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R K Hunt

Publications and source records attributed to R K Hunt.

35 records · Page 2Linked to original sources
PubMed1975

Neuronal locus specificity: trans-repolarization of Xenopus embryonic retina after the time of axial specification.

Signaling within an embryonic Xenopus eye comprised of two fused eye fragments can reprogram, in turn, the anteroposterior and dorsoventral axes of one of the fragments. The responding fragment, subsequently isolated and allowed to round up and innervate the brain, shows corresponding inversions in its retinotectal map. This is the first evidence of trans-repolarization of presumptive retina and provides an assay system for analysis of positional signaling within the retinal field.

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PubMed1975

Developmental programming for retinotectal patterns.

Programming events at stages 28-31 in Xenopus specify the cellular positional information that individual retinal ganglion cells will use to derive theri appropriate locus specificity for assembly of the retinotectal map. The 'programme' that emerges in the stage 31 retina affects the entire ganglion cell population (99 percent of which is generated later) and refers positional information to intraretinal axes (AP and DV) and zero-points. Its expression in intact retinae was not modified by repeated reintroduction into pre-stage 28 orbits, prolonged eye culture in vitro, or severe disruption of the timing and sequence of ganglion cell births or of optic fibre arrivals in the tectum. In contrase, intraretinal reorganizations did not produce major modifications (e.g. after transection, partial ablation, fragment fusion etc.) in the set of locus specificities arising in various retinal regions and, in some instances, in the reference axes themselves. The modified programmes were characteristic of the components undergoing reorganization, but were convergent (many sets of reorganizing components gave a few final patterns); they appeared to involve a stable and rapid reprogramming of certain components by others, in a hierarchical fashion. The remaining experiments focus on the problem of localizing the 'trigger' for the transition from the unspecified to the specified state in the retina at stages 28-31. Specification can occur in vitro, based on reversible AP and DV orientational markers which are present in the pre-stage 28 eye primordium; in heterochromically grafted eyes, specification was neither precipitated nor delayed by altering the stage of the host. Finally, chemical dissection of the differentiating eye primordium confirmed the inference (from Jacobson's [3-H] thymidine labelling kinetics) that a specific gangliogenic precursor cell type exists in the stage 28 retinal neuroepithelium, and suggested that differentiative events in these cells trigger the specification process.

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PubMed1974

Specification of positional information in retinal ganglion cells of Xenopus laevis: intra-ocular control of the time of specification.

Programming events in the stage 28-31 clawed frog embryo partly determine the organization of the future retinotectal map by specifying the permanent reference axes for cellular positional information in the retina. Thus, when transplanted in 180 degrees -rotated orientation into a stage 27/28 orbit, an unspecified (stage 28 or younger) eye can acquire new axes and develop a normally-oriented retinotectal map, whereas a specified (stage 31 or older) eye retains its original axes and develops an inverted map. We have used the retinotectal map to determine when (and under what conditions) specification with changes in orientation occurred in eye primordia serially transplanted between embryos of different stages. Specification was not precipitated when stage 22/23 eyes were grafted into stage 28-32 embryos or explanted in vitro, nor was specification delayed when stage 28 eyes were grafted into embryos younger than stage 28. Control experiments confirmed the general correlation between the time of specification and the stage of the eye primordium. We infer that intraocular regulatory mechanisms control the time of specification.

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PubMed1973

Neuronal locus specificity: altered pattern of spatial deployment in fused fragments of embryonic xenopus eyes.

Before optic nerve outgrowth in Xenopus laevis embryos, a change of state occurs in the differentiating retinal cell population which renders the cells refractory to information about subsequent changes in their positions, and commits individual ganglion cells to develop specific position-dependent properties (locus specificities) which subserve the formation of orderly retinotopic connections in the optic tectum. When different parts of eye primordia from stages before optic nerve outgrowth are fused, each piece in such a reconstructed eye does not generate ganglion cells with the partial-set of locus specificities normally arising from that region of the intact eye. It is inferred that separate parts of the early embryonic retina do not contain stable programs for spatial deployment of locus specificity, and that development of definitive locus specificities in retinal ganglion cells requires additional cellular interactions among the retinal cells later in development.

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PubMed1973

The acquisition of specificity in cutaneous sensory neurons: a reconsideration of the integumental specification hypothesis.

Neuronal specificity in cutaneous sensory nerve cells has been postulated to arise from "inductive interactions" between the cell's randomly outgrown peripheral neurite and local biochemical markers in the skin. Here was apply this integumental specification hypothesis to data recently obtained on the wiping-reflex behavior of frogs skin-grafted at various times during larval life. Deductions are generated about the developmental time course of the postulated nerve-skin interactions and two predictions are formulated and tested. Because the results of serial skin rotation experiments contradict the predictions, we conclude that the currently held hypothesis must be seriously questioned.

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PubMed1973

Specification of positional information in retinal ganglion cells of Xenopus: assays for analysis of the unspecified state.

The central connections of retinal ganglion cells are retinotopically organized, producing a "map" of the retina on the surface of the optic tectum. Exactly how and when individual ganglion cells develop the position-dependent properties (termed locus specificities) subserving formation of the map is unknown, but the positional information that each ganglion cell will use in this process is specified in the early Xenopus embryo during a critical period at stages 28-32. We report two methods for isolating eye primordia from the axial cues of the animals during this critical period and for then allowing the eyes to form retinotectal connections in a carrier embryo. The results show that, as early as optic vesicle stages 22-23, the eyes already contain orthogonal reference axes, that positional information can be specified with respect to these axes in vitro, and that the specification process itself may only entail a transition from a reversible to an irreversible state.

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PubMed1972

Specification of positional information in retinal ganglion cells of Xenopus: stability of the specified state.

In the normal development of retinotectal connections, the site in the tectum at which an optic fiber synapses is related to the position of its ganglion-cell body in the retina. How and when the ganglion cells acquire information about their positions is unknown, but the positional information that each ganglion cell will ultimately act upon is determined or specified at embryonic stages 28-32 in the clawed frog, Xenopus laevis. Here we report that once positional information has been so specified, it remains stable when the eye is "back-grafted" into the orbit of a stage-28 host, or cultured in vitro for up to 10 days before grafting into the orbit of a stage-38 host. Thus, the ganglion cells of these eyes form tectal connections appropriate to their original positions in the donor orbits and independent of their final positions in the host orbits. We conclude that specification of positional information involves stable changes in the phenotypic properties of the differentiating retinal cells that (i) render the cells refractory to information about changes in their positions after stage 32 and (ii) commit each ganglion cell to the development of a unique property (locus specificity) that predisposes its axon to synapse at a particular locus in the retinotectal map.

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PubMed1972

Development and stability of postional information in Xenopus retinal ganglion cells.

Neuronal specificity in retinal ganglion cells of Xenopus subserves the orderly connections of the optic nerve fibers in the tectum. This specificity derives from positional information acquired by the developing retina at embryonic stages 28-31. Here we report that ganglion cells of embryonic stage 28 eyes can acquire positional information with reference to the major axes of the body not only in the ocular orbit but also at other positions on the side of the body. When returned to the orbit this eye will form appropriate retinotectal connections. Conversely, retinal ganglion cells of stage 31 eyes, which have acquired positional information in the orbit, will retain their original neuronal positional specificities if the formation of retinotectal connections is delayed by grafting the eye to the flank for 30 days before returning it to the orbit. We conclude that neuronal specificity of retinal ganglion cells (a) does not derive from "inducers" unique to the periocular tissues; and (b) persists for some time independently of the establishment of retinotectal connections.

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PubMed1970

Brain enhancement in tadpoles: increased DNA concentration after somatotrophin or prolactin.

Frog tadpoles, injected with prolactin or somatotrophin during early stages when the brain cell population is rapidly increasing, exhibited marked increases over sham-injected controls in body weight, brain weight, and brain DNA, throughout subsequent development. Animals treated with somatotrophin attained the increase in brain DNA during the infection period. Prolactin had little effect during this period, but brain DNA accumulation continued at an accelerated rate over the next 15 days, when the rate of DNA increment normally declined. Patterns of incorporation of tritiated thymidine confirmed that both hormones increased DNA accumulation, suggesting increased cell proliferation rather than decreased cell death.

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