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Biomedical subjects

R Glenn Northcutt

Publications and source records attributed to R Glenn Northcutt.

10 recordsLinked to original sources
PubMed2006

Connections of the lateral and medial divisions of the goldfish telencephalic pallium.

Biotinylated dextran amine and fluorescent carbocyanine dye (DiI) were used to examine connections of the lateral (Dl) and medial (Dm) divisions of the goldfish pallium. Besides numerous intrinsic telencephalic connections to Dl and Dm, major ascending projections to these pallial divisions arise in the preglomerular complex of the posterior tuberculum, rather than in the dorsal thalamus. The rostral subnucleus of the lateral preglomerular nucleus receives auditory input via the medial pretoral nucleus, lateral line input via the ventrolateral toral nucleus, and visual input via the optic tectum, and it projects to both Dl and Dm. The anterior preglomerular nucleus and caudal subnucleus of the lateral preglomerular nucleus receive auditory input via the central toral nucleus and project to Dm. This pallial division also receives chemosensory information via the medial preglomerular nucleus. The central posterior (CP) nucleus, which receives both auditory and visual inputs, also projects to Dm and is the only dorsal thalamic nucleus projecting to the pallium. Thus, both Dl and Dm clearly receive multisensory inputs. Major projections of CP and projections of all other dorsal thalamic nuclei are to the subpallium, however. Descending projections of Dl are primarily to the preoptic area and the caudal hypothalamus, whereas descending projections of Dm are more extensive and particularly heavy to the anterior tuber and nucleus diffusus of the hypothalamus. The topography and connections of Dl are remarkably similar to those of the hippocampus of tetrapods, whereas the topography and connections of Dm are similar to those of the amygdala.

Afferent Pathways↗
PubMed2005

The new head hypothesis revisited.

In 1983, a new theory, the New Head Hypothesis, was generated within the context of the Tunicate Hypothesis of deuterostome evolution. The New Head Hypothesis comprised four claims: (1) neural crest, neurogenic placodes, and muscularized hypomere are unique to vertebrates, (2) the structures derived from these tissues allowed a shift from filter feeding to active predation, (3) the rostral head of vertebrates is a neomorphic unit, and (4) neural crest and neurogenic placodes evolved from the epidermal nerve plexus of ancestral deuterostomes. These claims are re-examined within the context of evolutionary developmental biology. The first may or may not be valid, depending on whether protochordates have these tissues in rudimentary form. Regarding the second, clearly, the elaboration of these tissues in vertebrates is correlated with a shift from filter feeding to active predation. The third claim is clarified, i.e., that the elaboration of the alar portion of the rostral brain and the development of olfactory organs and their associated connective tissues represent a neomorphic unit, which appears to be valid. The fourth is rejected. When the origin of neural crest and neurogenic placodes is examined within the context of developmental biology, it appears they evolved due to the rearrangement of germ layers in the blastulae of the deuterostomes that gave rise to chordates. Deuterostome evolution and the origin of vertebrates are also re-examined in the context of new data from developmental biology and taxonomy. The Tunicate Hypothesis is rejected, and a new version of the Dipleurula Hypothesis is presented.

Animals↗
PubMed2005

Taste bud development in the channel catfish.

Taste bud formation in channel catfish is first seen to occur in stage 39 embryos, when taste bud primordia (stage 1), consisting of three to five cells, including a single calretinin-positive cell, can be recognized within the oropharyngeal cavity and maxillary barbels. Within a short time (stage 40), stage 2 taste bud primordia are apparent and include two or three calretinin-positive cells. The number of calretinin-positive cells continues to increase (stage 3), and the primordia begin to erupt as mature taste buds (stage 4) by embryonic stage 48. This same pattern of taste bud development characterizes other regions of the head, with calretinin-positive cells first detected around the mouth and on the other barbels by stage 41 and on the rest of the head by stage 48. The development of trunk taste buds lags far behind that of the head, with the first calretinin-positive cells occurring on the lobes of the caudal fin by stage 48 and on the remaining fins by stage 50. Taste bud primordia on the trunk proper do not begin to appear until stage 53, when the larvae begin to feed, and these receptors begin to erupt only in 1-week-old larvae. Fibers of the facial nerve, which innervate all external taste buds, ramify within the ectoderm prior to the first appearance of taste bud primordia or their precursors.

Animals↗
PubMed2004

Development of the lateral line system in the shovelnose sturgeon.

The lateral line systems of aquatic amphibians and all chondrichthyan and osteichthyan fish present a similar array of mechanoreceptors. However, electroreceptors, the second major component of the lateral line system, have clearly undergone more significant evolutionary change. Chondrichthyans and non-neopterygian fish possess primitive ampullary organ electroreceptors, whereas significantly different 'new' ampullary organs and tuberous electroreceptors are found in a few groups of teleosts (mormyrids, gymnotids and some catfish). The pairing of mechano- and electroreceptors in the lateral line system, as well as the morphologically and physiologically distinct electroreceptors of teleosts have inspired several recent studies on the origin and evolution of the lateral line receptors. We described the development of the lateral line system in sturgeon (Scaphirhynchus platorynchus) as part of an outgroup analysis of lateral line development in three taxa: vertebrates that have both mechanoreceptive neuromasts and primitive electroreceptors; neopterygian fish that only have mechanoreceptors; and teleosts that have re-evolved new electroreceptors. Development in Scaphirhynchus was consistent with previously studied taxa in that the lateral line system developed from a series of six dorsolateral placodes. Interestingly, we found that the octaval placode was bound rostrally and caudally by large placodal fields, out of which the six lateral line placodes arose. This finding supports recent suggestions for a common placodal primordium for all placodes. Each of the six placodes gave rise to the lateral line nerves before elongating into sensory ridges, which contained neuromast primordia. The ampullary organ fields of Scaphirhynchus arose from the lateral zones of the anterodorsal, anteroventral, otic and supratemporal sensory ridges, which is also consistent with recently studied taxa. Comparisons of the lateral line system of Scaphirhynchus and close relatives, Acipenser and Polyodon, indicate that variation in some aspects of lateral line receptor numbers and distribution are related to changes in head morphology and feeding strategy, whereas other changes, such as a reduction in receptor number without a change in placode field size, indicate changes in placode development.

Animals↗
PubMed2004

A pallial visual area in the telencephalon of the bony fish Polypterus.

The dorsal (P2) and lateral (P3) pallial zones of bichirs receive a substantial projection from nucleus medianus of the posterior tuberculum. Although nucleus medianus does not receive a direct retinal input, its close proximity to ascending tectal efferents suggests that it might receive a tectal input and form a segment of a retinotectal-tubercular pathway to the pallium. In order to test this possibility, evoked responses to light flashes were electrophysiologically recorded and tectal efferents were experimentally determined in bichirs. These experiments suggest that a single visual field exists across the dorsal and lateral pallial zones and that this field is mediated by nucleus medianus, which does receive a direct tectal projection. This visual pathway appears to be uniquely derived and not homologous to any other known pathway in tetrapods. Furthermore, these results support the contention that the P2 and P3 pallial zones in bichirs are subdivisions of a single pallial zone.

Animals↗
PubMed2004

Taste buds: development and evolution.

The gustatory system in vertebrates comprises peripheral receptors (taste buds), innervated by three cranial nerves (VII, IX, and X), and a series of central neural centers and pathways. All vertebrates, with the exception of hagfishes, have taste buds. These receptors vary morphologically in different vertebrates but usually consist of at least four types of cells (dark, light, basal, and stem cells). An out-group analysis indicates that taste buds were restricted to the oropharynx, primitively, and that external taste buds, distributed over the head and, in some cases, even the trunk, evolved a number of times independently. The sensory neurons of the cranial nerves that innervate taste buds are believed to arise from epibranchial placodes, which are induced by pharyngeal endoderm, but it has never been demonstrated experimentally that these sensory neurons do, in fact, arise from these placodes. Although many details of the development of the innervation of taste buds are still unknown, it is now clear that taste buds are induced from either ecto- or endodermal epithelia, rather than arising from either placodes or neural crest. At present, there are two developmental models of taste bud induction: The neural induction model claims that peripheral nerve fibers induce taste buds, whereas the early specification model claims that oropharyngeal epithelium is specified by or during gastrulation and that taste buds arise from cell-cell interactions within the specified epithelium. There is now substantial evidence that the early specification model best describes the induction of taste buds.

Animals↗
PubMed2004

Retinoic acid repatterns axolotl lateral line receptors.

The effects of all-trans retinoic acid on the development of the lateral line placodes of axolotls was studied. Late gastrula and early neurula were exposed to 10(-7) to 10(-5) M retinoic acid for one hour and then reared until they would normally be feeding larvae. As in other vertebrates, the extent of the developmental abnormalities is concentration dependent. Those embryos exposed to the highest concentration of retinoic acid failed to form much of the forebrain and midbrain, including the olfactory, optic and otic organs, which were reduced or absent. Although all lateral line placodes continued to generate fully formed receptors and cranial nerves, the production of neuromasts and the organization of these receptors into lines were markedly reduced. Equally important, all of the placodes at the highest concentration of retinoic acid failed to generate ampullary organs, thereby indicating a strong posteriorizing effect of retinoic acid on these placodes.

Ambystoma mexicanum↗
PubMed2003

Connections of the pallial telencephalon in the Senegal bichir, Polypterus.

The connections of the dorsal telencephalon (pallium) of the Senegal bichir (Polypterus senegalus) were investigated in order to test the hypothesis that the simple everted pallium of polypterid fishes is directly comparable to the evaginated pallium of most vertebrates. Neuroanatomical tracers (DiI or biotinylinated dextran amines) were injected into the three traditionally recognized divisions of the bichir pallium to determine their afferent and efferent connections. The connections were corroborated with injections into regions identified as projecting to, or receiving input from, one or more pallial divisions. The results suggest that the bichir pallium consists not of three, but only two primary divisions: a dorsomedial one and a dorsolateral one. This reinterpretation refutes the hypothesis of a direct one-to-one comparison with the tripartite pallium of most vertebrates. Homologues of the bichir dorsomedial and dorsolateral pallium are recognized in the lateral and medial pallium of anuran amphibians, respectively. The pallium of bichirs is compared to that of derived ray-finned fishes as a link between derived ray-finned fishes and other vertebrates. The available information on the connections of the pallium of teleosts suggests that only the olfactory recipient (pars posterior) of the pallium can be directly compared to bichirs and amphibians and that the remaining divisions of the pallium in teleosts are uniquely derived features of the teleost telencephalon.

Animals↗
PubMedDate not supplied

Head size constrains forebrain development and evolution in ray-finned fishes.

In ray-finned fishes, which comprise nearly half of all vertebrate species, the telencephalon does not evaginate, as it does in other vertebrates, but instead everts. No detailed explanation for this species difference has ever been offered. Here we propose that telencephalic eversion evolved because ray-finned fish embryos are so small that their telencephalon cannot evaginate but must, instead, squeeze into the space just dorsal to the developing nasal epithelia and rostral to the eyes-morphogenetic movements that amount to eversion. Evidence for this hypothesis derives from cladistic analyses, which show that early ray-finned fishes reduced their adult body size and adopted a novel reproductive strategy, based on the production of myriad minute young. Because body size tends to be inversely proportional to brain:body ratio, this phylogenetic reduction in body size implies that embryonic ray-finned fishes should have proportionately larger brains than embryos of species whose telencephalons evaginate. This prediction was confirmed by comparing serially sectioned heads of representative ray-finned and cartilaginous fish embryos at several stages of development. The brain, excluding its ventricles, occupies 36-46% of the cranial cavity in embryonic ray-finned fishes, but less than 20% in embryonic sharks. Moreover, three-dimensional reconstructions show that in embryonic ray-finned fishes the telencephalon has no room for a full-fledged evagination; instead, it spreads into the spaces just dorsal and caudal to the developing nasal epithelia. These morphogenetic movements, in conjunction with a thinning of the forebrain roof, generate telencephalic eversion.

Ambystoma mexicanum↗