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R M Gaze

Publications and source records attributed to R M Gaze.

At least 37 records · Page 2Linked to original sources
PubMed1983

The visuotectal projections made by Xenopus 'pie slice' compound eyes.

In xenopus embryos at stages 28-32 one quarter to one third of the left eye rudiment was replaced by a similarly sized piece from a different position in a right eye rudiment. Three groups of operations were performed: (1) temporal tissue was placed in a nasal position; (2) nasal tissue was placed temporally; (3) ventral tissue was placed dorsally. The visuotectal projections made by these 'pie-slice' compound eyes were assessed electrophysiologically at 1 week to 6 months after metamorphosis. Of 97 animals, 71 yielded interpretable projections. In most cases two projections could be identified in each map. One, ascribed to the host part of the retina, extended over the entire tectal surface mapped. The other, identified as that from tissue derived from the pie-slice graft, projected to the tectum in register with that part of the host retina which matched the pie-slice in origin. Both projections were well ordered, and in the orientation expected if the corresponding piece of retinal tissue had participated in a normal projection. Consistent differences in pie-slice size and tectal coverage between the three groups were found. Pie-slices of nasal origin gave maps showing that they came from a relatively large portion of the retina and projected to a relatively large amount of the tectum; those of temporal origin occupied relatively small amounts of field and tectum. It was concluded that these results are further evidence for the existence of positional markers in the retina which are used for the assembly of the retinotectal map.

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PubMed1982

The innervation of a virgin tectum by a double-temporal or a double-nasal eye in Xenopus.

In Xenopus embryos at stage 32/33 one eye anlage was removed and the other made into a compound double-nasal or double-temporal eye. At metamorphosis the optic nerve was cut and the compound eye was permitted to regenerate fibres both to its own contralateral tectum and to the ipsilateral 'virgin' tectum. One month later the projection from the compound eye to the virgin tectum was assessed autoradiographically by use of tritiated proline. Projections from double-temporal eyes were found to be restricted to rostrolateral tectum, whereas projections from double-nasal eyes covered the entire tectal surface. It was concluded that the results did not suggest that positional markers existed on the tectum before the arrival of the optic fibres.

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PubMed1982

The retinotectal fibre pathways from normal and compound eyes in Xenopus.

Horseradish peroxidase was used to demonstrate the nature of the retinotectal fibre pathways from normal eyes and from compound double nasal (NN), double temporal (TT) and double ventral (VV) eyes in Xenopus. From normal eyes, nasal fibres were widespread in the optic tract and mostly entered the tectum through the medial and lateral brachia. Ventral retinal fibres approached the tectum via the medial brachium and dorsal retinal fibres passed through the lateral brachium, while temporal retinal fibres formed a narrow band in the centre of the tract and entered the tectum directly at its rostral border. Fibres from NN eyes formed a wide tract and strong medial and lateral brachia. Fibres from VV eyes all entered the tectum via the medial brachium and fibres from TT eyes formed a narrow tract and entered the tectum directly from its rostral extremity. Thus fibres from each type of compound eye followed pathways to the tectum that were appropriate to the embryonic origin of the retina forming the compound eye.

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PubMed1981

The organization of regenerating axons in the Xenopus optic nerve.

After regeneration of the optic nerve in Xenopus, with restoration of an orderly visuotectal map, HRP was applied to a small region of retina, or tectum, and the distribution of labelled fibres in the nerve investigated. As they passed the site of the crush, fibres became grossly disordered and remained so until they got to the chiasma.

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PubMed1981

The development of the retinotectal projections from compound eyes in Xenopus.

The retinotectal projections from double-nasal (NN), double-temporal (TT) and double-ventral (VV) compound eyes in Xenopus were studied autoradiographically and electrophysiologically during development. Early TT projections were confined to rostrolateral tectum and spread with advancing age to cover most of the tectum by shortly after metamorphosis. Early VV projections showed a decreased density of label on lateral tectum. Early NN projections appeared to extend across the entire rostrocaudal length of the available tectum at all stages of development, but showed a decrease in label density on rostral tectum. The results are discussed in relation to various hypotheses about the formation of retinotectal connexions.

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PubMed1980

Regeneration of optic nerve fibres from a compound eye to both tecta in Xenopus: evidence relating to the state of specification of the eye and the tectum.

Xenopus with one double-nasal (NN), double-temporal (TT) or double-ventral (VV) eye were induced to regenerate optic fibres from the compound eye to both tecta after metamorphosis. The extent of the projection from the compound eye was then estimated autoradiographically at various intervals after nerve section. The regenerated projection covered the whole of the contralateral tectum but covered only a restricted part of the ipsilateral tectum; thus NN eyes innervated caudomedial tectum only, TT eyes innervated rostrolateral tectum only and VV eyes innervated medial tectum only. Since the tectum ipsilateral to the compound eye also received the projection from the normal eye these results are taken to indicate that NN, TT and VV eyes are unregulated systems in terms of the retinal positional markers that they carry; each such eye bears only the appropriate half-set of such markers.

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PubMed1980

Stable programming for map orientation in fused eye fragments in Xenopus.

Compound eyes were formed in Xenopus embryos at stages 32/33 by fusion in the right orbit of (1) two right naso-ventral halves, (2) two right ventral halves, (3) two right temporo-ventral halves, (4) one right and one left naso-ventral half and (5) one right and one left temporo-ventral half. The contralateral visuotectal projections from the operated eyes later showed abnormalities reflecting the anatomical arrangement of the fused fragments. The experiments thus revealed considerable stability of the developmental programme leading to the later development of map orientation, in the face of operative disturbance of the types used.

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PubMed1980

Stable programming for map orientation in disarranged embryonic eyes in Xenopus.

In Xenopus embryos of stage 32/33 either the temporal or nasal half of the right eye anlage was replaced by a corresponding left half, giving a right eye in which the grafted half was inverted dorsoventrally. In other embryos either the dorsal or ventral half of the right eye anlage was replaced by a corresponding left half, giving an eye in which the grafted half was reversed nasotemporally. These foud types of operation were intended to produce eyes that were disarranged internally but which each had a complete range of positional values. The visuotectal projections from such eyes, recorded later in life, in most cases showed axial reversal of half of the map, reflecting the nature of the operation. The results thus demonstrate that the developmental programme in each of the fused retinal fragments is stable in relation to the eventual orientation of the map from that fragment. Operations to produce eyes with an inverted temporal half, if performed in operating solution of low ionic strength, may result in mirror reduplication and the formation of double nasal maps. It is suggested that this phenomenon may underlie previous reports of reprogramming of one eye fragment by another.

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PubMed1980

The retinotectal projections from surgically rounded-up half-eyes in Xenopus.

In Xenopus embryos of stage 32 half of one eye anlage was removed and the remaining half was surgically rounded-up. The visuotectal projections through such half-eyes, recorded after metamorphosis, showed in most cases a deformation of the map which corresponded to the deformation imposed on the half-eye at operation. Some eyes gave normal maps and some showed mirror-reduplication.

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PubMed1979

The central pathways of optic fibres in Xenopus tadpoles.

A cobalt chloride impregnation technique was applied to the optic nerve in Xenopus tadpoles and the central optic pathways were examined in cleared, whole-mounted preparations, and in thick sections. The overall plan of the optic input was visualized in relation to the outlines of the parts of the brain and details of the structure of the tectal optic neuropil, the neuropil of Bellonci and the basal optic neuropil were seen. The fibres in the main retinotectal tract maintained an orderly disposition with respect to each other, in contrast to the fibres of the basal optic tract, in which no order was apparent. Optic fibres were seen passing caudally from the region of the basal optic neuropil.

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PubMed1979

Selection of appropriate medial branch of the optic tract by fibres of ventral retinal origin during development and in regeneration: an autoradiographic study in Xenopus.

The formation of the branches of the optic tract has been studied with the use of [3H]proline autoradiography, during development and during regeneration of the optic nerve in Xenopus with one compound ventral (VV) eye made by the embryonic fusion of two ventral eye fragments. The formation of the optic pathway was abnormal in that the lateral branch failed to develop, suggesting that fibres from a VV retina selectively entered the tectum via the medial branch during development. Three months after section of the optic nerve of a VV eye, regenerated fibres were present both in the contralateral and ipsilateral tecta. On the ipsilateral side regenerated fibres entered the tectum via the medial branch only. Retinal fibres entered the contralateral tectum through both branches in some animals and through the medial branch only in others. It is concluded that mechanical factors alone are insufficient to explain the phenomenon of selection of the appropriate medial branch by fibres of ventral retinal origin either during development or in regeneration. Some form of fibre-substrate interaction seems to be necessary; and this ability of fibres from a VV eye to take the path appropriate for ventral retina argues strongly that the VV eye is not a regulated system in terms of cell specificities.

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PubMed1979

The relationship between retinal and tectal growth in larval Xenopus: implications for the development of the retino-tectal projection.

The modes of retinal and tectal histogenesis, as well as the patterns of terminal distribution of optic axons in larval Xenopus were studied, using anatomical techniques. We show that the retina grows by adding strips of cells at its ciliary margin. The pattern of retinal growth is asymmetrical along the dorso-ventral axis of the retina. On the other hand, the tectum grows by adding newly formed cells caudo-medially. The most rapid tectal growth takes place between stages 50 and 53, and thereafter only a small proportion of cells are added near the midline of the caudal tectum. Despite such incongruent modes of retinal and tectal growth, retinotopic order is maintained throughout larval life. We present here further evidence supporting the idea that connexions between the arrays of retinal and tectal cells shift progressively caudo-medially on the tectum during the period of growth. When the temporal pole of the retina is destroyed at various developmental stages, the density of degenerating synapses is always highest in the rostral pole of the tectum. Moreover, optic terminals stemming from the central retina spread caudally, invading newly maturing regions of the tectum. Quantitative analysis of the terminal distributions of optic axons suggest that orderly shifts in synaptic contacts between optic axons and tectal dendrites take place in the course of development. Ultrastructural observations on the maturing tectal neuropil are consistent with this view.

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PubMed1979

The orientation of the visuotectal map in Xenopus: developmental aspects.

Rotations and translocations of the eye anlage were performed in Xenopus embryos of stages ranging from 21/22 to 30. Some of the operations involved grafting wild-type eye anlagen into albino host orbits. Operations were performed under a variety of operating media and conditions. In later larval life, or after metamorphosis, the visuotectal maps from the operated eyes were recorded electrophysiologically. Results fell into two classes. In the majority, the orientation of the visuotopic map corresponded to the orientation of the eye at the time of recording. In the minority the visuotopic maps were 'compound', consisting of two parts each with its own independent orientation. The organization of the compound maps was such that one component was oriented in correspondence with the orientation of the eye, while the other component was normally oriented. Histological analysis and observations on genetically marked grafts indicated that the component parts of the compound eye were of dual cellular origin. The component giving the rotated (or translocated) map belonged to the originally operated eye tissue; whereas the component giving the normally oriented map was derived from newly grown eye tissue coming from the optic stalk. In no case was a normally oriented map obtained from a rotated or translocated eye. The results are discussed in relation to mechanisms proposed to account for the determination of map-related retinal specificity.

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PubMed1978

The diencephalic course of regenerating retinotectal fibres in Xenopus tadpoles.

The normal retinotectal path in the diencephalon of Xenopus tadpoles is widely distributed in the form of a wedge of fibres extending from the central grey to the outer margin of the diencephalon. Regenerating optic nerve fibres were shown, by silver-staining and proline autoradiography, to follow an abnormal path up the extreme lateral edge of the diencephalon. Study of tadpoles at various stages of development, and of optic nerves allowed to regenerate for various periods, indicates that all new incoming optic fibres pass up the lateral edge of the diencephalon. The inner/outer order of the fibres in the normal diencephalon thus reflects the radial distribution of the retinal cells of origin.

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