PubMed Health⌕ Search

Biomedical subjects

J C Glover

Publications and source records attributed to J C Glover.

At least 37 records · Page 2Linked to original sources
PubMed1995

Development of the longitudinal projection patterns of lumbar primary sensory afferents in the chicken embryo.

The literature on the anatomical organization of primary sensory afferents, though extensive, contains relatively little information about the longitudinal extent of the central collateral projections. Our understanding of intersegmental sensorimotor integration in the spinal cord and of the developmental mechanisms that establish its underlying circuitry could be significantly enhanced by a more complete description of these projections. To address this issue from a developmental perspective, we labeled the central projections of lumbar primary afferents in fixed preparations of the chicken embryo with the lipophilic tracer DiI. At late embryonic stages, the afferent projections had the following characteristics: Primary afferents originating from a single lumbar dorsal root ganglion bifurcated to project longitudinally in the dorsal funiculus or Lissauer's tract. Dorsal funiculus axons extended up to seven segments caudally and to at least ten segments rostrally, whereas axons in Lissauer's tract extended up to seven segments in each direction. Collaterals branched off the longitudinal axons over a range of about seven segments in each direction. Within this range, collaterals to specific terminal fields exhibited more restricted ranges. The development of these longitudinal patterns during earlier embryonic stages was followed from the time the afferents first reached the neural tube on day 4 of embryogenesis. The longitudinal axons lengthened as a single bundle up to day 10, with medial axons consistently longer than lateral axons. After day 10, the longitudinal axons were segregated into the dorsal funiculus and Lissauer's tract. Collaterals sprouted after about 2 days of longitudinal axon growth, by which time the axons had extended several segments in each direction. The segmental range over which collaterals were present reached a maximum of 20 segments at day 10. Collaterals to the different terminal areas differed in their segmental ranges already by this time. After day 10, the total segmental range of collaterals decreased to the stable level of about seven segments in each direction, which is characteristic of late-stage embryos.

Afferent Pathways↗
PubMed1995

The segmental precision of the motor projection to the intercostal muscles in the developing chicken embryo. A differential labelling study using fluorescent tracers.

Each skeletal muscle in the vertebrate is innervated by a group of motoneurons called a motoneuron pool. Retrograde labelling of single motoneuron pools has suggested that the arrangement of motoneuron pools innervating different limb muscles does not change during the embryonic period when more than 50% of the motoneurons die. In this study we retrogradely labelled neighbouring intercostal motoneuron pools differentially with latex microspheres or dextran amines coupled to fluorescent dyes. We then mapped the positions of the differentially labelled motoneurons in whole-mount preparations using a computer-aided drawing system. While the intercostal motoneuron pools are clearly segregated even at early stages, there is some intermingling at the rostral and caudal ends. We used a logistic regression to determine the extent of segmental overlap, and to facilitate a quantitative comparison of the overlap at different stages. Statistical analysis shows that the overlap (expressed as the percentage of the length of the overlapping motoneuron pools) decreases modestly during the period of motoneuron death. Computer simulations suggest that this decrease does not result from random motoneuron death alone; one alternative possibility is selective death of motoneurons in the overlap zone. Occasional "rogue" motoneurons, that is, motoneurons of one pool that scatter into the neighbouring pool, are still present at the end of the period of cell death, representing a potential source of "noise" in the establishment of segmental patterns of connectivity.

Animals↗
PubMed1994

The organization of vestibulo-ocular and vestibulospinal projections in the chicken embryo.

The functional neuroanatomy of vestibulo-ocular and vestibulospinal projections has been studied in the chicken embryo as a step towards elucidating the developmental mechanisms that determine the connectivity patterns of the vestibular system. Retrograde and anterograde axonal tracing, neurotransmitter immunohistochemistry, and retrograde transport of radiolabelled neurotransmitters or neurotransmitter analogs demonstrate that vestibular neurons are clustered into functionally specific populations. Each of these has a characteristic positional domain, projection pathway, and site of termination. At early embryonic stages, the positional domains of the various populations are systematically correlated with segmental and longitudinal territories of differential gene expression in the hindbrain. The intimate link between position and phenotype at these stages suggests that the functional identity of vestibular neurons is determined, at least in part, by mechanisms associated with early pattern formation of the neural tube.

Animals↗
PubMed1994

Segmental patterning of rat and chicken sympathetic preganglionic neurons: correlation between soma position and axon projection pathway.

The segmental organization of midthoracic rat and chicken sympathetic preganglionic neurons was examined by retrograde labeling in vivo and in vitro. The results demonstrate that individual sympathetic preganglionic neurons project only rostrally or caudally within the sympathetic chain, even though the spinal segment in which they reside provides innervation to both rostral and caudal ganglia. In addition, there is both a segmental and an intrasegmental pattern in the thoracic sympathetic column, in which the position of preganglionic neurons is related to the direction they project in the sympathetic chain. Thoracic sympathetic preganglionic neurons are organized into discrete segmental units, each of which exhibits an internal rostrocaudal polarity with respect to the direction of axon projection in the sympathetic chain. The rostrocaudal bias of this internal polarity is graded from segment to segment along the longitudinal axis. Since there is minimal overlap between thoracic segments, the transition from one segment to another entails a sharp change in the pathway choice of the preganglionic neurons. The organization of the preganglionic projections thus includes (1) segmental periodicity, (2) intrasegmental gradients of neuronal identity, and (3) an axial gradient of segment identity. The significance of these findings is twofold. First, they suggest a functional organization that may be related to the specificity of sympathetic reflex action. Second, they reveal a cellular organization that suggests an underlying segmental pattern in the developing spinal cord.

Amidines↗
PubMed1993

The development of brain stem projections to the spinal cord in the chicken embryo.

Recent studies of the development of brain stem projections to the spinal cord in the chicken embryo, with an emphasis on axon pathway selection, are reviewed. Neurons from medullary to mesencephalic levels project to the spinal cord along specific fiber tracts. Coherent, segregated neuron groups can be defined on the basis of which tract and which side of the brain stem they project on. The choice of axon trajectory is, therefore, correlated with neuron position. During development, these trajectory-defined brain stem groups project to the spinal cord in a stereotyped sequence. Early stages of this sequence reveal a potential homology between the reticulospinal systems of avians and lower vertebrates. The possibility that neuron position may be involved in determining axon pathway choice of brain stem projections is supported by complementary studies on vestibuloocular projections. The boundaries between vestibuloocular, vestibulospinal, and reticulospinal neuron groups coincide with rhombomere boundaries and boundaries between longitudinal cell columns. Axon trajectory-specific domains are, therefore, correlated with segmental and mediolateral patterns of differential gene expression.

Animals↗
PubMed1993

Clonal patterns of cell proliferation, migration, and dispersal in the brainstem of the chicken embryo.

Retroviral-mediated gene transfer was used to study clonal patterns of proliferation, migration, and dispersal in the brainstem of the chicken embryo. Clones were generated at stages 13-17 (Hamburger and Hamilton, 1951), a period of neurogenesis in the brainstem neural tube subsequent to the formation of rhombomeres. Clones were examined in separate experiments at stages 24-27, when many neurons migrate and differentiate; at stages 28-29, when brainstem nuclei begin to form; and at stages 34-35, when brainstem nuclei are fully formed. Stages 24-29 are characterized by a general variability in proliferative kinetics and migratory behavior. Clone sizes range from 1 to 29 cells, and migration patterns range from strictly radial (i.e., normal to the ventricular surface) to combined radial and tangential (i.e., perpendicular to the radial component). There is, however, an underlying systematic variation: (1) clones exhibiting tangential migration contain on average more cells than clones exhibiting only radial migration, and (2) the proportion of tangentially migrating clones increases from medial to lateral. By stages 34-35 some individual clones have apparently dispersed to disparate neuronal groups. The regional diversity observed among clones suggests that position along the mediolateral axis may determine the proliferative potential of progenitors and the migratory behavior and subsequent dispersal of their descendants.

Animals↗
PubMed1991

Regional specificity of developing reticulospinal, vestibulospinal, and vestibulo-ocular projections in the chicken embryo.

The regional mapping of reticulospinal, vestibulospinal, and vestibulo-ocular neuron groups onto specific axonal pathways was determined in the chicken embryo by retrograde axonal tracing. Experiments were performed on in vitro preparations of the brain stem to allow for precisely localized tracer injections combined with selective lesions of axon tracts. Brain-stem neuron groups were labelled from 3 days of embryonic development, when the first reticulospinal axons reached the cervical spinal cord, to 9 days of embryonic development, when each of the three systems studied had acquired a relatively mature organization. A striking feature at all stages was the spatial segregation of many neuron groups that projected along different trajectories. Examples of such segregation were found for neuron groups projecting in the same tract on different sides of the brain stem, in different tracts on the same side of the brain stem, and rostrally versus caudally. The occurrence of this segregation from early stages suggests that the choice of projection pathway by many brain-stem neurons is in some way linked to cell position. In some regions of the brain stem, neuron groups projecting along different pathways are intermingled. At least some of this intermingling, however, appears to occur subsequent to the initial establishment of axon projection patterns. Comparison of the mapping patterns at progressively older stages, and with previous mapping in the 11-day-old embryo (Glover and Petursdottir, 1988; Petursdottir, 1990) suggests that these projections are established with little error. The one obvious example of remodelling involved the pontine reticulospinal projection, in which an early contralateral axon population appeared to retract from spinal to medullary levels over the course of a few days. A similar phenomenon may be involved in the elimination of part of the contralateral reticulospinal projection from the midmedulla.

Animals↗
PubMed1991

Serotonin is released from isolated leech ganglia by potassium-induced depolarization.

1. The quantities of serotonin that are released from isolated leech ganglia in vitro were measured with the sensitive neurochemical techniques of HPLC-EC. 2. Segmental ganglia were exposed to elevated concentrations of potassium that depolarize leech serotonin-containing neurons by approximately 35 mV per decade. 3. Each segmental ganglion released on average 0.20 pmol of serotonin during 10 min of incubation in a solution containing 64 mM K+. 4. The rate of serotonin release increased nearly four-fold to 0.74 pmol/10 min when ganglia were incubated in 120 mM K+. 5. The rates of ganglionic serotonin release in 120 mM K+ were quantitatively similar in these three, experimentally important species of leeches: Hirudo medicinalis, Macrobdella decora and Haementeria ghilianii. 6. Ionic substitution experiments with the divalent cations Mg2+ and Co2+ indicated that the release of serotonin from leech ganglia is mediated by a Ca2+ dependent process. 7. The serotonin-uptake blockers, imipramine and chlorimipramine, did not increase the amount of serotonin released in elevated potassium. 8. Vitally staining the identified serotonin-containing neurons with Neutral Red dye did not reduce the quantity of serotonin that was released from the ganglia in elevated potassium. 9. This study demonstrates the capacity of leech ganglia to release the neurochemical serotonin, and the rates of transmitter release increase with the degree of depolarization of serotonin-containing neurons.

Animals↗
PubMed1988

Pathway specificity of reticulospinal and vestibulospinal projections in the 11-day chicken embryo.

The organization of the axonal pathways of reticulospinal and vestibulospinal projections in the 11-day chicken embryo was ascertained through retrograde tracing experiments. An in vitro preparation of the brainstem and cervical spinal cord facilitated precisely localized tracer applications. Single- and double-labelling experiments involving high cervical injections of tracers in combination with selective lesions defined the specific pathways by which different brainstem neurons project to the spinal cord. Coherent, and in many cases distinct, groups of reticulospinal and vestibulospinal neurons could thus be identified on the basis of their position and projection pathway. The organization of these groups and their projections in the 11-day chicken embryo is similar to that in avian and other vertebrate adults and therefore serves as a reference point for studies of pathfinding by bulbospinal axons during early development.

Animals↗
PubMed1988

Radial arrangement of clonally related cells in the chicken optic tectum: lineage analysis with a recombinant retrovirus.

A recently described method for lineage analysis in rodents uses a recombinant retrovirus to insert a foreign gene into the genome of a precursor cell; the gene's product is later detected histochemically in the infected cell's progeny. We have adapted this method for use in chicken embryos and used it to study the disposition of clonally related cells in the optic tectum. We report that descendants of a single precursor form narrow, radially oriented columns that span the thickness of the developing tectum. Analysis of embryos injected with virus at different stages suggests a developmental scheme in which early-born progeny are displaced laterally, late-born progeny are displaced radially, and cell mixing is limited. Many clones remain radially arrayed as laminae form and contribute neurons of diverse types of several laminae. In light of previous studies showing physiological relationships among vertically arrayed neurons, our results suggest that neurons descended from a single precursor are richly interconnected and functionally related.

Animals↗
PubMed1987

Serotonin storage and uptake by identified neurons in the leech Haementeria ghilianii.

Characterization of serotonin-containing neurons in the glossiphoniid leech Haementeria ghilianii was undertaken to provide a reference for developmental studies of their differentiation and for comparative studies of their distribution and function. Five types of serotonin-containing neurons were identified with an antiserum against serotonin and by radioenzymatic assay of individual isolated somata. They contain high concentrations of serotonin (in some cases exceeding saturation in aqueous solution) and their serotonin content increases with growth of the animal. Each type is capable of taking serotonin up from the extracellular fluid, as demonstrated autoradiographically. They exhibit segment-specific, and on comparison with hirudinid leeches, species-specific, differences in distribution, morphology, and the expression of serotonin metabolism.

Animals↗
PubMed1987

Development of neurotransmitter metabolism in embryos of the leech Haementeria ghilianii.

We have investigated the development of neurotransmitter metabolism in embryos of the glossiphoniid leech Haementeria ghilianii. The neurotransmitter content of dissected embryonic tissues was measured by means of radioenzymatic assays, while the presence of neurotransmitters in individual identified neurons was detected by means of immunocytochemical and monoamine histofluorescence techniques. The capacity for synthesis of neurotransmitters was measured by incubating dissected embryonic tissues in radiolabeled neurotransmitter precursors. A specific neurotransmitter uptake system present in some neurons was detected by means of an autoradiographic technique. At an early stage of development of the nervous system, when most neurons are just beginning process outgrowth, the nerve cord acquires the capacity to synthesize ACh, 5-HT, and GABA from their immediate precursors, and contains ACh. Moreover, 5-HT-immunoreactive neurons and neurons that are capable of GABA uptake can be identified. Dopamine-containing neurons are first detected by their histofluorescence at a slightly later stage, after process outgrowth is under way. As development continues, the content of and capacity for synthesis of these neurotransmitters increase, as does the number of neurons capable of GABA uptake. During the earlier stages of development, ACh content exceeds 5-HT content, which in turn exceeds dopamine content. By the end of embryogenesis, however, 5-HT and dopamine contents have greatly increased relative to ACh content, with 5-HT content exceeding ACh content by a factor of 2. Of the neurotransmitters thus far studied, 5-HT is present in the highest amount in the juvenile and adult nerve cord. Our results indicate that in the development of the leech nervous system neurotransmitter metabolism is one of the first neuronal characters to differentiate and that the subsequent levels of the different neurotransmitters are differentially regulated.

Acetylcholine↗
PubMed1986

Morphogenesis of an identified leech neuron: segmental specification of axonal outgrowth.

We have investigated the development of segmental diversity in an identified leech neuron, the Retzius cell. Retzius cells in the genital segments differ from those in other segments in lacking central axons and contacting different peripheral targets: the genitalia. These differences are not apparent during initial axon outgrowth, when all Retzius cells follow the same morphogenetic pattern. Rather, they first appear about the time the peripheral axons of the genital segment Retzius cells contact the genital primordia. This suggests that the pattern of central and peripheral axonal outgrowth may be modified by an interaction with peripheral targets.

Animals↗
PubMed1986

Fluorescent dextran-amines used as axonal tracers in the nervous system of the chicken embryo.

We have used recently developed fluorescein- and rhodamine-conjugated dextran-amines as axonal tracers in in vitro preparations of the nervous system of the chicken embryo. These substances are efficiently taken up by injured axons and transported rapidly to the cell bodies. They are also, albeit to a lesser degree, taken up by intact nerve terminals in skeletal muscle. They reveal the dendritic and axonal structure of labelled neurons, and are well suited for use in double-labelling experiments.

Animals↗
PubMed1982

Serotonin analog selectively ablates indentified neurons in the leech embryo.

Exposure of embryonic leeches to 5,7-dihydroxytryptamine a cytotoxic analog of the monoamine neurotransmitter serotonin, results in the selective ablation of serotonin-containing neurons in the ventral nerve cord. Other neurons appear to be unaffected by this treatment, including those that contain another monoamine neurotransmitter, dopamine. Embryos with ablations continue to develop into juvenile leeches, but as juveniles they are unable to make normal swimming movements. However, normal swimming movements can be instated in such leeches by injecting them with serotonin.

5,7-Dihydroxytryptamine↗