cerebellar hemispheres and the cerebellar vermis, but not the ventral pons, were smaller in older people at both measurements. Significant linear shrinkage was observed in all examined metencephalic regions. Annualized rate of shrinkage was the greatest in the cerebellar hemispheres, somewhat smaller in the vermis, and minimal in the pons. Neither sex differences, nor sex-specific aging trends were found. We conclude that differential age-related shrinkage of the metencephalic structures occurs in healthy adults, but its magnitude differs from cross-sectional estimates.

Adult↗

PubMed2001

Distribution of alpha-neoendorphin immunoreactivity in the diencephalon and the brainstem of the dog.

Alpha-neoendorphin (alpha-NE) is an opiate decapeptide derived from the prodynorphin protein. Its anatomical distribution in the brain of mammals other than the rat, particularly in carnivores, is less well known than for other opiate peptides. In the present work, we have charted the distribution of alpha-NE immunoreactive fibers and perikarya in the diencephalon and the brainstem of the dog. The highest densities of labeled fibers were found in the substantia nigra and in patches within the nucleus of the solitary tract. Moderate densities appeared in the arcuate nucleus (Ar), median eminence, entopeduncular nucleus, ventral tegmental area, retrorubral area, periaqueductal central gray, interpeduncular nucleus and lateral parabrachial nucleus. Groups of numerous labeled perikarya were localized in the magnocellular hypothalamic nuclei, Ar and in the central superior and incertus nuclei in the metencephalon. Moreover, less densely packed fibers and cells appeared widely distributed throughout many nuclei in the region studied. These results are discussed with regard to the pattern described in other species. In addition, the present results were compared with the distribution of met-enkephalin immunoreactivity in the diencephalon and the brainstem of the dog that we have recently described. Although the distributions of these two peptides overlap in many areas, the existence of numerous differences suggest that they form separate opiate systems in the dog.

Animals↗

PubMed2000

Hmx: an evolutionary conserved homeobox gene family expressed in the developing nervous system in mice and Drosophila.

Three homeobox genes, one from Drosophila melanogaster (Drosophila Hmx gene) and two from mouse (murine Hmx2 and Hmx3) were isolated and the full-length cDNAs and corresponding genomic structures were characterized. The striking homeodomain similarity encoded by these three genes to previously identified genes in sea urchin, chick and human, as well as the recently cloned murine Hmx1 gene, and the low homology to other homeobox genes indicate that the Hmx genes comprise a novel gene family. The widespread existence of Hmx genes in the animal kingdom suggests that this gene family is of ancient origin. Drosophila Hmx was mapped to the 90B5 region of Chromosome 3 and at early embryonic stages is primarily expressed in distinct areas of the neuroectoderm and subsets of neuroblasts in the developing fly brain. Later its expression continues in rostral areas of the brain in a segmented pattern, suggesting a putative role in the development of the Drosophila central nervous system. During evolution, mouse Hmx2 and Hmx3 may have retained a primary function in central nervous system development as suggested by their expression in the postmitotic cells of the neural tube, as well as in the hypothalamus, the mesencephalon, metencephalon and discrete regions in the myelencephalon during embryogenesis. Hmx1 has diverged from other Hmx members by its expression in the dorsal root, sympathetic and vagal nerve (X) ganglia. Aside from their expression in the developing nervous system, all three Hmx genes display expression in sensory organ development, and in the adult uterus. Hmx2 and Hmx3 show identical expression in the otic vesicle, whereas Hmx1 is strongly expressed in the developing eye. Transgenic mouse lines were generated to examine the DNA regulatory elements controlling Hmx2 and Hmx3. Transgenic constructs spanning more than 31 kb of genomic DNA gave reproducible expression patterns in the developing central and peripheral nervous systems, eye, ear and other tissues, yet failed to fully recapitulate the endogenous expression pattern of either Hmx2 or Hmx3, suggesting both the presence and absence of certain critical enhancers in the transgenes, or the requirement of proximal enhancers to work synergistically.

Amino Acid Sequence↗

PubMed2004

Regulation of Otx2 expression and its functions in mouse forebrain and midbrain.

Otx2 expression in the forebrain and midbrain was found to be regulated by two distinct enhancers (FM and FM2) located at 75 kb 5' upstream and 115 kb 3' downstream. The activities of these two enhancers were absent in anterior neuroectoderm earlier than E8.0; however, at E9.5 their regions of activity spanned the entire mesencephalon and diencephalon with their caudal limits at the boundary with the metencephalon or isthmus. In telencephalon, activities were found only in the dorsomedial aspect. Potential binding sites of OTX and TCF were essential to FM activity, and TCF sites were also essential to FM2 activity. The FM2 enhancer appears to be unique to rodent; however, the FM enhancer region is deeply conserved in gnathostomes. Studies of mutants lacking FM or FM2 enhancer demonstrated that these enhancers indeed regulate Otx2 expression in forebrain and midbrain. Development of mesencephalic and diencephalic regions was differentially regulated in a dose-dependent manner by the cooperation between Otx1 and Otx2 under FM and FM2 enhancers: the more caudal the structure the higher the OTX dose requirement. At E10.5 Otx1-/-Otx2DeltaFM/DeltaFM mutants, in which Otx2 expression under the FM2 enhancer remained, exhibited almost complete loss of the entire diencephalon and mesencephalon; the telencephalon did, however, develop.

Alleles↗

PubMed2000

LHRH neuronal migration: heterotypic transplantation analysis of guidance cues.

During embryonic development, the olfactory placode (OP) differentiates into the olfactory epithelium (OE). Luteinizing hormone-releasing hormone (LHRH) neurons migrate out of the OE in close association with the olfactory nerve (ON) to the telencephalon. LHRH neuronal migration and ON extension to the telencephalon may be independent events which are correlated but do not represent a causal relationship. However, we hypothesize that LHRH neurons are dependent on ON axons to migrate to the brain. To test this hypothesis, we ablated the right trigeminal placode and replaced it with an OP from another chick embryo. After several days' additional incubation, the embryos were fixed, sectioned, and immunostained with antibodies against LHRH or N-CAM. The ectopic OPs were well integrated into the host and developed into relatively normal appearing OEs. The ONs extended from the OE to several different sites: the lateral rectus of the eye, the ciliary ganglion, and the trigeminal ganglion. In all cases, LHRH neurons were found in the OE and ON, regardless of where the ON terminated. When the ON extended to the trigeminal ganglion, LHRH neurons could clearly be seen entering the metencephalon. Our results support the idea that LHRH neurons are dependent on the ON for guidance as they appear to follow the nerve even when it extends away from the brain. The cues which direct the ON and LHRH neurons to the telencephalon do not appear to be unique to this brain region.

Animals↗

PubMed1996

Urotensin II in the central nervous system of the frog Rana ridibunda: immunohistochemical localization and biochemical characterization.

Urotensin II (UII) is traditionally regarded as a product of the neurosecretory cells in the caudal portion of the spinal cord of jawed fishes. A peptide related to UII has been recently isolated from the frog brain, thereby providing the first evidence that UII is also present in the central nervous system of a tetrapod. In the present study, we have investigated the distribution of UII-immunoreactive elements in the brain and spinal cord of the frog Rana ridibunda by immunofluorescence using an antiserum directed against the conserved cyclic region of the peptide. Two distinct populations of UII-immunoreactive perikarya were visualized. The first group of positive neurons was found in the nucleus hypoglossus of the medulla oblongata, which controls two striated muscles of the tongue. The second population of immunoreactive cell bodies was represented by a subset of motoneurons that were particularly abundant in the caudal region of the cord (34% of the motoneuron population). The telencephalon, diencephalon, mesencephalon, and metencephalon were totally devoid of UII-containing cell bodies but displayed dense networks of UII-immunoreactive fibers, notably in the thalamus, the tectum, the tegmentum, and the granular layer of the cerebellum. In addition, a dense bundle of long varicose processes projecting rostrocaudally was observed coursing along the ventral surface of the brain from the midtelencephalon to the medulla oblongata. Reversed-phase high-performance liquid chromatography analysis of frog brain, medulla oblongata, and spinal cord extracts revealed that, in all three regions, UII-immunoreactive material eluted as a single peak which exhibited the same retention time as synthetic frog UII. Taken together, these data indicate that UII, in addition to its neuroendocrine functions in fish, is a potential regulatory peptide in the central nervous system of amphibians.

Animals↗

PubMed1996

Expression of cadherin-11 delineates boundaries, neuromeres, and nuclei in the developing mouse brain.

Cadherin-11 (cad11 or OB-cadherin) was previously identified as a mesenchymal cell-cell adhesion molecule. Here we studied the expression of cad11 transcripts in developing brains derived from E11.5 to E16.5 mouse embryos. In the brains at these stages, cad11 was expressed in various patterns, which could be grouped into three categories. First, cad11 expression occurred along boundaries between certain brain subdivisions, including those between the ventral and dorsal thalamus and between the mesencephalon and metencephalon. At these boundaries, cad11-positive cells were localized in a narrow, columnar compartment of the neuroepithelium. Second, cad11 expression delineated particular neuromeric compartments at the ventricular zone of the neuroepithelium. A typical example of this pattern was observed in the hypothalamus. Third, cad11 was expressed in differentiating or differentiated brain nuclei. The former included the thalamus, epithalamus, and pretectum; the latter included the mammillary, red, trigeminal motor, facial motor, prepositus hypoglossal, and inferior olive nuclei, as well as the substantia nigra. Furthermore, developing nuclei and the superficial zone of the cerebellum expressed cad11, and the cortical plate of the developing cerebrum also did so. We compared the expression pattern of cad11 with that of R-cadherin in the diencephalon and found that each cadherin delineated a unique set of diencephalic subdivisions. These findings suggest that cad11-mediated specific cell-cell adhesion plays roles in segmentation or compartmentalization of the developing brain in various ways. We also discussed the possibility that cad11 might be involved in neuronal connections between specific nuclei.

Animals↗

PubMed1999

Monoclonal antibody GL1 and its possible involvement in the morphogenesis of the otic vesicle.

In a previous study, a monoclonal antibody (MAB) named GL1 was identified that is expressed in a precise pattern during gastrulation and early neurulation stages in chick embryos. In this article we have further investigated the expression pattern of this MAB in the chick embryo. GL1 antigen is present in several organs that seem not to be related developmentally. Among them, GL1 is present during the early steps of the otic placode formation, in the pharyngeal endoderm, in some neural crest cells, in the somites, and in the ventricular surface of the nervous system. The distribution in the nervous system is well patterned with two broad lines of expression in the ventricular side of the metencephalic region, a unique and centered expression in the border between the metencephalon and the myelencephalon and again in two lines running along the myelencephalon and the rostral spinal cord. Additionally, GL1 can be induced by members of the FGF family, and we have used this system to elucidate its role in otic placode formation. The results obtained reveal that GL1 can be a useful marker for the study of developmental processes in the endoderm, the otic anlage, and the apical surface of the nervous system. Biochemical analysis of the antigen recognized by this MAB must be carried out to elucidate the molecular nature of the antigen.

Animals↗

PubMedDate not supplied

Gestational administration of cadmium alters the neurotransmitter levels in newborn rat brains.

The effects of gestational and early lactational intoxication by cadmium (Cd) were studied in the brain of young Wistar rats. Pregnant rats were exposed to 10 mg of cadmium acetate per litre of drinking water, from initiation of pregnancy to parturition or until postnatal day 5. At birth or on postnatal day (PND) 5 the pups were weighed, sacrificed and brains were removed and frozen for later study. Protein, lipid and nucleic acid contents were measured and the brain Cd concentration was determined. Levels of dopamine (DA), 5-hydroxytryptamine (5-HT) and their respective metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) were measured in neonatal pup brain by higher performance liquid chromatography with electrochemical detection. The results from this experiment showed that Cd increased the 5-HT and 5-HIAA contents in all areas of the brain and the DA and DOPAC levels in mesencephalon, but decreased the DA and DOPAC levels in the metencephalon. On the other hand, Cd intoxication did not modify the other biochemical parameters measured, with the exception of a decrease in nucleic acids on PND 5.

Animals↗

PubMed1992

Changes in dorsoventral but not rostrocaudal regionalization of the chick neural tube in the absence of cranial notochord, as revealed by expression of engrailed-2.

Notochord has been implicated in previous studies in both the dorsoventral and rostrocaudal patterning of the developing neural tube. This possibility has been further explored by analyzing the expression of Engrailed-2 in chick embryos developing with cranial notochord defects. Control embryos containing intact notochords expressed Engrailed-2 protein within the neural tube and in a subset of the neural crest and overlying surface ectoderm at the future mesencephalon and cranial metencephalon levels. Within the neural tube, expression was confined to cell nuclei in the roof plate and lateral walls; floor plate nuclei directly overlying the notochord typically failed to show expression. After surgical removal of Hensen's node, the source of notochord precursor cells, embryos were cultured through neurulation and assayed for expression of Engrailed-2 protein. All embryos that partially or completely lacked cranial notochord expressed Engrailed-2 in a pattern similar to that of control embryos containing intact notochords, except that when notochord and floor plate were absent, Engrailed-2 was also expressed in the most ventral part of the neural tube. These results indicate that 1) Engrailed-2 expression is suppressed in the most ventral neural tube owing to induction of the floor plate by the notochord, and 2) that the presence of an underlying notochord is not required for correct rostrocaudal expression, suggesting that multiple pathways act in the patterning of the rudiment of the central nervous system.

Animals↗

PubMed1994

Developmental relations between sixth nerve motor neurons and their targets in the chick embryo.

The developmental relations between abducens (VI) nerves and their targets, the lateral rectus, quadratus, and pyramidalis muscles, have been examined in the chick embryo from early neural tube stages through 10 days of incubation. Sites of myoblast origins were determined by microinjection of replication-incompetent retroviruses containing the LacZ reporter into paraxial mesoderm corresponding to somitomeres 3-5. Motor neurons and axons were identified by Bodian staining, immunocytochemistry, and application of DiI and DiO to dissected peripheral nerves. Anlage of the dorsal oblique originate in somitomere 3, close to the ventrolateral margin of the mid-to-caudal mesencephalon. Precursors of the lateral rectus arise deep within somitomere 4, beside the future metencephalon (rhombomere "A"). Quadratus and pyramidalis precursors are located between and partially segregated from these other two anlage. VIth nerve axons exit rhombomeres 5 and 6 via multiple median roots, fasciculate, and by stage 17 have elongated rostrally beneath the hindbrain. Immediately caudal to a mesenchymal pre-muscle condensation located deep to rhombomere 2, the VIth nerve separates into two branches. One branch enters the rostral portion of the condensation, from which quadratus and pyramidalis muscles will segregate. This branch projects exclusively from rhombomere 5 and is the accessory abducens nerve. The other branch enters the caudal, presumptive lateral rectus, region of the condensation. This is the abducens nerve, and it projects from cells located in both rhombomeres 5 and 6. These findings indicate that specific matching of motor nerves with their presumptive targets begins prior to the differentiation and segregation of myogenic populations, and that spatial organization of developing eye muscles is initiated well before they interact with connective tissue precursors derived from the neural crest.

Abducens Nerve↗