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The retinotectal projection of quarter eyes in Xenopus laevis
141
Developmental Brain Research, 29 (1986) 141-143 Elsevier
BRD 60161
Short Communications The retinotectal projection of quarter eyes in Xenopus laevis NORBERT DEGEN and KURT BRANDLE Arbeitsgruppe Neurophysiologie, Fachbereich Biologie der Johann Wolfgang Goethe-Universiti~t Frankfurt am Main, Frankfurt am Main (F. R. G.)
(Accepted May 6th, 1986) Key words: retinotectal projection - - eye fragment --Xenopus
Following extirpation of three quarters of the eye anlage of Xenopus laevis in stage 33/34, the remaining quadrants developed into small eyes of normal shape. Irrespective of the different portions of the eye anlage the quarter eyes originated from, the retinotectal projection was alwaysconfined to the rostrolateral part of the tectum, forming an orderly map, and its dimensions corresponded to the size of the contralaterai eye. Therefore, it was suggested that during development both size and location of the projection are independent of the existence of tectal markers. Instead, they are determined by the number of ingrowing optic fibers and the direction of their growth from the rostral origin.
Recent investigations of surgically reassembled compound eyes and eye fragments, such as half eyes, indicated that each part of the retina in Xenopus still retains its original specificity 1,3,6,7. Nevertheless, the retinotectal projections were not restricted to the appropriate portion of the tectum, but covered the whole tectal surface. In all experiments compound eyes and half eye fragments invariably developed into eyes of normal shape and size. Therefore the observation may be explained in two ways: (1) initially there are no markers in the tectum; the optic fibers grow to occupy the entire surface available on the tectum; (2) markers do exist, interconnection is correct when the first fibers grow in but later projection is expanded due to the mass of ingrowing fibers. In order to determine which of the two possible explanations is valid we reduced the size of the retina fragments to such a degree that development of normal size eyes was no longer possible. Which of the two above hypotheses would hold true? In stage 33/34 (ref. 5) we removed three quarters of the eye anlage. The remaining quadrants originated from the nasoventral (NV), ventral (V), or
temporo-ventral (TV) retina (see Fig. 2, left). Within a few days, ca. 40% of the eye fragments rounded up and formed eyes of normal shape. The remaining 60% developed into extremely small globular fragments without lenses and were consequently excluded from further study. The so-called quarter eyes never achieved the size of eyes encountered in untreated animals. They never extended 2 5 % - 7 5 % of a normal eye. Neither did they show any sign of accelerated or delayed growth if compared to growth of normal eyes. The optic input to the contralateral rectum was studied in 41 tadpoles (stage 58-62) 5 and 5 metamorphosed specimens of Xenopus. The animals were anaesthetized by MS 222 (Sandoz) and placed in the center of a perimeter, the operated eye facing the fixation point in the perimeter. Electrophysiological recording was done with glass-coated tungsten electrodes 4. Multiunit spike responses to spot-light stimuli were obtained from 16 animals with one-quarter TV-eyes, from 14 animals with one quarter Veyes, and from 16 animals with one-quarter NVeyes.
Correspondence: Arbeitsgruppe Neurophysiologie, Fachbereich Biologie der Johann Wolfgang Goethe Universit~it Frankfurt am Main, Siesmayerstrasse80, 6000 Frankfurt am Main 11, F.R.G.
0165-3806/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)
142
Fig. t. Wholemounts of mesencephalic region (indicated by dotted lines). Optic projections marked by cobalt filling. Top view, stage 61. a: normal projection, b: projection of NV-eye c: projection of TV-eye.
With 9 animals responses could only be o b t a i n e d by stimulation with intermittently changing intensity of light. But in all cases it was, nevertheless, possible to d e t e r m i n e both dimensions and location of the projection field in the tectum. For 37 specimens we were able to m a p the response to stimulation of small areas in the visual field. As a rule, the p r o j e c t i o n field r e t a i n e d its retinotopical organization, even though partly e n m e s h e d or double projections could mostly be observed. This was also true for projections after rotation of the eye fragments by 90 ° or 180 ° . In all such cases the m a p was r o t a t e d at the same angles. In one respect, however, the responses differed considerably from those o b t a i n e d in half eyes and c o m p o u n d eyes. The projection did not cover m o r e than one third to about one half of the tectal surface. T h e r e was a linear correlation b e t w e e n the dimensions of the projection and the size of the o p e r a t e d
143
left tectum
(TV) T ~ N b)
(V) T O N
(NV) T Q
N
types of eye fragments (right eye)
c|
Fig. 2. Scheme of the expected retinotectal projections of quarter eyes. T, temporal; N, nasal, a: presence of tectal markers, respecified retina, b: presence of tectal markers, retina retaining its original specification, c: absence of tectal markers. Only (c) is in accordance with experimental results.
eye. The reduced projection field was always confined to the rostrolateral portion of the tectum, despite the different origin of the fragments. In some cases these findings could be histologically supported by cobalt filling of the eye stalk after recording was finished (Fig. l a - c ) . Gaze et al. 2 have shown a similar reduction in projection during development of normal eyes in Xenopus. For the following reasons, however, it appears fairly unlikely that the reduced size of the projection field should have been caused by developmental delay. (1) In recordings from 5 animals up to two years after metamorphosis, we never found the projection field extended into the caudo-medial portion of the tectum. (2) Analysis of the size increase of the eye
1 Feldman, J.D. and Gaze, R.M., The development of halfeyes in Xenopus tadpoles, J. Comp. Neurol., 162 (1975) 13-22. 2 Gaze, R.M., Keating, M.J. and Chung, S.H., The evolution of the retinotectal map during development in Xenopus, Proc. R. Soc. London Set. B, 185 (1974) 301-303. 3 Gaze, R,M. and Straznicky, C., 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, J. Embryol. Exp. Morphol., 60 (1980) 125-140. 4 Merril, E.G. and Ainsworth, A., Glass-coated platinum-
anlage between stage 33/34 and metamorphosis yielded sigmoidal growth curves. Comparison of normal eyes, half eyes and quarter eyes did not reveal any shift during the period of growth which would have provided some indication of developmental delay. Similarly, detailed histological study of the developing retina failed to show any time shift with respect to differentiation. (3) At the time of recording no size differences with regard to the receptive fields of both normal and operated eyes were noticeable. If development were delayed, larger receptive fields and less retinotopic organization would have be expected. Our findings, then, do not support the theoretical possibility that location and size of the tectal projection field are determined by markers present in the tectum during first ingrowth of optic fibers. Even if markers were present and the eye fragment did respecify to a full set of positional values, the projection would, nevertheless, have to spread over the whole tectal surface in accordance with the complementary specification of the tectum. If the eye did retain its original specificity, the projection would have to occupy only the corresponding part of the tectum. Neither configuration was found to be true (Fig. 2a-c). As a consequence, we suggest that the projection field is only determined by the number of ingrowing optic fibers and the direction they take during ingrowth. If the number of fibers is small compared to the size of the tectum they have to occupy, all the fibers gather near their entrance into the tectum, i.e. at the rostrolateral pole. The pattern of fiber configuration within the projection field of the tectum may be determined by interactions among the optic fibers in accordance with their specificity. This study was supported by a grant from the Deutsche Forschungsgemeinschaft to K.B.
plated tungsten microelectrodes, Med. Biol. Eng., 10 (1972) 662-672. 5 Nieuwkoop, P.D. and Faber, J., A Normal Table of Xenopus laevis (Daudin), North-Holland, Amsterdam, 1956. 6 Straznicky, C., Gaze, R.M. and Keating, M.J., The retinotectal projections from surgically rounded-up half-eyes in Xenopus, J. Embryol. Exp. Morphol., 58 (1980) 79-91. 7 Willshaw, D.J,, Fawcett, J.W. and Gaze, R.M., The visuotectal projections made by Xenopus 'pie slice' compound eyes, J. EmbryoL Exp. Morphol., 74 (1983)29-45.
Developmental Brain Research, 29 (1986) 141-143 Elsevier
BRD 60161
Short Communications The retinotectal projection of quarter eyes in Xenopus laevis NORBERT DEGEN and KURT BRANDLE Arbeitsgruppe Neurophysiologie, Fachbereich Biologie der Johann Wolfgang Goethe-Universiti~t Frankfurt am Main, Frankfurt am Main (F. R. G.)
(Accepted May 6th, 1986) Key words: retinotectal projection - - eye fragment --Xenopus
Following extirpation of three quarters of the eye anlage of Xenopus laevis in stage 33/34, the remaining quadrants developed into small eyes of normal shape. Irrespective of the different portions of the eye anlage the quarter eyes originated from, the retinotectal projection was alwaysconfined to the rostrolateral part of the tectum, forming an orderly map, and its dimensions corresponded to the size of the contralaterai eye. Therefore, it was suggested that during development both size and location of the projection are independent of the existence of tectal markers. Instead, they are determined by the number of ingrowing optic fibers and the direction of their growth from the rostral origin.
Recent investigations of surgically reassembled compound eyes and eye fragments, such as half eyes, indicated that each part of the retina in Xenopus still retains its original specificity 1,3,6,7. Nevertheless, the retinotectal projections were not restricted to the appropriate portion of the tectum, but covered the whole tectal surface. In all experiments compound eyes and half eye fragments invariably developed into eyes of normal shape and size. Therefore the observation may be explained in two ways: (1) initially there are no markers in the tectum; the optic fibers grow to occupy the entire surface available on the tectum; (2) markers do exist, interconnection is correct when the first fibers grow in but later projection is expanded due to the mass of ingrowing fibers. In order to determine which of the two possible explanations is valid we reduced the size of the retina fragments to such a degree that development of normal size eyes was no longer possible. Which of the two above hypotheses would hold true? In stage 33/34 (ref. 5) we removed three quarters of the eye anlage. The remaining quadrants originated from the nasoventral (NV), ventral (V), or
temporo-ventral (TV) retina (see Fig. 2, left). Within a few days, ca. 40% of the eye fragments rounded up and formed eyes of normal shape. The remaining 60% developed into extremely small globular fragments without lenses and were consequently excluded from further study. The so-called quarter eyes never achieved the size of eyes encountered in untreated animals. They never extended 2 5 % - 7 5 % of a normal eye. Neither did they show any sign of accelerated or delayed growth if compared to growth of normal eyes. The optic input to the contralateral rectum was studied in 41 tadpoles (stage 58-62) 5 and 5 metamorphosed specimens of Xenopus. The animals were anaesthetized by MS 222 (Sandoz) and placed in the center of a perimeter, the operated eye facing the fixation point in the perimeter. Electrophysiological recording was done with glass-coated tungsten electrodes 4. Multiunit spike responses to spot-light stimuli were obtained from 16 animals with one-quarter TV-eyes, from 14 animals with one quarter Veyes, and from 16 animals with one-quarter NVeyes.
Correspondence: Arbeitsgruppe Neurophysiologie, Fachbereich Biologie der Johann Wolfgang Goethe Universit~it Frankfurt am Main, Siesmayerstrasse80, 6000 Frankfurt am Main 11, F.R.G.
0165-3806/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)
142
Fig. t. Wholemounts of mesencephalic region (indicated by dotted lines). Optic projections marked by cobalt filling. Top view, stage 61. a: normal projection, b: projection of NV-eye c: projection of TV-eye.
With 9 animals responses could only be o b t a i n e d by stimulation with intermittently changing intensity of light. But in all cases it was, nevertheless, possible to d e t e r m i n e both dimensions and location of the projection field in the tectum. For 37 specimens we were able to m a p the response to stimulation of small areas in the visual field. As a rule, the p r o j e c t i o n field r e t a i n e d its retinotopical organization, even though partly e n m e s h e d or double projections could mostly be observed. This was also true for projections after rotation of the eye fragments by 90 ° or 180 ° . In all such cases the m a p was r o t a t e d at the same angles. In one respect, however, the responses differed considerably from those o b t a i n e d in half eyes and c o m p o u n d eyes. The projection did not cover m o r e than one third to about one half of the tectal surface. T h e r e was a linear correlation b e t w e e n the dimensions of the projection and the size of the o p e r a t e d
143
left tectum
(TV) T ~ N b)
(V) T O N
(NV) T Q
N
types of eye fragments (right eye)
c|
Fig. 2. Scheme of the expected retinotectal projections of quarter eyes. T, temporal; N, nasal, a: presence of tectal markers, respecified retina, b: presence of tectal markers, retina retaining its original specification, c: absence of tectal markers. Only (c) is in accordance with experimental results.
eye. The reduced projection field was always confined to the rostrolateral portion of the tectum, despite the different origin of the fragments. In some cases these findings could be histologically supported by cobalt filling of the eye stalk after recording was finished (Fig. l a - c ) . Gaze et al. 2 have shown a similar reduction in projection during development of normal eyes in Xenopus. For the following reasons, however, it appears fairly unlikely that the reduced size of the projection field should have been caused by developmental delay. (1) In recordings from 5 animals up to two years after metamorphosis, we never found the projection field extended into the caudo-medial portion of the tectum. (2) Analysis of the size increase of the eye
1 Feldman, J.D. and Gaze, R.M., The development of halfeyes in Xenopus tadpoles, J. Comp. Neurol., 162 (1975) 13-22. 2 Gaze, R.M., Keating, M.J. and Chung, S.H., The evolution of the retinotectal map during development in Xenopus, Proc. R. Soc. London Set. B, 185 (1974) 301-303. 3 Gaze, R,M. and Straznicky, C., 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, J. Embryol. Exp. Morphol., 60 (1980) 125-140. 4 Merril, E.G. and Ainsworth, A., Glass-coated platinum-
anlage between stage 33/34 and metamorphosis yielded sigmoidal growth curves. Comparison of normal eyes, half eyes and quarter eyes did not reveal any shift during the period of growth which would have provided some indication of developmental delay. Similarly, detailed histological study of the developing retina failed to show any time shift with respect to differentiation. (3) At the time of recording no size differences with regard to the receptive fields of both normal and operated eyes were noticeable. If development were delayed, larger receptive fields and less retinotopic organization would have be expected. Our findings, then, do not support the theoretical possibility that location and size of the tectal projection field are determined by markers present in the tectum during first ingrowth of optic fibers. Even if markers were present and the eye fragment did respecify to a full set of positional values, the projection would, nevertheless, have to spread over the whole tectal surface in accordance with the complementary specification of the tectum. If the eye did retain its original specificity, the projection would have to occupy only the corresponding part of the tectum. Neither configuration was found to be true (Fig. 2a-c). As a consequence, we suggest that the projection field is only determined by the number of ingrowing optic fibers and the direction they take during ingrowth. If the number of fibers is small compared to the size of the tectum they have to occupy, all the fibers gather near their entrance into the tectum, i.e. at the rostrolateral pole. The pattern of fiber configuration within the projection field of the tectum may be determined by interactions among the optic fibers in accordance with their specificity. This study was supported by a grant from the Deutsche Forschungsgemeinschaft to K.B.
plated tungsten microelectrodes, Med. Biol. Eng., 10 (1972) 662-672. 5 Nieuwkoop, P.D. and Faber, J., A Normal Table of Xenopus laevis (Daudin), North-Holland, Amsterdam, 1956. 6 Straznicky, C., Gaze, R.M. and Keating, M.J., The retinotectal projections from surgically rounded-up half-eyes in Xenopus, J. Embryol. Exp. Morphol., 58 (1980) 79-91. 7 Willshaw, D.J,, Fawcett, J.W. and Gaze, R.M., The visuotectal projections made by Xenopus 'pie slice' compound eyes, J. EmbryoL Exp. Morphol., 74 (1983)29-45.