Identification of visual pallial telencephalon in the goldfish, Carassius auratus: a combined cytochrome oxidase and electrophysiological study.

A strategy based upon a comparative decrease in bilateral symmetry of cytochrome oxidase (COX) histochemistry following unilateral eye enucleation was used to identify the primary visual area in the area dorsalis of the telencephalon of the goldfish, Carassius auratus. The lateral zone of area dorsalis (Dl) at about the level of the anterior commissure exhibits such a bilateral difference. A parallel decline in the symmetry COX reactivity was observed in the associated part of the central zone (Dc). Electrophysiological activity using extracellular techniques confirmed the visually-driven activity of neurons in these areas. Lesions confirmed the loci in the lateral zone of area dorsalis, including both its dorsal and ventral parts. Single- and multi-unit recordings exhibited a variety of responses to different light stimuli. Single unit latency measures proved not to be a reliable measure of target areas. Responses habituated to stimuli repeated within 5 s and were only reliably evoked with intervals greater than several seconds.

Sensory system evolution at the origin of craniates.

The multiple events at the transition from non-craniate invertebrate ancestors to craniates included the gain and/or elaboration of migratory neural crest and neurogenic placodes. These tissues give rise to the peripherally located, bipolar neurons of all non-visual sensory systems. The brain was also elaborated at or about this same time. Were the peripheral and central events simultaneous or sequential? A serial transformation hypothesis postulates that paired eyes and an enlarged brain evolved before the elaboration of migratory neural crest placodal sensory systems. Circumstantial evidence for this scenario is derived from the independent occurrence of the combination of large, paired eyes plus a large, elaborated brain in at least three taxa (cephalochordates, arthropods and craniates) and partly from the exclusivity of the diencephalon for visual system-related distal sensory components versus the restricted distribution of migratory neural crest-placodal sensory systems to the remaining parts of the neuraxis. This scenario accounts for the similarity of all central sensory system pathways due to the primary establishment of descending visual pathways via the diencephalon and midbrain tectum to brainstem motor regions and the subsequent exploitation of the same central beachhead by the migratory neural crest-placodal systems as a template for their organization.

Topography and topology of the teleost telencephalon: a paradox resolved.

Analysis of vasculature in the telencephalic pallium of a teleost allows the considerable depth of the sulcus externus, which lies at the lateral extent of the ependymal attachment, to be appreciated. The depth of this sulcus is compelling evidence for a simple eversion process (an outfolding of the pallial wall of each hemisphere) during telencephalic development in all ray-finned fishes that is not complicated in teleosts by secondary migration of pallial cell groups. A simple eversion process is known to occur in some ray-finned fishes with relatively simple telencephalic cytoarchitecture but has been disputed in teleosts based on the pattern of olfactory tract projections. A resolution to the conflicting hypotheses of pallial relationships across ray-finned fishes and in comparison with other craniate radiations is presented here, based on a re-examination of hodological and histochemical data mandated by this sulcal anatomy.

Defining sameness: historical, biological, and generative homology.

Current debate concerning homology arises from three different research interests-phylogenetics, character evolution, and generative pathways. Phylogenetic homology focuses on descent of the character from a common ancestor. Biological homology addresses character evolution and diversification. Exceptions to the general case complicate these two approaches: historically and biologically homologous characters may be produced by different generative pathways, and minutely similar characters produced by the same generative pathways may have a sporadic phylogenetic distribution. We suggest that for studies of comparative developmental biology, new descriptive terms are needed to distinguish similar structures that result from the same generative pathways from those that result from different generative pathways. The terms syngeny, meaning "same genesis", and allogeny, meaning "different genesis", allow the acknowledgement of sameness at the generative level and can be used in combination with the terminology of historical homology and biological homology to describe any given character.

Chordate evolution and the origin of craniates: an old brain in a new head.

The earliest craniates achieved a unique condition among bilaterally symmetrical animals: they possessed enlarged, elaborated brains with paired sense organs and unique derivatives of neural crest and placodal tissues, including peripheral sensory ganglia, visceral arches, and head skeleton. The craniate sister taxon, cephalochordates, has rostral portions of the neuraxis that are homologous to some of the major divisions of craniate brains. Moreover, recent data indicate that many genes involved in patterning the nervous system are common to all bilaterally symmetrical animals and have been inherited from a common ancestor. Craniates, thus, have an "old" brain in a new head, due to re-expression of these anciently acquired genes. The transition to the craniate brain from a cephalochordate-like ancestral form may have involved a mediolateral shift in expression of the genes that specify nervous system development from various parts of the ectoderm. It is suggested here that the transition was sequential. The first step involved the presence of paired, lateral eyes, elaboration of the alar plate, and enhancement of the descending visual pathway to brainstem motor centers. Subsequently, this central visual pathway served as a template for the additional sensory systems that were elaborated and/or augmented with the "bloom" of migratory neural crest and placodes. This model accounts for the marked uniformity of pattern across central sensory pathways and for the lack of any neural crest-placode cranial nerve for either the diencephalon or mesencephalon. Anat Rec (New Anat) 261:111-125, 2000.

An atypical diencephalic nucleus in actinopterygian fishes: visual connections and sporadic phylogenetic distribution.

Nucleus rostrolateralis is a distinctive diencephalic nucleus in some ray-finned fishes. In the osteoglossomorph Pantodon, it is a large ovoid nucleus with visual system connections. A topologically and cytoarchitectonically similar nucleus has been found in six species of non-osteoglossomorph fishes, two species of gars and four euteleosts. Of the latter, two are ostariophysans of the genus Danio, one an atherinomorph, and one a notothenioid percomorph. A variety of characteristic similarities can be found in nucleus rostrolateralis among all these species. The present study reports on these similarities in Danio and in the euteleost Xiphophorus as compared with the nucleus in Pantodon. While this nucleus has a phylogenetic distribution that might imply convergent evolution, the high degree of similarity in its features across species strongly suggests that its genetic basis may be the same despite the lack of phenotypic homology.

Visual connections of the atypical diencephalic nucleus rostrolateralis in Pantodon buchholzi (Teleostei, Osteoglossomorpha).

Among the many thousands of teleost fish species, a diencephalic nucleus rostrolateralis (RL) has been identified in only six widely divergent species. In one of these, Pantodon buchholzi, its retina projects to both RL and the optic tectum, the latter in a visuotopic manner. The ventral part of the retina beneath a horizontal black-pigmented septum connects to the dorsomedial optic tectum and nucleus RL. The dorsal part of the retina connects to the lateroventral optic tectum and little, if at all, to RL. Using DiI tracing, the position of RL in the optic pathway of this fish has been directly demonstrated. Cells in the stratum periventriculare of the dorsomedial optic tectum contribute to the afferent input of RL (bilaterally); cells in the ventrolateral tectum do not. RL is also reciprocally connected with the torus longitudinalis and may project to three nuclei of the preglomerular complex. Ganglion cells in the retina that project to RL are sparsely distributed throughout the ventral hemiretina compared with ganglion cells that project to the optic tectum. Since this fish is an obligatory surface feeder, the neuroanatomical connectivity of nucleus RL in P. buchholzi suggests a role in the fish's visual identification of targets for feeding behavior.

Evolution of sensory pathways in vertebrates.

Sensory receptor evolution is a function of the array of events in the physical world that are detectable by biological systems. Examples of both conservation and innovation occur across vertebrates in the organization of sensory systems for the reception of photic, positional, chemical, tactile, mechanosensory and electrosensory lateral line, acoustic, and magnetic stimuli. Recent findings in genetics and ontogeny allow new approaches to questions of how new sensory receptors and their corresponding central nervous system pathways evolve, how sensory specialization arises and its effects on other sensory systems, the role of cell-adhesion molecules in the ontogeny of sensory pathways and their topological organization, and the occurrence of reorganization and co-option of developmental modules over sensory system evolution. Relatively simple alterations at the genetic and ontogenetic levels often can result in alterations in the phenotype of far greater complexity.

The dorsal thalamus of jawed vertebrates: a comparative viewpoint.

In anamniotes, the dorsal thalamus comprises: (1) a caudal division, the collothalamus, which receives its predominant input from the midbrain roof and projects ipsilaterally to the telencephalon, predominantly to the striatum, and (2) a rostral division, the lemnothalamus, which predominantly receives a direct retinal (lemniscal) input and projects bilaterally to the telencephalon, predominantly to the pallium. In amniotes, collothalamic nuclei relay visual, auditory, and somatosensory-multisensory inputs from the midbrain roof to the ipsilateral telencephalon, terminating in both striatum and pallium. For example, the collothalamic visual nuclei consist of the LP-pulvinar complex in mammals and nucleus rotundus in diapsid reptiles, birds, and turtles. Among amniotes, the latter nuclei are homologous to each other as discrete nuclei, as are the collothalamic auditory and collothalamic somatosensory-multisensory nuclei. Lemnothalamic nuclei (and nuclear groups) in amniotes predominantly (and/or plesiomorphically) receive lemniscal inputs; some project to the telencephalon bilaterally, and most, in contrast to collothalamic nuclei, do not project to the striatum. In mammals, the lemnothalamic nuclei include most of those in the anterior, medial, intralaminar, and ventral nuclear groups and the dorsal lateral geniculate nucleus. In diapsid reptiles, they include the dorsomedial and dorsolateral anterior nuclei and the dorsal lateral optic nucleus; comparable nuclei are present in birds and turtles, with birds additionally having a discrete somatosensory lemniscal relay nucleus. These lemnothalamic nuclei in each amniote radiation are homologous as a field to the lemnothalamus (i.e., nucleus anterior) in anamniotes. Both divisions of the dorsal thalamus were elaborated to some degree in the common ancestral amniote stock. A further major elaboration of the lemnothalamus characterized the ancestral stock of mammals and may have been one of the key events in early mammalian evolution. Birds have independently, to a lesser degree, elaborated the lemnothalamus.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis.

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

The evolution of the dorsal pallium in the telencephalon of amniotes: cladistic analysis and a new hypothesis.

The large body of evidence that supports the hypothesis that the dorsal cortex and dorsal ventricular ridge of non-mammalian (non-synapsid) amniotes form the dorsal pallium and are homologous as a set of specified populations of cells to respective sets of cells in mammalian isocortex is reviewed. Several recently taken positions that oppose this hypothesis are examined and found to lack a solid foundation. A cladistic analysis of multiple features of the dorsal pallium in amniotes was carried out in order to obtain a morphotype for the common ancestral stock of all living amniotes, i.e., a captorhinomorph amniote. A previous cladistic analysis of the dorsal thalamus (Butler, A.B., The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis, Brain Res. Rev., 19 (1994) 29-65; this issue, previous article) found that two fundamental divisions of the dorsal thalamus can be recognized--termed the lemnothalamus in reference to predominant lemniscal sensory input and the collothalamus in reference to predominant input from the midbrain roof. These two divisions are both elaborated in amniotes in that their volume is increased and their nuclei are laterally migrated in comparison with anamniotes. The present cladistic analysis found that two corresponding, fundamental divisions of the dorsal pallium were present in captorhinomorph amniotes and were expanded relative to their condition in anamniotes. Both the lemnothalamic medial pallial division and the collothalamic lateral pallial division were subsequently further markedly expanded in the synapsid line leading to mammals, along with correlated expansions of the lemnothalamus and collothalamus. Only the collothalamic lateral pallial division--along with the collothalamus--was subsequently further markedly expanded in the non-synapsid amniote line that gave rise to diapsid reptiles, birds and turtles. In the synapsid line leading to mammals, an increase in the degree of radial organization of both divisions of the dorsal pallium also occurred, resulting in an 'outside-in' migration pattern during development. The lemnothalamic medial division of the dorsal pallium has two parts. The medial part forms the subicular, cingulate, prefrontal, sensorimotor, and related cortices in mammals and the medial part of the dorsal cortex in non-synapsid amniotes. The lateral part forms striate cortex in mammals and the lateral part of dorsal cortex (or pallial thickening or visual Wulst) in non-synapsid amniotes. Specific fields within the collothalamic lateral division of the dorsal pallium form the extrastriate, auditory, secondary somatosensory, and related cortices in mammals and the visual, auditory, somatosensory, and related areas of the dorsal ventricular ridge in non-synapsid amniotes.

The diencephalon of the Pacific herring, Clupea harengus: retinofugal projections to the diencephalon and optic tectum.

The pattern of retinofugal projections to nuclei in the diencephalon and to the optic tectum was analyzed with horseradish peroxidase and autoradiographic methods in Clupea harengus, a clupeomorph teleost, for comparison with osteoglossomorph, elopomorph, and euteleost teleosts and with non-teleost actinopterygians. Most retinal fibers decussate in the optic chiasm and project to nuclei in the preoptic area, ventral and dorsal thalamus, posterior tuberculum, synencephalon, and pretectum, as well as to the accessory optic nuclei and optic tectum. Some ipsilateral projections do not decussate in the optic chiasm, while others decussate and recross via the supraoptic (minor) and posterior commissures. The pattern of projections is similar to that seen in other actinopterygian fishes with several exceptions. The terminal field usually present lateral to nucleus anterior in the dorsal thalamus is extremely reduced despite the relatively large size of the nucleus. A dense terminal field lies within the cell plate of nucleus corticalis in the pretectum rather than dorsal to it. The tectal hemisphere is composed of two distinct lobules, and the dorsal optic tract projects to the more rostromedial lobule while the ventral optic tract projects to the more caudolateral lobule. The lack of a significant projection to nucleus anterior and the lobular morphology of the optic tectum appear to be apomorphic for Clupea. Other features of the pattern of retinal projections are also analyzed in actinopterygian fishes including Clupea, and several hypotheses are advanced as to which traits are plesiomorphic for actinopterygians and/or for teleosts.

The diencephalon of the Pacific herring, Clupea harengus: cytoarchitectonic analysis.

The cytoarchitecture of nuclei in the preoptic area, ventral thalamus, dorsal thalamus, epithalamus, hypothalamus, posterior tuberculum, synencephalon, and pretectum and the accessory optic nuclei was analyzed in the clupeomorph teleost, Clupea harengus. Plesiomorphic (evolutionarily primitive) and apomorphic (evolutionarily derived) features of nuclei were identified by cladistic analysis. Plesiomorphic features include the cytoarchitectonic organization of most of the preoptic nuclei, the somewhat scattered cells of nucleus ventrolateralis, the compact, oval shape of nucleus intermedius, the presence of dorsoventrally oriented laminae in the central posterior nucleus, and most features of the hypothalamic nuclei. Also plesiomorphic are the presence of a thick, prominent paraventricular organ, a nucleus of the paraventricular organ, a nucleus tuberis posterior, and a preglomerular complex in which the boundaries between multiple nuclei are relatively difficult to distinguish. Additionally, the cytoarchitecture of the three synencephalic nuclei present in Clupea, the presence of small cells in nucleus pretectalis superficialis pars parvicellularis and of larger, scattered cells in nucleus pretectalis superficialis pars magnocellularis, the presence of large cells in the dorsal accessory optic nucleus that form a rostrocaudally oriented column, and the feature of a small, cell-sparse ventral accessory optic nucleus are plesiomorphic. Apomorphic features include the presence of a single, large, circular lamina that surrounds a central neuropil in all but the most caudal part of nucleus anterior, a lack of bilateral asymmetry in the habenular nuclei, the relatively small size of the periventricular nucleus of the posterior tuberculum, the presence of two, distinguishable caudomedial nuclei in the posterior tuberculum, elongation and folding of the neuropil of nucleus pretectalis superficialis pars parvicellularis, and the relatively large size of nucleus pretectalis superficialis pars magnocellularis and the posterior pretectal nucleus.

The diencephalon and optic tectum of the longnose gar, Lepisosteus osseus (L.): cytoarchitectonics and distribution of acetylcholinesterase.

The cytoarchitecture of nuclei in the diencephalon and the distribution of acetylcholinesterase (AChE) in the diencephalon and optic tectum were analyzed in the longnose gar, Lepisosteus osseus, a non-teleost actinopterygian fish. Nuclei were identified in the preoptic area, thalamus, posterior tubercle, hypothalamus, synencephalon, and pretectum which are homologous to like-named nuclei in teleosts and other non-teleost actinopterygians. Of particular note, a nucleus in the rostral diencephalon, nucleus rostrolateralis, which has previously been identified only in the osteoglossomorph Pantodon, is present in the long-nose gar. The posterior pretectal nucleus, previously identified in teleosts and in the bowfin Amia, is also present in gars. The small size of the posterior pretectal nucleus in gars supports the hypothesis that this nucleus was small plesiomorphically. The distribution of AChE in the diencephalon and optic tectum corresponds in most respects to that found in teleosts. The superficial pretectal nuclei, including the posterior pretectal nucleus, are strongly positive for AChE. In contrast, most of the nuclei within the preglomerular complex are negative for AChE. Acetylcholinesterase is present in some of the fibers in the optic tracts and in most retinorecipient nuclei, as well as in some other nuclei and tracts.

Tectal projection to an unusual nucleus in the diencephalon of a teleost fish, Pantodon buchholzi.

Nucleus rostrolateralis, a newly identified nucleus, has been found to date in only three species of ray-finned fishes, two of which are osteoglossomorphs. It is relatively large and well developed in only one of the osteoglossomorphs, Pantodon buchholzi, in which it receives a relatively sparse, primarily contralateral, input from the retina. The present report describes a relatively intense, bilateral projection from the optic tectum to nucleus rostrolateralis.

Retinal projections in the bowfin, Amia calva: cytoarchitectonic and experimental analysis.

The retinofugal projections in the bowfin, a non-teleost actinopterygian, were studied by autoradiographic and horseradish peroxidase methods, and the cytoarchitecture of retinorecipient regions of the diencephalon was analyzed with serially sectioned, Bodian stained material. Nuclei were identified in the thalamus, the periventricular portion of the posterior tuberculum, synencephalon, and pretectum which are homologous to like-named nuclei in teleosts and other non-teleost actinopterygian fishes. Of particular note, a posterior pretectal nucleus and, possibly, a homologue of nucleus corticalis were found to be present in the pretectum. These nuclei have previously been identified only in teleosts. The posterior pretectal nucleus is relatively small in the bowfin, and the distribution of a small, versus a large, posterior pretectal nucleus in Teleostei and Halecomorphi suggests that this nucleus was small plesiomorphically. The pattern of retinofugal projections in the bowfin is similar to that in other non-teleost actinopterygian fishes and in teleosts in most regards. Contralaterally, the retina projects to nuclei in the dorsal and ventral thalamus, superficial and central pretectum, dorsal and ventral accessory optic nuclei, and to the optic tectum. Additionally, there are sparse projections to the suprachiasmatic nucleus in the preoptic area, the periventricular nucleus of the posterior tuberculum, and the dorsal and ventral periventricular pretectal nuclei. Ipsilateral projections are sparse and are derived from fibers which do not decussate in the optic chiasm. Undecussated ipsilateral retinal projections, as present in the bowfin, are a widely distributed character in vertebrates and appear to be plesiomorphic for vertebrates.

Variation of tectal morphology in teleost fishes.

The morphological organization of the optic tectum was analyzed in a number of divergent teleost species. Particular attention was focused on the organization of the periventricular gray zone and of afferent and efferent fiber systems within or adjacent to it. A dorsal part of the periventricular gray zone is recognized which is in register with the distribution of tangentially and radially oriented fascicles. A lateral part of the periventricular gray can be identified as being lateral to the lateral-most of the tangentially and radially oriented fascicles and being in register with a large number of circular fascicles in the deep white zone. Circular fascicles pass between the lateral part of the periventricular gray and a more ventral part. There are few if any circular fascicles in the part of the deep white zone that is in register with the ventral part of the gray. These parts of the periventricular gray are most clearly apparent in clupeomorph fishes, due to an exaggerated separation related to the presence of two tectal lobes within the hemisphere. It is hypothesized that the dorsal part of the gray, as can be recognized cytoarchitectonically, is in register with the projection of the dorsal optic tract and that the lateral and ventral parts of the gray are in register with the projection of the ventral optic tract.