Afferent and efferent connections of the secondary general visceral sensory nucleus in goldfish.

The secondary general visceral sensory nucleus (SVN) receives ascending fibers from the commissural nucleus of Cajal (NCC), or the primary general visceral sensoru in the medulla oblongata of teleosts. However, the full set of fiber connections of the SVN have been studied only in the Nile tilapia. We have investigated the connections of the SVN in goldfish by tracer injection experiments to the nucleus. We paid special attention to the possible presence of spinal afferents, since the spinal cord projects to the lateral parabrachial nucleus, or the presumed homologue of SVN, in mammals. We found that the SVN indeed receives spinal projections. Spinal terminals were restricted to a region ventrolaterally adjacent to the terminal zone of NCC fibers, suggesting that the SVN can be subdivided into two subnuclei: the commissural nucleus-recipient (SVNc) and spinal-recipient (SVNsp) subnuclei. Tracer injections to the SVNc and SVNsp as well as reciprocal injections to the diencephalon revealed that both subnuclei project directly to diencephalic structures, such as the posterior thalamic nucleus and nucleus of lateral recess, although diencephalic projections of the SVNsp were rather sparse. The SVNsp appears to send fibers to more wide-spread targets in the preoptic area than the SVNc does. The SVNc projects to the telencephalon, while the SVNsp sends scarce or possibly no fibers to the telencephalon. Another notable difference was that the SVNsp gives rise to massive projections to the dorsal diencephalon (ventromedial thalamic, central posterior thalamic, and periventricular posterior tubercular nuclei). These differential connections of the subnuclei may reflect discrete functional significances of the general visceral sensory information mediated by the medulla oblongata and spinal cord.

Fiber Connections of the Caudal Corpus Cerebelli, with Special Reference to the Intrinsic Circuitry, in a Teleost (Oreochromis niloticus).

The caudal part of the corpus cerebelli of Nile tilapia can be divided into dorsal and ventral regions. The granule cell layer of the dorsal (dGL) and ventral (vGL) regions of the caudal corpus cerebelli is known to receive indirect inputs from the telencephalon relayed by the nucleus paracommissuralis. The descending pathways are topographically organized, and the dGL and vGL receive inputs from different dorsal telencephalic parts. The caudal corpus cerebelli, in turn, projects extracerebellar efferents. However, it remains unknown how the descending telencephalic inputs are processed within the cerebellum. Therefore, the present study investigated intrinsic connections of the caudal corpus cerebelli by injecting neural tracers into the molecular layer of dorsal and ventral regions. Injections of tracers into the ventral molecular layer resulted in labeled cells in the vGL and the ganglionic layer of the ventral corpus. The axonal trajectories from labeled cells in the ganglionic layer were analyzed in detail via single-axon reconstructions, which suggested that the terminal portions were confined to the ganglionic layer of the dorsal corpus. No labeled terminals were observed outside the caudal corpus cerebelli. Tracer applications to the dorsal molecular layer resulted in labeled cells not only in the ganglionic layer and the granule cell layer of the dorsal corpus but also in the ganglionic layer of the ventral corpus. The latter finding confirms the presence of intrinsic projections from the ventral region to the dorsal region in the caudal corpus cerebelli. We further revealed that the intrinsic projection neurons are Purkinje cells by immunohistochemistry for zebrin II (aldolase C), which is a marker of Purkinje cells, combined with tracer injections into the dorsal corpus. Unlike injections into the ventral corpus, injections into the dorsal corpus resulted in labeled terminals in extracerebellar structures, such as the nucleus of the medial longitudinal fascicle and reticular formation. The present study suggests that indirect inputs from different telencephalic parts received and processed by distinct regions of caudal corpus cerebelli are sent out of the corpus through the efferent neurons in the dorsal corpus.

The Parapineal Is Incorporated into the Habenula during Ontogenesis in the Medaka Fish.

The parapineal is present in many teleost families, while it is absent in several others. To find out why the parapineal is absent at adult stages in the latter families, the development of the epithalamus was examined in the medaka fish (Oryzias latipes). For this purpose, a green fluorescent protein-transgenic medaka line, in which the pineal complex (pineal and parapineal) is visible fluorescently, was used. We found that a distinct parapineal was present in the roof plate at early developmental stages. Subsequently, however, the parapineal and the associated roof plate began to be incorporated into the habenula between embryonic stages 28 and 29. Between embryonic stages 29 and 30, the entire parapineal was incorporated into the habenula. That is, the parapineal became a small caudomedial region (termed the 'parapineal domain') within the left habenula in the majority of embryos, resulting in the left-sided asymmetry of the epithalamus. Thereby the left habenula became larger and more complex than its right counterpart. In the minority of embryos, the parapineal was incorporated into the right habenula or into the habenulae on both sides. In the majority of embryos, the parapineal domain projected a fiber bundle to a subnucleus (termed the 'rostromedial subnucleus') in the left habenula. The rostromedial subnucleus sent axons, through the left fasciculus retroflexus, to the rostral region of the left half of the interpeduncular nucleus. We further found that the ratio of the left-sided phenotype was temperature dependent and decreased in embryos raised at a high temperature. The present study is the first demonstration that the supposed lack of a distinct parapineal in adult teleost fishes is due to ontogenetic incorporation into the habenula.

The primary brain vesicles revisited: are the three primary vesicles (forebrain/midbrain/hindbrain) universal in vertebrates?

It is widely held that three primary brain vesicles (forebrain, midbrain, and hindbrain vesicles) develop into five secondary brain vesicles in all vertebrates (von Baer's scheme). We reviewed previous studies in various vertebrates to see if this currently accepted scheme of brain morphogenesis is a rule applicable to vertebrates in general. Classical morphological studies on lamprey, shark, zebrafish, frog, chick, Chinese hamster, and human embryos provide only partial evidence to support the existence of von Baer's primary vesicles at early stages. Rather, they suggest that early brain morphogenesis is diverse among vertebrates. Gene expression and fate map studies on medaka, chick, and mouse embryos show that the fates of initial brain vesicles do not accord with von Baer's scheme, at least in medaka and chick brains. The currently accepted von Baer's scheme of brain morphogenesis, therefore, is not a universal rule throughout vertebrates. We propose here a developmental hourglass model as an alternative general rule: Brain morphogenesis is highly conserved at the five-brain vesicle stage but diverges more extensively at earlier and later stages. This hypothesis does not preclude the existence of deep similarities in molecular prepatterns at early stages.

Ascending general visceral sensory pathways from the brainstem to the forebrain in a cichlid fish, Oreochromis (Tilapia) niloticus.

Fiber connections of the general visceral sensory centers in the brainstem were studied with tract-tracing methods in a percomorph teleost, tilapia Oreochromis niloticus. General visceral afferents of the vagal nerve from abdominal viscera terminated bilaterally in the commissural nucleus of Cajal (NCC) and area postrema (AP). The NCC and AP projected bilaterally to the secondary general visceral nucleus (SVN), four diencephalic nuclei (the preglomerular general visceral nucleus [pVN], nucleus of the lateral recess, posterior thalamic nucleus, and lateral tuberal area), preoptic area, and ventral telencephalon (supracommissural, dorsal, and ventral parts) in addition to the glossopharyngeal and vagal lobes and medullary reticular formation. Injections to the SVN resulted in labeled terminals in the forebrain structures that receive fibers from the primary centers and additionally in the diffuse nucleus of the inferior lobe, lateral torus, and inferior subdivision of lateral torus. The present study suggests that the ascending general visceral projections arising from the brainstem centers in teleosts are quite similar to those in mammals and birds. Descending pathways were also notable. In addition to descending projections from the SVN and medullary structures to the primary centers, long descending pathways to the SVN, NCC, and AP were found to originate from the pVN, nucleus of the lateral recess, posterior thalamic nucleus, and preoptic area. The SVN was found to receive fibers from the ventral telencephalon as well. Therefore, the present study indicates that most of the general visceral structures in the forebrain are reciprocally connected with the brainstem centers.

Morphogenesis of the medaka cerebellum, with special reference to the mesencephalic sheet, a structure homologous to the rostrolateral part of mammalian anterior medullary velum.

We have examined cerebellar morphogenesis after neural tube stage in medaka (Oryzias latipes), a ray-finned fish, by conventional histology and immunohistochemistry using anti-proliferating cell nuclear antigen (PCNA) and anti-acetylated tubulin antibodies. Our results indicate that the medaka cerebellum is formed in 4 successive stages: (1) formation and enlargement of the cerebellar primordia; (2) rostral midline fusion of the left/right halves of the cerebellar primordia; (3) formation of the cerebellar matrix zones in the midline and caudalmost regions of the primitive cerebellum, and (4) growth and differentiation of the cerebellum. Our results also show that cerebellar morphogenesis is different from that in mammals in 3 important points: the developmental origins of the primordia, directions along which cerebellar fusion proceeds, and number, locations and duration of the cerebellar matrix zones. During the course of this study, an alar-derived membranous structure between the cerebellum and the midbrain in the adult medaka brain was identified as the structure homologous to the rostrolateral part of the mammalian anterior medullary velum. We have named this structure in the adult teleostean brains as the 'mesencephalic sheet'. The present study indicates that there exists both conserved and divergent patterns in cerebellar morphogenesis in vertebrates.

Rapid and simple method for quantitative evaluation of neurocytotoxic effects of radiation on developing medaka brain.

We describe a novel method for rapid and quantitative evaluation of the degree of radiation-induced apoptosis in the developing brain of medaka (Oryzias latipes). Embryos at stage 28 were irradiated with 1, 2, 3.5, and 5 Gy x-ray. Living embryos were stained with a vital dye, acridine orange (AO), for 1-2 h, and whole-mount brains were examined under an epifluorescence microscope. From 7 to 10 h after irradiation with 5 Gy x-ray, we found two morphologically different types of AO-stained structures, namely, small single nuclei and rosette-shaped nuclear clusters. Electron microscopy revealed that these two distinct types of structures were single apoptotic cells with condensed nuclei and aggregates of apoptotic cells, respectively. From 10 to 30 h after irradiation, a similar AO-staining pattern was observed. The numbers of AO-stained rosette-shaped nuclear clusters and AO-stained single nuclei increased in a dose-dependent manner in the optic tectum. We used the number of AO-stained rosette-shaped nuclear clusters/optic tectum as an index of the degree of radiation-induced brain cell death at 20-24 h after irradiation. The results showed that the number of rosette-shaped nuclear clusters/optic tectum in irradiated embryos exposed to 2 Gy or higher doses was highly significant compared to the number in nonirradiated control embryos, whereas no difference was detected at 1 Gy. Thus, the threshold dose for brain cell death in medaka embryos was taken as being between 1-2 Gy, which may not be so extraordinarily large compared to those for rodents and humans. The results show that medaka embryos are useful for quantitative evaluation of developmental neurocytotoxic effects of radiation.

Early development of the cerebellum in teleost fishes: a study based on gene expression patterns and histology in the medaka embryo.

The cerebellar structures of teleosts are markedly different from those of other vertebrates. The cerebellum continues rostrally into the midbrain ventricle, forming the valvula cerebelli, only in ray-finned fishes among vertebrates. To analyze the ontogenetic processes that underlie this morphological difference, we examined the early development of the cerebellar regions, including the isthmus (mid/hindbrain boundary, MHB), of the medaka (Oryzias latipes), by histology and in-situ hybridization using two gene (wnt1 and fgf8) probes. Isthmic wnt1 was expressed stably in the caudalmost mesencephalic region in the neural tube at all developmental stages examined, defining molecularly the caudal limit of the mesencephalon. The wnt1-positive mesencephalic cells became located rostrally to the isthmic constriction at Iwamatsu's stages 25-26. Isthmic fgf8 expression changed dynamically and became restricted to the rostralmost metencephalic region at stage 24. The rostralmost part (prospective valvula cerebelli) of the fgf8-positive rostral metencephalon protruded rostrally into the midbrain ventricle, bypassing the isthmic constriction, at stages 25-26. Thus, the isthmic constriction shifted caudally with respect to the molecularly defined MHB at stages 25-26. Paired cerebellar primordia were formed from the alar plates of the fgf8-positive rostral metencephalon and the fgf8-negative caudal metencephalon in the medaka neural tube. Our results show that cerebellar development differs between teleosts and murines: both the rostral and caudal metencephalic alar plates develop into the cerebellum in medaka, whereas in the murines only the caudal metencephalic alar plate develops into the cerebellum, and the rostral plate is reduced to a thin membrane.

Fiber connections of the corpus glomerulosum pars rotunda, with special reference to efferent projection pattern to the inferior lobe in a percomorph teleost, tilapia (Oreochromis niloticus).

Fiber connections of the corpus glomerulosum pars rotunda (GR) in a teleost, tilapia Oreochromis niloticus, were studied by biotinylated dextran amine injections into the GR and inferior lobe. After tracer injections into the GR, major groups of labeled somata were found bilaterally in the cortical nucleus and ipsilaterally in the nucleus intermedius. Numerous labeled terminals were found ipsilaterally in the central nucleus, nucleus of lateral recess, and diffuse nucleus (NDLI) of the inferior lobe. Some other connections were also elucidated in the present study, although these were less abundant. Notably, efferent projections to the inferior lobe were not evenly distributed within each lobar nucleus. Labeled terminals were confined to the cell body zone of central nucleus and the outer cell-sparse layer of the nucleus of lateral recess. The rostrolateral portion of NDLI and ventrolateral portion of middle to caudal NDLI received few GR fibers, the rostromedial portion of NDLI a moderate density of fibers, and the rest of the nucleus numerous fibers. These different portions of the NDLI, to some extent, also differed in other afferent and efferent connections, suggesting regional specialization of the nucleus. Furthermore, restricted injections to the lobar nuclei suggest different efferent projections of the component cells of the GR: large and small cells. The large cells project only to the central nucleus, whereas the small cells project to the NDLI and nucleus of lateral recess. Therefore, the two types of GR cells appear to constitute parallel pathways from the pretectum to the inferior lobe.

Diversity of brain morphology in teleosts: brain and ecological niche.

Modern teleosts have more copies of developmental regulatory genes than other vertebrates, probably due to a whole genome duplication that occurred specifically at the base of the lineage of ray-finned fishes. The genome duplication generates duplicated genes (including their regulatory regions), and one of the duplicates might become redundant and free from selective pressures. These redundant genes might be more easily mutated during evolution. Brain morphogenesis is a process that is dependent on a large genetic program in which a subprogram for the regionalization of the brain is coupled with that for cell-proliferation control. If beneficial mutations took place in key genes within the genetic program for brain morphogenesis, it might result in the enhancement of region-specific cell proliferation and cell survival in the corresponding brain subdivisions. This mechanism might account for the appearance of various forms of teleost brains, which have been preserved under selection pressure in diverse environments. It is conceivable that variously modified brains might evolve under the conditions of natural selection so that the brains help fit the teleost species for diverse ecological niches.

Developmental origin of diencephalic sensory relay nuclei in teleosts.

We propose here a novel interpretation of the embryonic origin of cells of diencephalic sensory relay nuclei in teleosts based on our recent studies of gene expression patterns in the medaka (Oryzias latipes) embryonic brain and comparative hodological studies. It has been proposed that the diencephalic sensory relay system in teleosts is unique among vertebrates. Teleost relay nuclei, the preglomerular complex (PG), have been assumed to originate from the basal plate (the posterior tuberculum) of the diencephalon, whereas relay nuclei in mammals are derived from the alar plate (dorsal thalamus) of the diencephalon. Our results using in situ hybridization show, however, that many pax6- or dlx2-positive cells migrate laterally and ventrocaudally from the diencephalic alar plate to the basal plate during development. Massive clusters of the migrated alar cells become localized in the mantle layer lateral to the posterior tubercular neuroepithelium, from which main nuclei of the PG appear to differentiate. We therefore consider most if not all neurons in the PG to be of alar, not basal, origin. Thus, the teleost PG, at least in part, can be regarded as migrated alar nuclei. Developmental and hodological data strongly suggest that the teleost PG is homologous to a part of the mammalian dorsal thalamus. The organization and origin of the diencephalic sensory relay system might have been conserved across vertebrates.

A new interpretation on the homology of the teleostean telencephalon based on hodology and a new eversion model.

Various hypotheses regarding the homology of the teleostean telencephalon with that of other vertebrates have been proposed to date. However, a firm conclusion on this issue has yet to be drawn. We propose here a new hypothesis with a new eversion model. Our hodological data and the analysis of dorsal telencephalic organization in adult cyprinids suggest that: (1) the area dorsalis pars posterior corresponds to the lateral pallium; (2) ventral region of area dorsalis pars lateralis to the medial pallium; (3) pars medialis, dorsal region of pars lateralis, pars dorsalis, and pars centralis of the area dorsalis to the dorsal pallium, and (4) nucleus taenia to the ventral pallium. We propose in a three dimensional model that the eversion process occurs not only dorsolaterally but also caudolaterally. We consider that the caudally directed component dominates for ventral zones of the pallium, or the lateral and ventral pallia; and the periventricular surface of these zones shift caudally, laterally, and then rostrally in teleosts with pronounced telencephalic eversion. This new model fits well with the putative homology based on hodology and the organization of telencephalic divisions in the adult brain.

The medial branch of the lateral branch of the posterior ramus of the spinal nerve.

UNLABELLED: In the needle insertion of epidural anesthesia with the paramedian approach, the needle can pass through the longissimus muscle in the dorsum of the patients. When the needle touches a nerve in the muscles, the patients may experience pain in the back. Obviously, the needle should avoid the nerve tract. To provide better anesthetic service, analysis of the structure and where the concerned nerves lie in that region is inevitable. MATERIAL AND METHOD: We studied five cadavers in this study. Two cadavers were fixed with Thiel's method. With these cadavers, we studied the nerve running of the posterior rami of the spinal nerve from the nerve root to the distal portion. Three of them were used for the study of transparent specimen, with which we studied the course and size of the nerve inside the longissimus muscle. RESULTS: We observed there were three branches at the stem of the posterior rami of the spinal nerves between the body segment T3 and L5, i.e. medial branch, medial branch of the lateral branch and lateral branch of the lateral branch. The medial branch of the lateral branch supplied to the longissimus muscle. With the transparent specimen, we found that there were different nerve layouts between the upper thoracic, lower thoracic, upper lumbar, and lower lumbar segments in the medial branch of the lateral branch in the longissimus muscle. In the lower thoracic and upper lumbar segments, the medial branch of the lateral branch of the upper lumbar segments produced layers nerve network in the longissimus muscle. L1 and L2 nerves were large in size in the muscle. CONCLUSION: In the upper lumbar segments the medial branch of the lateral branch of the posterior rami of the spinal nerve produced dense network in the longissimus muscle, where the epidural needle has high possibility to touch the nerve. Anesthetists have to consider the existence of the medial branch of the lateral branch of the posterior rami of the spinal nerve when they insert the needle in the paramedical approach to the spinal column.

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus).

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.

Central connection of the optic, oculomotor, trochlear and abducens nerves in medaka, Oryzias latipes.

Medaka (Oryzias latipes) is one of the few vertebrate experimental animals in which inbred lines have been established. It is also a species that has advanced in genetic studies in a manner comparable to zebrafish. This fish is therefore a good model for studying functional organization of the nervous system, but anatomical analysis of its nervous system has been limited to embryonic stages. In the present study, we investigated anatomy of cranial nerves in adult fish focusing on the visual function, using an inbred strain of medaka. Cranial nerves of medaka were labeled using biocytin, revealing a central distribution of retinofugal terminals, retinopetal neurons, and oculomotor, trochlear and abducens motor neurons. The optic nerve of the adult medaka was of a complete decussation type. Retinofugal terminals were located in 8 brain nuclei, the suprachiasmatic nucleus, nucleus pretectalis superficialis, nucleus dorsolateralis thalami, area pretectalis pars dorsalis (APd), area pretectalis pars ventralis (APv), nucleus of the posterior commissure (NPC), accessory optic nucleus, and the tectum opticum. Retinopetal neurons were identified in 6 brain nuclei, the ganglion of the terminal nerve, preoptic retinopetal nucleus, nucleus dorsolateralis thalami, APd, APv, and NPC. The oculomotor neurons were mostly labeled ipsilaterally and were located dorsomedially, abutting the fasciculus longitudinalis medialis in the mesencephalon. The trochlear nucleus was located contralaterally and dorsolaterally adjacent to the fasciculus longitudinalis medialis in the mesencephalon. The abducens nucleus was located ipsilaterally in a ventrolateral part of the rhombencephalic reticular formation. These results, generally similar to those in other teleosts, provide the basis for future behavioral and genetic studies in medaka.

Somatotopic organization of the trigeminal ganglion cells in a cichlid fish, Oreochromis (Tilapia) niloticus.

Somatotopic organization of the trigeminal ganglion is known in some vertebrates. The precise pattern of somatotopy, however, seems to vary in different vertebrate groups. Furthermore, the somatotopic organization remains to be studied in teleosts. From an evolutionary point of view, the morphology and somatotopic organization of the trigeminal ganglion of a percomorph teleost, Tilapia, were investigated by means of the tract-tracing method using biocytin and three-dimensional reconstruction models with a computer. The trigeminal ganglion was one cell aggregate elongated in the dorsoventral direction, which was separate from the facial and anterior lateral line ganglia. Biocytin applications to the trigeminal nerve root labeled ordinary ganglion cells in the trigeminal ganglion and a few displaced trigeminal ganglion cells in the facial ganglion. Biocytin applications to three primary branches (the ophthalmic, maxillary, and mandibular nerves) revealed that trigeminal ganglion cells were somatotopically distributed in the ganglion reflecting the dorsoventral order of the three branches. Ganglion cells of the ophthalmic nerve were distributed in the dorsal part of the trigeminal ganglion, those of the mandibular nerve in the ventral part, and those of the maxillary nerve in the intermediate part. Some of maxillary and mandibular ganglion cells appear to overlap in their boundary region, whereas ophthalmic ganglion cells did not intermingle with ganglion cells of other branches. Labeled-primary fibers terminated in the sensory trigeminal nucleus, descending trigeminal nucleus, medial funicular nucleus, a ventral part of the facial lobe, reticular formation, and trigeminal motor nucleus. Labeled cells were observed in the mesencephalic trigeminal nucleus and the trigeminal motor nucleus. The results suggest that the morphology and somatotopic organization of the trigeminal ganglion of tilapia are similar to those of mammals, except that the axis of the somatotopic organization of the ganglion in mammals is a mediolateral direction reflecting the mediolateral order of the ophthalmic, maxillary, and mandibular nerves.

Three cases of retroesophageal right subclavian artery.

We have experienced three cases of retroesophageal right subclavian artery. Two cases were cadavers, and one case was a live human. In the two cadavers of a 68-year-old and a 76-year-old, respectively Japanese and European males, the right subclavian artery originated from the aorta after the aorta branched the right carotid artery, the left carotid artery and the left subclavian artery. The right carotid artery immerged solely from the aorta. Where the right subclavian artery originated from the aorta, the artery took a dorsal direction. It passed between the esophagus and the vertebral column. The esophagus was compressed from the dorsal side by the right subclavian artery. The structural anomaly of the right subclavian artery accompanied the cephalad recurrence of the branch from the right vagal nerve toward the larynx. In the live human case, we obtained CT views. The patient was a 41-year-old Japanese, who complained of dysphagia lusoria. We found that the right subclavian artery was anomalous and originated from the aorta as the last cardinal branch in the thorax.

Morphogenesis and regionalization of the medaka embryonic brain.

We examined the morphogenesis and regionalization of the embryonic brain of an acanthopterygian teleost, medaka (Oryzias latipes), by in situ hybridization using 14 gene probes. We compared our results with previous studies in other vertebrates, particularly zebrafish, an ostariophysan teleost. During the early development of the medaka neural rod, three initial brain vesicles arose: the anterior brain vesicle, which later developed into the telencephalon and rostral diencephalon; the intermediate brain vesicle, which later developed into the caudal diencephalon, mesencephalon, and metencephalon; and the posterior brain vesicle, which later developed into the myelencephalon. In the late neural rod, the rostral brain bent ventrally and the axis of the brain had a marked curvature at the diencephalon. In the final stage of the neural rod, ventricles began to develop, transforming the neural rod into the neural tube. In situ hybridization revealed that the brain can be divided into three longitudinal zones (dorsal, intermediate, and ventral) and many transverse subdivisions, on the basis of molecular expression patterns. The telencephalon was subdivided into two transverse domains. Our results support the basic concept of neuromeric models, including the prosomeric model, which suggests the existence of a conserved organization of all vertebrate neural tubes. Our results also show that brain development in medaka differs from that reported in other vertebrates, including zebrafish, in gene-expression patterns in the telencephalon, in brain vesicle formation, and in developmental speed. Developmental and genetic programs for brain development may be somewhat different even among teleosts.

Axonogenesis in the medaka embryonic brain.

In order to know the general pattern of axonogenesis in vertebrates, we examined axonogenesis in the embryonic brain of a teleost fish, medaka (Oryzias latipes), and the results were compared with previous studies in zebrafish and mouse. The axons and somata were stained immunocytochemically using antibodies to a cell surface marker (HNK-1) and acetylated tubulin and visualized by retrograde and anterograde labeling with a lipophilic dye. The fiber systems developed correlating with the organization of the longitudinal and transverse subdivisions of the embryonic brain. The first axons extended from the synencephalic tegmentum, forming the first fiber tract (fasciculus longitudinalis medialis) in the ventral longitudinal zone of the neural rod, 38 hours after fertilization. In the neural tube, throughout the entire brain two pairs of longitudinal fiber systems, one ventral series and one dorsal or intermediate series, and four pairs of transverse fiber tracts in the rostral brain were formed sequentially during the first 16 hours of axon production. In one of the dorsal longitudinal tracts, its branch retracted and disappeared at later stages. One of the transverse tracts was found to course in the telencephalon and hypothalamus. The overall pattern of the longitudinal fiber systems in medaka brain is similar to that in mouse, but apparently different from that in zebrafish. We propose that a ventral tract reported in zebrafish partially belongs to the dorsal fiber system, and that the longitudinal fiber systems in all vertebrate brains pass through a common layout defined by conserved genetic and developmental programs.

Fiber connections of the lateral valvular nucleus in a percomorph teleost, tilapia (Oreochromis niloticus).

Fiber connections of the lateral valvular nucleus were investigated in a percomorph teleost, the tilapia (Oreochromis niloticus), by tract-tracing methods. Following tracer injections into the lateral valvular nucleus, neurons were labeled in the ipsilateral dorsal part of dorsal telencephalic area, corpus glomerulosum pars anterior, dorsomedial thalamic nucleus, central nucleus of the inferior lobe, mammillary body, semicircular torus, valvular and cerebellar corpus, in the bilateral rostral regions of the central part of dorsal telencephalic area, dorsal region of the medial part of dorsal telencephalic area, habenula, anterior tuberal nucleus, posterior tuberal nucleus, and spinal cord, and in the contralateral lateral funicular nucleus. Labeled fibers and terminals were found in the ipsilateral cerebellar corpus and bilateral valvula of the cerebellum. Tracers were injected into portions of the telencephalon, pretectum, inferior lobe, and cerebellum to confirm reciprocally connections with the lateral valvular nucleus and to determine afferent terminal morphology in the lateral valvular nucleus. Telencephalic fibers terminated mainly in a dorsolateral portion of the lateral valvular nucleus. Terminals from the corpus glomerulosum pars anterior, central nucleus of the inferior lobe, and mammillary body showed more diffuse distributions and were not confined to particular portions of the lateral valvular nucleus. Labeled terminals in the lateral valvular nucleus were cup-shaped or of beaded morphology. These results indicate that the lateral valvular nucleus receives projections from various sources including the telencephalon, pretectum, and inferior lobe to relay information to the valvular and cerebellar corpus. In addition, the corpus glomerulosum pars anterior in tilapia is considered to be homologous to the magnocellular part of superficial pretectal nucleus in cyprinids.