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D Bentley

Publications and source records attributed to D Bentley.

At least 73 records · Page 4Linked to original sources
PubMed1989

Pre-axonogenesis migration of afferent pioneer cells in the grasshopper embryo.

In insects, afferent neurons arise primarily from the ectodermal epithelium in the periphery and differentiate at the site of their precursor mitosis. Here we describe ectodermally derived cells that migrate away from their site of origin and initiate axonogenesis at a distant location. In embryonic grasshopper limb buds, the first two pairs of afferents to differentiate are the pair of Ti1 pioneers at the limb tip and the pair of Cx1 cells found at the base of the limb. While the Ti1 pioneers arise from the mitosis of a pioneer mother cell at the limb tip, the Cx1 cells are shown to emerge from the epithelium at circumferential positions that are approximately 150 degrees apart and that belong to different embryonic compartments. The cells migrate into contact with each other before initiating axonogenesis, and their axons then extend in a new direction that is orthogonal to the route of cell migration.

Afferent Pathways↗
PubMed1989

Selective recognition between embryonic afferent neurons of grasshopper appendages in vitro.

Selective affinity between afferent neurons has been proposed as a major mechanism underlying the assembly of the insect peripheral nervous system during development. Afferent insect neurons establish adhesive interactions in vivo that are resistant to proteolytic degradation by elastase and independent of the basal lamina. We have tested whether afferent neurons express selective affinity for one another under more simplified and controlled conditions in vitro. We report here that (1) afferent neurons from dissociated embryonic tissue selectively aggregate within 80 min when incubated with rotation, (2) afferent axons establish and maintain fasciculation in vitro, and (3) afferent neuronal processes in vitro preferentially contact the somata of other afferent neurons in a mixed field of cells.

Afferent Pathways↗
PubMed1989

Pioneer neuron pathfinding from normal and ectopic locations in vivo after removal of the basal lamina.

The contribution of the basal lamina to Ti1 pioneer axon guidance in grasshopper limb buds was investigated by allowing growth cones to migrate in 30%-31% stage limbs from which the basal lamina had been removed by enzymatic treatment. When the Ti1 axons extended from their normal location, the pathways established in the absence of basal lamina were normal. This indicates that the basal lamina is not required for initial proximal axon outgrowth, recognition of limb segment boundaries, or selective interaction with neuronal somata. Removal of the basal lamina from slightly older (32% stage) embryos resulted in displacement of the Ti1 somata to ectopic locations in approximately 50% of the limbs. Pathfinding from ectopic locations was aberrant in 45% of the cases observed. This demonstrates that if orienting information is present in the basal lamina-free epithelium at this stage, it is not the predominant factor in determining growth cone orientation from ectopic locations.

Animals↗
PubMed1989

Organization of cytoskeletal elements and organelles preceding growth cone emergence from an identified neuron in situ.

The purpose of this study was to investigate the arrangement of cytoskeletal elements and organelles in an identified neuron in situ at the site of emergence of its growth cone just before and concurrent with the onset of axonogenesis. The Ti1 pioneer neurons are the first pair of afferent neurons to differentiate in embryonic grasshopper limbs. They arise at the distal tip of the limb bud epithelium, the daughter cells of a single precursor cell, the Pioneer Mother Cell (PMC). Using immunohistochemical markers, we characterized the organization of microtubules, centrosomes, Golgi apparatus, midbody, actin filaments, and chromatin from mitosis in the PMC through axonogenesis in the Tils. Just before and concurrent with the onset of axonogenesis, a characteristic arrangement of tubulin, actin filaments, and Golgi apparatus is localized at the proximal pole of the proximal pioneer neuron. The growth cone of the proximal cell stereotypically arises from this site. Although the distal cell's axon generally grows proximally, occasionally it arises from its distal pole; in such limbs, the axons from the sister cells extend from mirror symmetric locations on their somata. In the presence of cytochalasin D, the PMC undergoes nuclear division but not cytokinesis and although other neuronal phenotypes are expressed, axongenesis is inhibited. Our data suggest that intrinsic information determines the site of growth cone emergence of an identified neuron in situ.

Actins↗
PubMed1989

Removal of the basal lamina in vivo reveals growth cone-basal lamina adhesive interactions and axonal tension in grasshopper embryos.

The Ti1 afferent neurons are the first cells to undergo axonogenesis in embryonic grasshopper limbs. The Ti1 growth cones migrate between the limb epithelium and its basal lamina. We have investigated the nature of growth conebasal lamina interactions in vivo by removing the basal lamina with mild enzymatic digestion. Treatment with elastase, ficin, or papain removes the basal lamina when viewed in scanning electron microscopy. Trypsin and chymotrypsin leave the basal lamina intact. If the basal lamina is removed after the Ti1 growth cones have extended over intrasegmental epithelium but are not yet in contact with either differentiated segment boundaries or neurons, the growth cones retract to the cell somata. If the basal lamina is removed by elastase, and the Ti1 neurons are allowed to extend axons after treatment, a second elastase digestion does not cause the axons to retract. It is therefore unlikely that axon retraction is due to general proteolysis. These results suggest that if Ti1 growth cones have initially extended in the presence of an intact basal lamina, they are dependent on the lamina to remain extended over this region of the limb. The retraction of the Ti1 axons after removal of the basal lamina is inhibited by cytochalasin D, suggesting that microfilament-based cytoskeletal components underlie this event. This result indicates that the axons are under tension in vivo. The ability of the Ti1 growth cones to resist axonal tension suggests that adhesive interactions between the growth cones and the basal lamina underlie normal axon outgrowth in vivo.

Animals↗
PubMed1989

Pioneer growth cone adhesion in vivo to boundary cells and neurons after enzymatic removal of basal lamina in grasshopper embryos.

The Ti1 pioneer neurons of embryonic grasshopper limbs extend axons between the limb epithelium and its basal lamina. Their growth cones exhibit high affinity for both limb segment boundaries and immature neurons. We have investigated the role of the basal lamina in growth cone adhesion to neurons and segment boundaries by removing the basal lamina with mild enzymatic digestion when the Ti1 growth cones are in contact with different cellular substrates. If the basal lamina is removed when the Ti1 growth cones are in contact with other neurons, the growth cones remain in contact with the neuronal somata, and the Ti1 cell bodies in contact with the neuronal somata, and the Ti1 cell bodies reposition proximally. This suggests that the basal lamina is involved in the adhesion of the Ti1 somata to the substrate but not in growth cone-neuronal adhesion. This is the first direct evidence that growth cones establish adhesive cell-cell interactions with other neurons in vivo. Enzymatic treatments that remove the basal lamina also cause embryonic limbs to elongate. If the Ti1 axons are strongly apposed to 2 segment boundaries prior to protease treatment, their somata reposition to the nearest segment boundary, yet their axons do not retract off of the segment boundaries, despite severe stretching by the enzyme-induced limb expansion. These results indicate that the affinity of the Ti1 cells for segment boundaries is due at least in part to adhesive cell-cell interactions that are resistant to proteolytic digestion and independent of the basal lamina.

Animals↗
PubMed1988

Platelet phenolsulphotransferase activity and 'abdominal migraine'.

Low platelet phenolsulphotransferase activity has been reported in adult patients with dietary sensitive migraine. Platelet activity of this enzyme was therefore measured in children having 'abdominal migraine' with probable dietary trigger and in controls. No significant difference was found in activity between the two groups. There was no significant correlation between platelet phenolsulphotransferase activity and age.

Abdomen↗
PubMed1987

Mutations in the first exon are associated with altered transcription of c-myc in Burkitt lymphoma.

The c-myc proto-oncogene is involved in chromosomal translocations that are specifically and consistently found in Burkitt lymphoma. Although these translocations are thought to lead to a deregulation of c-myc expression, the structural and functional basis of this phenomenon has not been identified. Mutations in a specific region spanning approximately 70 base pairs and located at the 3' border of the first exon of translocated c-myc alleles were consistently detected in Burkitt lymphoma cells carrying classic (8:14) as well as variant (8:22 and 2:8) translocations. These structural alterations were accompanied by an altered pattern of c-myc transcription, namely, the removal of a block to transcriptional elongation that has been mapped to the same region. Thus, specific c-myc mutations leading to the alleviation of this block to transcriptional elongation may represent a general mechanism causing c-myc activation in Burkitt lymphoma.

Burkitt Lymphoma↗
PubMed1987

Pioneer growth cone behavior at a differentiating limb segment boundary in the grasshopper embryo.

The first neurons to extend axons through embryonic grasshopper limbs are a pair of sibling pioneer neurons. After migrating proximally along the limb axis, the pioneer growth cones normally make an abrupt ventral turn. In some cases (less than 20%) this turn is directly toward the proximo-ventrally located Cx1 guidepost neurons. However, in the majority of cases (greater than 80%) the pioneer growth cones make a more acute ventral turn along a single circumferential line which lies distal to the Cx1 neurons. Growth cones from other afferent neurons orient along the same line. Growth cones can extend along this line around more than half of the circumference of the limb and can grow in either direction along it. The circumferential line appears to be the prospective trochanter-coxa segment boundary. Afferent axons on the segment boundary leave it and contact the proximo-ventrally located Cx1 neurons. The site at which pioneer growth cones leave the boundary is variable and appears to be the point from which filopodial contact with Cx1 cells is first established. In addition to the trochanter-coxa segment boundary, the pioneer growth cones and axons also respond to the tibia-femur and femur-trochanter segment boundaries. The role of segment boundaries as barriers to growth cone movement and the effect of such barriers on the timing and placement of differentiation of pioneer neurons are discussed.

Animals↗
PubMed1987

Pathfinding by pioneer neurons in isolated, opened and mesoderm-free limb buds of embryonic grasshoppers.

The Ti1 afferent neurons are the first neurons to undergo axonogenesis in limb buds of embryonic grasshoppers. Their growth cones pioneer a stereotyped pathway through the limb which becomes the route of one of the major leg nerve trunks. The growth cones appear to be oriented by several kinds of guidance cues, including guidepost neurons, a developing limb segment boundary, and an additional proximally orienting cue(s). In the experiments reported here, we have investigated the possible nature and source of proximally orienting and segment boundary cues by surgical manipulations of the limb. Before the onset of pioneer axonogenesis, limbs were isolated from the body, opened longitudinally and pinned out flat, or stripped of mesoderm. Pioneer axon routes in cultured, surgically manipulated limb buds were compared to routes in cultured control limbs. The results indicate that proximal extension of pioneer growth cones along the limb axis does not require (during the period of growth) tissue extrinsic to the limb, contact guidance by the limb contour, an axial electrical field, a diffusion gradient generated by a localized source, mesodermal cells, or guidepost neurons; adequate guidance information for proximal growth apparently can be provided by the limb epidermal epithelium (including the basal lamina) and/or by internal polarity of the pioneer neurons. Adequate guidance information for the segment boundary portion of the pioneer route apparently can be provided by the limb epithelium.

Animals↗
PubMed1987

Programmed death of peripheral pioneer neurons in the grasshopper embryo.

The Ti1 pioneer neurons arise at the distal tip of the metathoracic leg in the grasshopper embryo, and are the first neurons in the limb bud to extend axons to the central nervous system (C. M. Bate (1976) Nature (London) 260, 54-56; H. Keshishian (1980) Dev. Biol. 80, 388-397). By providing a neural pathway along which growth cones of later arising neurons migrate, these pioneer axons establish the route of one of the major nerve trunks in the leg (Keshishian, 1980; R. K. Ho and C. S. Goodman (1982) Nature (London) 297, 404-406; D. Bentley and H. Keshishian (1982) Science 218, 1082-1088). Here, we demonstrate that at the 55-59% stage of development, the two Ti1 pioneer neurons undergo programmed death. The role which these pioneers serve in establishing a nerve route appears to be their only function, and may be important for the normal development of the peripheral nervous system. The Ti1 pioneers provide an example of a previously hypothesized class (J. W. Truman (1984) Annu. Rev. Neurosci. 7, 171-188) of programmed neuron death: obsolete neurons whose function was developmental rather than behavioral.

Animals↗
PubMed1986

Epithelial cell specialization at a limb segment boundary in the grasshopper embryo.

Epithelial cells at the developing femur-trochanter limb segment boundary in the grasshopper embryo are specialized with respect to nonboundary cells. They are elongated, with the long axis oriented along the limb circumference. Some cells along the boundary preferentially bind an antibody (anti-HRP), and so are molecularly specialized as well. The specialized cells are the most proximal cells of the more distal segment.

Animals↗
PubMed1986

Pioneer growth cone steering along a series of neuronal and non-neuronal cues of different affinities.

We have analyzed the morphology of over 5000 Ti1 pioneer growth cones labeled with anti-HRP, which reveals the disposition of axons, growth cone branches, and filopodia. Ti1 axon pathways typically consist of a sequence of 7 characteristically oriented segments, with a single, distinct reorientation point between each segment. Growth cones exhibit the same orientations and reorientations in a given region as do axon segments at later stages. The single, distinct reorientations suggest that growth cones make discrete switches between guidance cues as they grow. Ti1 growth cones are guided by various types of cues. A set of 3 immature identified neurons serves as nonadjacent guidepost cells and lies at the proximal end of 3 of the axon segments. To form another segment, growth cones reorient along a limb segment boundary within the epithelium. Growth cones also respond consistently to, and orient toward, a specific mesodermal cell, which may be a muscle pioneer. Thus, growth cones respond to at least 3 different types of cells in the leg. Ti1 growth cones exhibit a hierarchy of affinity for these cues. Guidepost neurons are the dominant cues in that contact with them reorients growth cones from guidance by the other types of cues. Growth cone branches are exclusively oriented to specific cues. Growth cones reorient by extending a branch directly to the cue of highest affinity and by withdrawing any branches that are extended to a cue of lesser affinity. A single filopodium in direct contact with a guidepost neuron can reorient a growth cone that still has multiple filopodia or even prominent branches specifically oriented to a previous cue of lesser affinity. These observations suggest that growth cone steering may not result simply from passive adhesion and filopodial traction, but may involve more active processes.

Axons↗
PubMed1986

Pioneer growth cone morphologies reveal proximal increases in substrate affinity within leg segments of grasshopper embryos.

We have compared the morphologies of approximately 5000 antibody-labeled afferent pioneer growth cones fixed at various stages of growth along their characteristic path over the epithelium in the legs of grasshopper embryos, and have used growth cone morphology as an indicator of differences in the affinity of the epithelial substrate for pioneer growth cones in vivo. Growth cone morphologies differ markedly between different locations in limb buds, and also in the same location in limbs at different stages of differentiation. Growth cones characteristically extend branches and lamellae circumferentially along segment boundaries, and filopodia and lamellae are retained (or extended) longer there. Where they contact a relatively well-differentiated segment boundary, the growth cones also abruptly reorient circumferentially. In the proximal regions of limb segments, growth cones consistently have a high degree of branching and lamellae; previously formed axons also extend secondary branches and spread there as development progresses. Low incidence of these morphologies is observed at all stages in the distal regions of limb segments. Thus, neuronal morphologies correlate both spatially and temporally with the differentiation of limb segmentation. These results suggest the following: Detailed growth cone morphology is a reliable indicator of differences in extrinsic guidance cues. The affinity of the epithelial substrate for afferent pioneer growth cones increases proximally within segments, with a peak at the segment boundary. (This affinity could be based on surface density of adhesion molecules or on nonadhesive molecules that actively regulate growth cone extension.) Increasing epithelial affinity within segments appears to act as a proximal guidance cue for afferent pioneer growth cones. Pioneer growth cones are observed to navigate proximally in circumstances where proximally located guidepost cells differentiate too late to guide them.

Animals↗