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Biomedical subjects

L A Barlow

Publications and source records attributed to L A Barlow.

11 recordsLinked to original sources
PubMed2001

Specification of pharyngeal endoderm is dependent on early signals from axial mesoderm.

The development of taste buds is an autonomous property of the pharyngeal endoderm, and this inherent capacity is acquired by the time gastrulation is complete. These results are surprising, given the general view that taste bud development is nerve dependent, and occurs at the end of embryogenesis. The pharyngeal endoderm sits at the dorsal lip of the blastopore at the onset of gastrulation, and because this taste bud-bearing endoderm is specified to make taste buds by the end of gastrulation, signals that this tissue encounters during gastrulation might be responsible for its specification. To test this idea, tissue contacts during gastrulation were manipulated systematically in axolotl embryos, and the subsequent ability of the pharyngeal endoderm to generate taste buds was assessed. Disruption of both putative planar and vertical signals from neurectoderm failed to prevent the differentiation of taste buds in endoderm. However, manipulations of contact between presumptive pharyngeal endoderm and axial mesoderm during gastrulation indicate that signals from axial mesoderm (the notochord and prechordal mesoderm) specify the pharyngeal endoderm, conferring upon the endoderm the ability to autonomously differentiate taste buds. These findings further emphasize that despite the late differentiation of taste buds, the tissue-intrinsic mechanisms that generate these chemoreceptive organs are set in motion very early in embryonic development.

Ambystoma↗
PubMed2000

Distribution and innervation of taste buds in the axolotl.

Adult axolotls have approximately 1,400 taste buds in the epithelium of the pharyngeal roof and floor and the medial surfaces of the visceral bars. These receptors are most dense on the lingual surfaces of the upper and lower jaws, slightly less dense throughout lateral portions of the pharyngeal roof and floor, and more sparse within medial portions of the pharyngeal roof and floor, except for a median oval patch of receptors located rostrally between the vomerine tooth fields. Each taste bud is a pear-shaped organ, situated at the center of a raised hillock and averaging 80 and 87 microm in height and width, respectively. Each comprises 50 to 80 cells, which can be classified as basal, dark fusiform, or light fusiform, based on differences in their morphology. The distal ends of the apical processes of the fusiform cells reach the surface of each hillock, forming a single taste pore with an average diameter of 15 microm. Each apical process terminates in one of three ways: as short, evenly spaced microvilli; as long clustered microvilli; or as large, stereocilia-like microvilli. The pharyngeal epithelium and associated taste buds in axolotls are innervated solely by rami of the facial, glossopharyngeal and vagal nerves. Approximately, the rostral one half of the pharyngeal roof is innervated by the palatine rami of the facial nerve, whereas the caudal one half of the pharyngeal roof is innervated by the pharyngeal rami of the glossopharyngeal and vagal nerves. The lingual surface of the lower jaw is innervated by the pretrematic (mandibular) ramus of the facial nerve. The dorsal two-thirds of the visceral arches, and the ventral one-third of the visceral arches and the pharyngeal floor, are innervated by both the pretrematic and post-trematic rami of the glossopharyngeal and vagal nerves, respectively.

Ambystoma↗
PubMed1998

The role of innervation in the development of taste buds: insights from studies of amphibian embryos.

Amphibian embryos have long been model organisms for studies of development because of their hardiness and large size, as well as the ease with which they can be experimentally manipulated. These particular advantages have allowed us recently to test the role of innervation in the development of vertebrate taste buds using embryos of an aquatic salamander, the axolotl. The predominant model of taste bud genesis has been one of neural induction, in which ingrowing sensory neurites induce taste bud differentiation in the epithelium that lines the mouth and pharynx. However, when we prevented embryonic sensory neurons from contacting the oropharyngeal epithelium by using transplantation or tissue culture techniques, we found that taste bud differentiation was independent of nerve contact. Additionally, using similar types of experimental manipulations, we have recently shown that taste bud differentiation is not a result of interactions of the oropharyngeal epithelium with craniofacial mesenchyme. Surprisingly, we found that although taste bud genesis occurs very late in embryonic development, it is an intrinsic feature of the presumptive oropharyngeal epithelium extremely early, in fact as early as the completion of gastrulation. These data have prompted us to propose a new model for the development of amphibian taste buds: (i) The presumptive oropharyngeal epithelium is specified by the time gastrulation is complete; (ii) Subsequently, a distributed population of taste bud progenitors is set up within this epithelium via local cell-cell interactions. These progenitor cells give rise to taste buds, which are distributed throughout the mouth and pharynx. How widely applicable this model might be for the genesis of taste buds in other vertebrates remains to be seen. However, since it is likely that the taste system of axolotls more closely resembles the ancestral state from which both the amphibian and mammalian taste systems have evolved, it is possible that many of the same developmental mechanisms that give rise to amphibian taste buds are also used to generate the receptor organs in mammals.

Ambystoma↗
PubMed1998

Amphibians provide new insights into taste-bud development.

Until recently, the predominant model of taste-bud development was one of neural induction: ingrowing sensory fibers were thought to induce taste-bud differentiation late in embryonic development. Recent experimental studies, however, show that the development of taste buds is independent of their innervation. In amphibian embryos, the ability to generate taste buds is an intrinsic feature of the oropharyngeal epithelium long before the region becomes innervated. These studies indicate that patterning of the oropharyngeal epithelium occurs during gastrulation, and suggest that taste buds or their progenitors play the dominant role in the development of their own innervation.

Amphibians↗
PubMed1997

Taste buds develop autonomously from endoderm without induction by cephalic neural crest or paraxial mesoderm.

Although it had long been believed that embryonic taste buds in vertebrates were induced to differentiate by ingrowing nerve fibers, we and others have recently shown that embryonic taste buds can develop normally in the complete absence of innervation. This leads to the question of which tissues, if any, induce the formation of taste buds in oropharyngeal endoderm. We proposed that taste buds, like many specialized epithelial cells, might arise via an inductive interaction between the endodermal epithelial cells that line the oropharynx and the adjacent mesenchyme that is derived from both cephalic neural crest and paraxial mesoderm. Using complementary grafting and explant culture techniques, however, we have now found that well-differentiated taste buds will develop in tissue completely devoid of neural crest and paraxial mesoderm derivatives. When the presumptive oropharyngeal region was removed from salamander embryos prior to the onset of cephalic neural crest migration, taste buds developed in grafts and explants coincident with their appearance in intact control embryos. Similarly, explants from neurulae in which movement of paraxial mesoderm had not yet begun also developed taste buds after 9-12 days in vitro. We conclude that neither cranial neural crest nor paraxial mesoderm is responsible for the induction of embryonic taste buds. Surprisingly, the ability to develop taste buds late in embryonic development seems to be an intrinsic feature of the oropharyngeal endoderm that is determined by the completion of gastrulation.

Ambystoma↗
PubMed1996

Embryonic taste buds develop in the absence of innervation.

It has been hypothesized that taste buds are induced by contact with developing cranial nerve fibers late in embryonic development, since descriptive studies indicate that during embryonic development taste cell differentiation occurs concomitantly with or slightly following the advent of innervation. However, experimental evidence delineating the role of innervation in taste bud development is sparse and equivocal. Using two complementary experimental approaches, we demonstrate that taste cells differentiate fully in the complete absence of innervation. When the presumptive oropharyngeal region was taken from a donor axolotl embryo, prior to its innervation and development of taste buds, and grafted ectopically on to the trunk of a host embryo, the graft developed well-differentiated taste buds. Although grafts were invaded by branches of local spinal nerves, these neurites were rarely found near ectopic taste cells. When the oropharyngeal region was raised in culture, numerous taste buds were generated in the complete absence of neural elements. Taste buds in grafts and in explants were identical to those found in situ both in terms of their morphology and their expression of calretinin and serotonin immunoreactivity. Our findings indicate that innervation is not necessary for complete differentiation of taste receptor cells. We propose that taste buds are either induced in response to signals from other tissues, such as the neural crest, or arise independently through intrinsic patterning of the local epithelium.

Ambystoma↗
PubMed1995

Embryonic origin of amphibian taste buds.

Despite numerous descriptive studies, the embryonic origin of vertebrate taste buds has never been experimentally determined. A number of different alternatives have been suggested for taste bud origins, including epibranchial placodes, the neural crest, and the local epithelium of the oropharyngeal cavity. The role of a series of epibranchial placodes and the cephalic neural crest, which together give rise to the cranial nerves innervating taste buds, was examined with regard to the development of oropharyngeal taste buds in an ambystomatid salamander, the axolotl. When pigmented placodal ectoderm or neural folds were grafted isotopically and isochronically into nonpigmented host embryos, known derivatives of each tissue contained pigmented cells, but labeled taste buds were never encountered. Thus, neither epibranchial placodes nor neural crest contribute cells to taste buds during embryogenesis. The majority of the oropharyngeal cavity of ambystomatid salamanders is lined by an endodermal epithelium. In order to demonstrate conclusively that taste buds arise from this local epithelium, the presumptive cephalic endoderm of early axolotl gastrulae was microinjected with the lipophilic dye, DiI. In the oropharyngeal epithelium of all larvae examined, both taste buds and general epithelial cells were labeled with DiI, indicating their common endodermal origin. Our findings are novel in that this is the first experimental demonstration of the endodermal origin of a vertebrate sensory receptor cell class.

Ambystoma↗
PubMed1994

Analysis of the embryonic lineage of vertebrate taste buds.

In all vertebrates, taste buds are the last sensory receptors to appear late in embryonic development. They are thought to arise locally from the oropharyngeal epithelium, although this hypothesis has not been tested experimentally. Alternatively, taste buds have been proposed to arise from neuroectodermal cells that migrate from peripheral neurogenic sources to the oropharyngeal epithelium and give rise to taste bud precursor cells. In order to determine the exact embryonic lineage of the cells of vertebrate taste buds, we have employed a combination of endogenous and exogenous cell marking techniques to follow neuroectodermal and endodermal cells through development. We find, in the ambystomatid salamander used in our studies, taste buds arise locally within the endodermally-derived epithelium lining the oropharyngeal cavity, and do not receive a contribution from neuroectodermal sources, i.e. ectodermal placodes or cephalic neural crest.

Animals↗
PubMed1992

Patterns of serotonin and SCP immunoreactivity during metamorphosis of the nervous system of the red abalone, Haliotis rufescens.

Larvae of the red abalone, Haliotis rufescens, rely on external chemical cues to trigger metamorphosis; thus, the timing of metamorphosis is dependent upon the larva's chance encounter with the appropriate substrate. We examined the effect of the timing of metamorphosis on the development of the central nervous system (CNS), concentrating on the pattern of serotonin and small cardioactive peptide- (SCP) immunopositive neurons in the cerebral ganglia. By 4 days postfertilization the cerebral ganglion has five pairs of serotonin-immunoreactive (IR) neurons, one pair of which (the V cells) innervate the velum. This complement of cells remains stable for as long as the larval stage persists but metamorphosis causes the rapid loss of the V cells. In the case of SCP-IR neurons, one pair is present prior to metamorphic competency, but as larvae continue to age in the absence of inducing cues, additional pairs are gradually added. Metamorphosis causes an acceleration in SCP-IR neuron addition. This separation of developmental patterns is well adapted for the inherent uncertainty of the timing of metamorphosis in abalone larvae.

Animals↗
PubMed1986

Evaluation of acute bioassays for assessing toxicity of polychlorinated biphenyl-contaminated soils.

Proposed State of California regulations use fish toxicity information as one criterion in municipal or industrial waste hazard evaluation. Static 96-hr bioassays were performed using fathead minnows (Pimephales promelas), blacksmith (Chromis punctipinnis), and glass shrimp (Palaemonetes kadiakensis) exposed to soil experimentally contaminated with up to 500 ppm polychlorinated biphenyl (PCB) capacitor fluid added at a concentration of 500 mg liter-1. Other bioassays were conducted with a 6-day mixing period prior to the bioassay or with acetone added to solubilize the PCBs. No mortality attributable to PCB toxicity was observed in definitive bioassays using the two fish and one invertebrate species. PCB levels leached from soil containing 500 ppm Aroclor 1242 ranged from less than 0.6 to 3.4 ppb in freshwater tests to 3.5 ppb in seawater bioassays. Using these data as the basis for waste classification, soils contaminated with up to 500 ppb PCBs during capacitor spills would be designated nonhazardous. PCBs are known to be environmentally persistent and to bioaccumulate. Acute toxicity tests, therefore, do not adequately evaluate the general toxicity of PCB-contaminated soils. Hazardous waste regulations for hydrophobic compounds such as PCBs should instead be based upon chronic toxicity data and should also consider bioaccumulation potential.

Animals↗