/ . Embryol. exp. Morph. Vol. 61, pp. 259-276, 1981
Printed in Great Britain © Company of Biologists Limited 1981
259
Spreading of hemiretinal projections
in the ipsilateral tectum following unilateral
enucleation: a study of optic nerve
regeneration in Xenopus with one
compound eye
By CHARLES STRAZNICKY1 AND DAVID TAY
From the Centre for Neuroscience and Department of Human Morphology,
School of Medicine, The Flinders University of South Australia
SUMMARY
Right compound eyes were formed in Xenopus embryos at stages 32-33 by the fusion
of two nasal (NN), two ventral (VV) or two temporal (TT) halves. Shortly after metamorphosis the optic nerve from the compound eye was sectioned and the left intact eye
removed. The retinotectal projections from the compound eye to the contralateral and
ipsilateral tecta were studied by [3H]proline autoradiography and electrophysiological
mapping between 6 weeks and 5 months after the postmetamorphic surgery. The results
showed that NN and VV eyes projected to the entire extent of both tecta. In contrast, optic
fibre projection from TT eyes, although more extensive than the normal temporal hemiretinal
projection, failed to cover the caudomedial portion of the tecta. The visuotectal projections
in all three combinations corresponded to typical reduplicated maps to be expected from
such compound eyes, where each of the hemiretinae projected across the contralateral and
ipsilateral tecta in an overlapping fashion. The rapid expansion of the hemiretinal projections
of the compound eyes in the ipsilateral tectum following the removal of the resident optic
fibre projection suggests that tectal markers may be carried and deployed by the incoming
optic fibres themselves.
INTRODUCTION
In Xenopus the entire retina projects topographically to the whole extent
of the contralateral tectum (Gaze, 1958). Considerable work has been devoted
to the understanding of how the orderly connexions between the axonal
processes of the retinal ganglion cells and tectal neurons are formed during
development and are reformed following optic nerve regeneration in adult
animals. The most comprehensive hypothesis, advanced by Sperry (1951, 1963)
suggests that the establishment of the topographic retinotectal connexions is
based on selective cytochemical affinities between the axons of retinal ganglion
1
Author's address: School of Medicine, The Flinders University of South Australia,
Bedford Park, S.A. 5042, Australia.
260
C. STRAZNICKY AND D. TAY
B
Left eye
Left tectum
Right eye
Right tectum
Fig. 1. Drawings of the pathways and terminations of regenerating optic fibres from
compound eyes to the contralateral and ipsilateral tecta following 3HP injection
into the compound eye. Note that in contrast to the complete projections to the
contralateral tectum only about half of the ipsilateral tectum is innervated by optic
fibres from a TT (A), NN (B) and VV (C) eye.
cells and the corresponding tectal neurons; it is presumed that these positional
specificities are generated independently from one another during embryogenesis.
Experimental evidence indeed supports the notion that individual ganglion
cells differ from one another on the basis of their position in the retina and
that these positional differences are expressed with reference to the tectal
termination of their optic axons (Gaze, 1978). The polarization of the retina
in relation to the future termination of the optic fibres has been shown to
occur during early embryogenesis (Gaze, Feldman, Cook & Chung, 1979;
Sharma & Hollyfield, 1980). The time at which similar tectal polarization
occurs is not known. Experiments involving tectal rotation in metamorphic
frogs have demonstrated the existence of tectal positional markers serving as
targets for ingrowing optic axons (Levine & Jacobson, 1974). Since these
studies were carried out in juvenile animals, where the retinotectal connexions
are already formed, they failed to furnish information about the nature and
the mode of generation of tectal positional markers.
Recent experiments in goldfish involving optic nerve regeneration from
half retinae suggest that positional markers may be induced by the ingrowing
Optic nerve regeneration in Xenopus with one compound eye
261
optic fibres themselves, imprinting their position-related retinal label on the
tectal cells (Schmidt, 1978; Schmidt, Cicerone & Easter, 1978). Consequently,
an abnormal optic fibre input either during development or in regeneration
could alter the tectal markers by generating a set appropriate to the nature of
the incoming optic fibres.
The retinotectal projections from compound eyes (eyes made by the fusion
of two nasal (NN), two temporal (TT) or two ventral (VV) halves) are abnormal
in that each similar retinal half spreads across the whole extent of the tectum
instead of being restricted to the corresponding part (Gaze, Jacobson &
Szekely, 1963; Straznicky, Gaze & Keating, 1974). Recent observations have
demonstrated that the polarity of the retinal developmental programme is
kept unaltered in fused eye fragments (Straznicky & Gaze, 1980; Gaze &
Straznicky, 1980 a). Further evidence for the stability of the retinal positional
markers is derived from studying the patterns of optic fibre regeneration from
compound eyes. The optic fibre regeneration from such eyes to the contralateral
and ipsilateral tecta are strikingly different (Fig. 1). The compound-eye
projection to the ipsilateral tectum (also innervated by the normal eye) is
highly selective in that it is restricted to one half, corresponding to the type
of the compound eye (Tay & Straznicky, 1978; Craze & Straznicky, 1979,
19806). The projection from the same eye extends across the entire contralateral
tectum as it did before the nerve was cut. The difference in the optic fibre
distributions in the two tecta suggests that the compound eye possesses retinal
positional markers characteristic of the two (similar) halves. It is also apparent
from these studies that in the ipsilateral tectum the normal retinotopic ordering
of fibre projections from both eyes is maintained in contrast to the contralateral
tectum where it is not. It is thus conceivable that the topographic ordering of
the incoming fibres on the tectum ipsilateral to the compound eye may be the
result of their interaction with the resident fibres from the contralateral eye.
If the basis of selective regeneration from a compound eye to the ipsilateral
tectum is fibre-fibre recognition, one does not have to assume the existence
of tectal markers independent from the optic fibre projection.
The experiments reported here attempt to test the validity of such a mechanism
in animals with a right compound eye, where the left normal eye was removed
2 weeks after metamorphosis with the simultaneous sectioning of the optic
nerve from the compound eye. The resultant spreading of the hemiretinal
projections of the compound eye in the ipsilateral tectum supports the idea
that positional markers may be carried and deployed in the tectum by the
incoming optic fibres themselves.
262
C. STRAZNICKY AND D. TAY
METHODS
Laboratory bred Xenopus laevis were used in this study.
Surgery
Eye operations
The right eyes of stages -32 to -33 embryos (Nieuwkoop & Faber, 1956) were
operated on in full-strength Niu-Twitty solution to obtain double-nasal (NN),
double-temporal (TT) or double-ventral (VV) eyes. Embryos were anaesthetized
with 0-001 % solution of MS 222 (Tricaine Methanesulphonate, Sandoz) and
the temporal half of the eye anlage was removed by suction and substituted
by a left nasal half from another embryo to form an NN eye. Care was taken
that neither the host's nor the donor's eye fragments included parts from the
temporal half. Similarly TT and VV eyes were formed in other embryos.
Twenty-four hours after the microsurgery, animals were checked and the
unsuccessful cases as well as animals with grossly asymmetric compound eyes
(60 % or more for one half) were excluded from further experiments. The
NN, and TT and VV eye animals were kept separately and reared to
metamorphosis and beyond.
Optic nerve transection and enucleation
Two weeks after metamorphosis animals were anaesthetized by immersion
in a 0-1 % solution of MS 222. The optic nerve from the compound eye was
approached through the roof of the mouth and cut 0-5 mm rostral to the
chiasma. The proximal and distal stumps of the nerve were approximated to
facilitate regeneration. Following the optic nerve transection, the left normal
eye was enucleated. Autoradiographic and electrophysiological studies were
performed between 42 and 150 days after optic nerve section and enucleation.
Electrophysiology
Animals of 60 days or longer postoperative survival were anaesthetized by
intraperitoneal injection of 0-1 % aqueous solution of MS 222 and immobilized
with 0-05 mg tubocurarine (10 mg/ml, Wellcome) administered intramuscularly.
The tectum was exposed by opening the skull and the dorsal surface photographed at 50 x magnification. The animal was then set up with right eye
centred on the optic axis of the perimeter for visuotectal recording. The mapping
of visuotectal projections was similar to that previously described for adult
Xenopus (Straznicky, Gaze & Keating, 1971). Single or multiunit action
potentials were recorded from terminal arborizations of optic fibres at predetermined tectal positions, and the corresponding receptive fields in the visual
field were mapped. Visuotectal maps were based on about 30-40 different
tectal electrode positions in each aimal. After the visuotectal mapping the
brains of animals were processed for autoradiography.
Optic nerve regeneration in Xenopus with one compound eye
263
Histology
(i) Antoradiography
Twenty-four hours before recording and sacrifice 10/*Ci L-5-[3H]proline
(3HP) (Amersham, specific activity 21 Ci per m-mole) was injected into the
compound eye. The head of the animal was fixed in Bouins' solution, the
dissected brain embedded in paraffin, serially sectioned at 10 fim in the
transverse (coronal) plane and mounted on slides. The closely spaced, deparaffinized sections were coated with K2 emulsion (Ilford) exposed in light-tight
boxes at 4 °C for 14 days, developed in Kodak D19 developer and counterstained
with Harris's haematoxylin.
(ii) Analysis of autoradiograms
In selected animals, every tenth serial section of the brain was drawn using
a microscope camera-lucida attachment to record the size of the tecta and the
distribution of 3HP-labelled axons in the contralateral and ipsilateral tecta.
The extent of tectum covered with silver grains and the total tectal surface area
were measured by a Hewlett & Packard digitizer (987A) and the two measurements were correlated. The autoradiographic grain density was established
along the mediolateral extent of the optic-fibre-receiving layer of the tectum
by measuring the reflected incident light over an oblong area of 20 x 60 /im
(Straznicky, Gaze & Keating, 19796). By moving the oblong medio-laterally in
steps of 50 fim the grain density profile of the compound-eye projection was
established. About 25 measurements were made on each of the three subsequent
sections of the selected tectal area. Background activity was measured on the
same section over non-visual brain tissue.
RESULTS
The autoradiographic visualization of the VV eye projection has been
described previously (Straznicky, Gaze & Horder, 1979 c); however, no
anatomical description on NN and TT eye projections has been published,
therefore a short account of the autoradiographic optic fibre projections from
NN and TT eyes is first given here to serve as reference to retinotectal projections to be described in this paper.
(A) Retinotectal projections from NN and TT eyes {optic nerve intact)
Three animals of each combination were injected with 3HP 5 months after
metamorphosis. The autoradiographic silver grains filled the superficial layer
evenly across the rostrocaudal and mediolateral extents of the tectum in
NN eye animals (Fig. 2 A), as in the normal retinal projection. Unexpectedly, in none of the three TT eye animals did the silver grains cover the
whole tectal surface; a wedge-shaped area in the caudomedial tectum was
systematically devoid of silver grains (Fig. 2B). Graphical reconstructions and
264
C. STRAZNICKY AND D. TAY
v ^-m
<»•.
Fig. 2. Bright-field 3HP autoradiographs of contralateral retinotectal projections
from a NN (A) and a TT (B) eye 5 months after metamorphosis and graphical
reconstruction of the extent of the TT eye projection from serial sections (C). The
plane of section is transverse; right is to the left, dorsal at the top. Note in (B) that
the retinotectal projection from a TT eye does not extend to the midline. Stippled
area in (C) indicates caudomedial tectum which is not innervated. Arrow points
rostrally on the tectum. Scale 300 /*m applies to (A), (B) and (C).
Optic nerve regeneration in Xenopus with one compound eye
265
Table 1. Retinotectal and visuotectalprojections to the contralateral and ipsilateral
tecta in animals with right NN eye
VisuotectalI projections
A
Frog
Autorad iography
postmetamorphiir*is f — ^ — — — — ^ — — — —_—_^ — A
surgery
Contralateral tectum Ipsilateral tectum
NN1
NN2
NN3
42
72
72
Complete projection
Complete projection
Complete projection
NN4
80
Complete projection
Complete projection*
Complete projection*
Slight rostrolateral
projection deficit
Complete projection*
Complete
Complete
Complete
Complete
Complete
Complete
Complete
Complete
Complete
Complete
Complete
Complete
NN5
NN6
NN7
NN8
NN9
NN10
120
120
135
135
136
137
projection
projection
projection
projection
projection
projection
Contralateral
tectum
Ipsilateral
tectum
Not tested
Not tested
NN
NN*f
NN
Few points
only
NN
NN
projection
NN
NN
projection*
NN
NN
projection
NN
NN
projection
projection Unsuccessful recording
NN
NN
projection
* Lower grain density in the rostrolateral part of the ipsilateral tectum.
t Deficient: visual projection to the; ipsilateral tectum.
planimetric measurements (Fig. 2C) indicated a consistent 10-13 % deficit
of TT-eye projection in the caudomedial tectum in animals without optic
nerve section (Table 4).
(B) Retinotectal projections from compound eyes following optic-nerve section
and left eye enucleation
NN eye projections
Ten animals were included in this group (Table 1). 3HP autoradiography
demonstrated that both the contralateral and ipsilateral tecta were fully covered
with silver grains, suggestive of a continuous retinotectal projection from the
NN eye across the whole extent of both tecta. In five animals slightly lower
grain density was measured in the rostral than in the caudal tectum (Fig. 3).
Since 3HP autoradiography does not distinguish between fibres of passage
and fibre termination it could not be determined whether or not the measured
lower grain density corresponded to sparser innervation in the rostrolateral
tectum. Silver grains were evenly distributed over the medial and lateral
branches of the optic tract in the rostral pole of the tectum, indicating that
regenerated optic fibres entered into the tectum via both these branches. Seven
animals were recorded electrophysiologically 72-137 days after optic nerve
section. The contralateral and ipsilateral visuotectal projections were typical
NN maps in five animals (Fig. 4). The centre of the visual field projected to
the rostral part, and the nasal and temporal poles to the caudal part of both
2660
C. STRAZNICK Y AND D. TAY
r 400
1
1
•300
1
1
1—'—i
•5 2 0 0
,
1
• 100
•—
Right
Left
Fig. 3. Grain density counts over the optic-fibre-receiving layer of the rostral and
caudal (stippled) parts of the right and left tecta in animal NN4: background
activity is indicated by a broken line. The units of the ordinate represent number of
grains over 1200/^m2 area. Note lower grain counts rostrally in both tecta. Each
segment of the histogram represents 140 fim. M = medial, L = lateral.
tecta. In one animal an apparent projection deficit from the central visual field
to the rostral part of the ipsilateral tectum was observed (Fig. 5) despite the
presence of autoradiographic silver grains in this sector. In the seventh animal
mapped in this group, only few points were recorded from the ipsilateral
tectum, whilst the contralateral tectum had a typical NN map.
VV eye projections
In all ten animals studied in this group (Table 2) the anatomical retinotectal
projection spread across the whole rostrocaudal and mediolateral extents of
the contralateral and ipsilateral tecta. In a few animals, however, the silver-grain
density over the tectum decreased laterally, confirming our earlier report on
VV eye projections during development and in regeneration (Straznicky et al.
1979a). Silver grains were more or less evenly distributed over the medial and
lateral branches of the optic tract in eight animals. In the remaining two
animals optic fibres entered the tecta selectively via the medial branch. These
observations indicate that during regeneration optic fibres of ventral retinal
origin are not able to choose the appropriate medial branch for entry into the
tectum without the presence of pre-existing fibre tracts. These findings provide
indirect support for our previous suggestion that selective regeneration may
be based on fibre-fibre recognition, recognizing the course of similar fibres in
the tract and following them enroute to the tectum (Straznicky et al. 1979 a).
In all four animals recorded, both the contralateral and ipsilateral visuotectal
projections were typical VV maps (Fig. 6).
TT eye projections
In contrast to the previous two groups 14 out of 15 animals with TT eyes
showed a caudomedial deficiency in the autoradiographically demonstrable
contralateral and ipsilateral retinotectal projections (Table 3).
Optic nerve regeneration in Xenopus with one compound eye
267
Fig. 4. Visual-field projections from the right NN eye to the contralateral and
ipsilateral tecta in animal NN6. For this and all subsequent maps the conventions
are the following. Right ipsilateral tectum is on the left and the left contralateral
tectum is on the right. On the tectal diagram the small filled arrow points rostrally.
The visual-field charts are centred on the optic axis of the eye extending out 100°
radially. D, dorsal; V, ventral; T, temporal and N, nasal. The dorsal surface of the
tecta shows rows of electrode positions, the corresponding rows of stimulus positions
in the visualfieldare indicated with the same number. Open circles indicate electrode
positions from where no visual responses were obtained. The large open arrows
indicate the orientation of the nasotemporal axis of the visual field in relation to
the rostrocaudal tectal axis. Note that each of the halves of the visual field projects
to the entire contralateral and ipsilateral tectum.
The autoradiographic studies in these animals indicated that the caudomedial
margins of both tecta were not innervated. The distribution of the silver grains
in one representative animal, is shown in a p{ioto-montage (Fig. 7). The rostral
parts of both tecta are fully covered with silver grains, whereas, more caudally
the silver grains are restricted to the lateral two thirds of the tecta. Although
no strict boundary can be seen, a steep decrease of silver-grain density was
established by grain density measurements (Fig. 8).
Reconstructions on the optic fibre projections from TT eyes revealed
a consistent projection deficit varying between 7-32 % of the whole tectal
268
C. S T R A Z N I C K Y AND D. TAY
Fig. 5. Visual-field projections in animal NN3. Note that no visual responses were
obtained from the rostral part of the ipsilateral tectum.
Table 2. Retinotectal and visuotectal projections to the contralateral and ipsilateral
tecta in animals with right VV eye
Visuotectal projections
Frog
Days from
postAutoradiography
metamorphic
A
surgery
Contralateral tectum Ipsilateral tectum
VV1
VV2
VV3
VV4
VV5
VV6
42
56
56
84
84
92
VV7
VV8
VV9
VV10
94
112
120
140
Complete projection
Complete projection
Complete projection*
Complete projection
Complete projection
Complete projection*
Complete
Complete
Complete
Complete
Complete
Complete
A
I
\
Contralateral
tectum
Ipsilateral
tectum
projection
Not tested
projection*
Not tested
projection*
Not tested
projection
VV
VV
projection* Unsuccessful recording
projection*
VV
VV
Essentially normal
Complete projection Complete projection
projection
Complete projection Complete projection
VV
VV
Complete projection Complete projection* Unsuccessful recording
Complete projection Complete projection
VV
VV
* Decreasing grain density Laterally.
Optic nerve regeneration in Xenopus with one compound eye
269
Fig. 6. Visual-field projections in animal VV8. The large open arrows indicate the
orientation of the dorsoventral axis of the visual field in relation to the mediolateral
tectal axis.
area (Table 4). Although the number of animals is insufficient for more detailed
analysis, the data show a tendency for the projection deficit to reduce with
time after optic nerve section.
In 11 out of 12 animals recorded, the contralateral and ipsilateral visuotectal
maps correspond to typical TT eye projections, confirming that we had dealt
with TT eye animals. In five animals with TT maps the caudomedial tecta
(Fig. 9) failed to yield evoked potentials, substantiating the autoradiographic
observations that this part of the tecta had not been innervated. In the other
six animals the caudomedial deficiency of the TT eye projection was less
extensive and consequently it was not demonstrable with a visuotectal recording.
It is worth noting that animal TT3 had a normal visuotectal projection with
accompanying complete autoradiographic retinotectal projections.
DISCUSSION
The aim of the present study was to determine whether regenerated optic
fibres from the hemiretinae of a compound eye expand their assigned projection
270
C. STRAZNICKY AND D. TAY
Table 3. Retinotectal and visuotectal projections to the contralateral and ipsilateral
tecta in animals with right TT eye
Visuotecta 1 projections
Frog
Days from
postmetaAutoradiography
morphic f
surgery Contralateral tectum
Ipsilateral tectum
TT1
TT2
TT3
42
42
56
Caudomedial deficiency
Caudomedial deficiency
Complete projection
TT4
TT5
TT6
56
72
72
TT7
TT8
TT9
A
f
Contralateral
tectum
Ipsilateral
tectum
Not tested
Not tested
Caudomedial deficiency
Caudomedial deficiency
Caudomedial deficiency
Caudomedial deficiency
Caudomedial deficiency
Slight caudomedial
deficiency
Caudomedial deficiency
Caudomedial deficiency
Caudomedial deficiency
TT
TT
TT
84
Caudomedial deficiency
Caudomedial deficiency
TT
TT10
90
110
138
TT11
TT12
138
140
TT13
TT14
140
149
TT
TT
Caudomedial deficiency
TT
Caudomedial deficiency
Not tested
TT
TT
Slight caudomedial
deficiency
TT
TT
Caudomedial deficiency
Slight caudomedial
Unsuccessful recording
deficiency
TT
TT
Caudomedial deficiency
TT
TT
Caudomedial deficiency
TT15
150
Caudomedial deficiency
Caudomedial deficiency
Slight caudomedial
deficiency
Caudomedial deficiency
Slight caudomedial
deficiency
Caudomedial deficiency
Almost complete
projection
Almost complete
projection
Slight caudomedial
deficiency
Normal
TT
TT
TT
Host's
Temporal
half
projection
Few points
only
TT
in the ipsilateral tectum, following the elimination of resident optic fibres of
the normal eye. The compound-eye projection patterns were assessed anatomically and electrophysiologically and the two results were comparable. The main
and consistent finding of this study was that each nasal, ventral and temporal
half of the compound retina expanded its normal termination over the ipsilateral
tectum.
Retinotectal-size-disparity experiments in adult fish and frogs indicate that
the optic fibre projection is capable of reorganization in the form of compression
of the entire retinal projection to a half tectum (Gaze & Sharma, 1970; Udin,
1977) and the expansion of the hemiretinal projection across the whole tectum
(Straznicky, Tay & Lunam, 1978; Schmidt et al. 1978). Creating size disparity
in the retinotectal relationship during early embryogenesis in animals with
one compound eye have been reported to cause similar modification in visual
projections (Gaze et al. 1963; Straznicky et al. 1974). The projection from each
retinal half of a compound eye has been found to spread across the entire
Optic nerve regeneration in Xenopus with one compound eye
Fig. 7. Bright-field photographs of autoradiographs of animal TT8. Rostral is on the
bottom, caudal is at the top. Note the deficient innervation of the caudomedial
tectum. Inset shows the extent of the TT-eye projection as seen from above and
reconstructed from serial sections. Arrows point rostrally on the tectum. Scale
500 /mi.
271
C. STRAZNICKY AND D. TAY
272
_j
1
1
300
200
100
Fig. 8. Silver-grain-density counts from the midtectal region of animal TT8 (A).
Each segment on the histogram represents 140/*m. Broken line gives background
activity over non-visual brain tissue. Arrows on the bright-field autoradiography
of the corresponding tectal section marks the sites of grain density measurements (B).
Scale 300 /tm.
tectum when investigated in adult animals. Since the positional values of
retinal ganglion cells in a perturbed eye remain stable (Straznicky & Gaze, 1980;
Gaze & Straznicky, 1980a, b) the altered retinotectal topographic relationship
must therefore involve changes at the tectal level.
In order to account for the results of developmental and regeneration studies
on retinotectal connexions, Willshaw & von der Malsburg (1979) have proposed
an elegant model for a mechanism of tectal marker induction. According to
this model the tectum is initially devoid of markers. As optic fibres arrive into
the tectum they interact with each other and with the tectal cells resulting in
the generation of tectal positional markers. The model presumes the decay
of the tectal markers after the removal of the resident optic fibre projection.
The expanded representation of the hemiretinae of the compound eyes in
the ipsilateral tectum in the present study stands in contrast to our previous
observations where these projections were restricted, in the presence of optic
fibres from the normal eye, to the corresponding portion of the ipsilateral
tectum (Tay & Straznicky, 1978; Gaze & Straznicky, 1979, 19806). These
observations indicate that the resident optic fibre projection from the normal
Optic nerve regeneration in Xenopus with one compound eye
273
Table 4. Retinotectal projection and tectal surface areas given in mm2 in animals
with right TT eye
Contralateral tectum
Ipsilateral tectum
> r
Tectal
surface
area
Area of
TT-eye
projection
Projection
deficit
(%)
Tectal
surface
area
Area of
TT-eye
projection
Projection
deficit
(%)
TT10
TT11
TT12
TT13
TT14
TT15
0-850
0-639
0-744
1150
1-154
1-257
1017
0-915
0-806
1-293
1178
0-850
0-552
0-621
0-945
0-958
0-990
0-863
0-897
0-716
1171
1094
13-6
16-5
17-8
16-9
21-2
151
11-3
10-9
9-2
7-4
0-682
0-691
0-782
1053
1041
1183
0-967
0-889
0-683
1-052
1011
0-618
0-469
0-642
0-791
0-637
0-900
0-704
0-799
0-561
0-759
0-856
10-8
320
18-5
24-8
38-8
23-9
27-1
9-8
17-7
27-0
14-9
TTNlf
TTN2f
TTN3f
1179
1023
1095
Frog
TT3*
TT4
TT5
TT6
TT7
•
t
1040
13-5
0-937
10-2
0-982
12-7
Projection from right operated eye was normal.
Right optic nerve was not cut.
eye maintains the whole range of tectal markers to which the hemiretinal
projections from compound eyes align themselves. Since the ipsilateral tectum
had been innervated by the intact eye up to two weeks after metamorphosis
it had to carry the normal complement of markers at the time of the arrival
of optic fibres from the compound eye. The first check point 6 weeks after
operation however, showed an expanded hemiretinal projection across the
entire ipsilateral tectum from NN and VV eyes and partial expansion from
TT eyes. The very early regeneration pattern was not studied in this series of
experiments.
Comparable experiments in fish following partial retinal ablations have
shown that the initial regeneration from a residual hemiretina was restricted to
the appropriate tectal half, followed later by a subsequent expansion of this
projection across the entire tectum (Schmidt, 1978). In contrast, when the
optic fibres from a hemiretina regenerated into a tectum which had been
denervated for many months, the projection expanded initially across the
entire tectum. These results indicate that tectal markers, serving as targets for
regenerated optic fibres disappear during long-term denervation.
Signs of rostral, lateral and caudal expansion of the projections are noticeable
in some animals in the present study as judged by the lower grain densities in
the rostrolateral and lateral portions of the ipsilateral tectum in animals with
274
C. S T R A Z N I C K Y AND D. TAY
Fig. 9. Visual-field projection in animal TT11. Broken line indicates the approximate
extent of autoradiographic silver-grain deposition in the tecta.
NN or VV eyes and by the consistent caudomedial projection deficit in TT eye
animals. From these observations we may infer that the early regeneration may
have also been selective, as with the goldfish results. It is quite apparent however,
that the decay of tectal positional markers in Xenopus is more rapid than in
goldfish, allowing the spreading of the hemiretinal projection to occur across
the tectum within a shorter time. The earliest visuotectal mappings were carried
out 56-84 days after optic-nerve section. All the recordings showed the
restoration of characteristic NN, TT and VV projections in both tecta. Thus
not only had the extent of the hemiretinal projections in the ipsilateral tectum
changed, but a new retinotopic ordering was also generated corresponding to
the nature of the compound eye. The remarkably rapid change (within 6-8
weeks) in the set of tectal markers induced by the hemiretinal projection makes
it unlikely that positional markers, independent from the previous innervation,
existed.
The interpretation of the present results on the deficient TT eye projections
with intact as well as with regenerated optic nerve is difficult. None of the
previous studies on TT eye projections in adult frogs reported a discernible
Optic nerve regeneration in Xenopus with one compound eye
275
projection deficit by visuotectal mapping. We think that the phenomenon is
transient, present during larval and young postmetamorphic life. Temporal
fibres prefer rostrolateral tectum both during development and in regeneration
(Straznicky et al. 1979; Gaze & Straznicky, 1980b). Consequently the expansion
of the temporal hemiretinal projection is towards the caudomedial tectum.
This tectal sector is furthest away from the incoming fibres and hence the last
to be innervated.
In view of the goldfish results (Schmidt, 1978) and of the present observations,
it is likely that detailed tectal markers are induced by the ingrowing optic
fibres, yet rostrocaudal and mediolateral polarity cues are also needed for the
orientation of the retinotectal map. Previous experiments with tecta which
were completely deprived of optic fibre input throughout development have
shown that regenerating optic fibres form an orderly but rotated projection on
a rotated tectum (Straznicky, 1978). The presence of at least minimum tectal
polarity cues prior to initial innervation is indicated also by observations on
the development of the compound-eye projection (Straznicky et al. \979b).
The results show that the initial outgrowth of optic fibres from TT, NN and VV
eyes is to the corresponding half of the growing tectum. It appears conceivable
that tectal polarity cues determine the orientation and fibre-fibre interactions
the extent and the orderliness of the retinotectal projection. The possible
underlying mechanisms, instrumental in retinotectal map formation have been
discussed in detail elsewhere (Straznicky, Gaze & Keating, 1981).
Ms Teresa Clark's skilled assistance in preparing the autoradiography, Ms Jenny
Hiscock's assistance in the planimetric measurements and Miss Laima Visockis' secretarial
assistance are gratefully acknowledged. The research was supported by a grant from the
Australian Research Grants Committee and from the Flinders University Research Budget.
REFERENCES
R. M. (1958). The representation of the retina on the optic lobe of the frog. Q.J. exp.
Physiol. Cogn. Med. Sci 43, 209-214.
GAZE, R. M. (1978). The problem of specificity in the formation of nerve connections.
In Specificity of Embryological Interactions (ed. D. R. Garrod, pp. 53-93. London:
Chapman and Hall.
GAZE, R. M. & SHARMA, S. C. (1970). Axial differences in the reinnervation of goldfish
optic tectum by regenerating optic nerve fibres. Expl Brain Res. 10, 171-181.
GAZE,R. M. & STRAZNICKY, C. (1979). Selective regeneration ofopticfibers fromacompound
eye to the ipsilateral tectum in Xenopus. J. Physiol. 293, 57-58.
GAZE, R. M. & STRAZNICKY, C. (1980a). Stable programming for map orientation in
disarranged embryonic eyes in Xenopus. J. Embryol. exp. Morph. 55, 143-165.
GAZE, R. M. and STRAZNICKY, C. (1980/)). 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. Morph. 60, 125-140.
GAZE, R. M., JACOBSON, M. and SZEKELY, G. (1963). The retinotectal projection in Xenopus
with compound eyes. /. Physiol. 165, 484-489.
GAZE, R. M., FELDMAN, J. D., COOK, J. & CHUNG, S. H. (1979). The orientation of the
visuotectal map in Xenopus: developmental aspects. /. Embryol. exp. Morph. 53, 39-66.
GAZE,
276
C. STRAZNICKY AND D. TAY
R. L. & JACOBSON, M. (1974). Development of optic nerve fiber projection is
determined by positional markers in the frog's tectum. Expl Neurol. 43, 527-538.
NIEUWKOOP, P. D. & FABER, J. (1956). Normal Table of Xenopus laevis. Amsterdam:
(Daudin) North Holland Publ. Co.
SCHMIDT, J. T. (1978). Retinal fibers alter tectal positional markers during the expansion
of the half retinal projection in goldfish. /. comp. Neurol. 177, 279-300.
SCHMIDT, J. T., CICERONE, C. M. & EASTER, S. S. (1978). Expansion of the half retinal
projection to the tectum in goldfish. An electrophysiological and anatomical study.
J. comp. Neurol. 177, 257-278.
SHARMA, S. C. & HOLLYFIELD, J. G. (1980). Specification of retinotectal connexions during
development of the toad Xenopus laevis. J. Embryol. exp. Morph. 55, 77-92.
SPERRY, R. W. (1951). Mechanism of neural maturation. In Handbook of Experimental
Psychology (ed. S. S. Stevens), pp. 236-280. New York: Wiley.
SPERRY, R. W. (1963). Chemoaffinity in the orderly growth of nerve fiber patterns and
connections. Proc. natn. Acad. Sci., U.S.A. 50, 703-710.
STRAZNICKY, K. (1978). The acquisition of tectal positional specification in Xenopus.
Neurosci. Letts. 9, 177-184.
STRAZNICKY, C. & GAZE, R. M. (1980). Stable programming for map orientation in fused
eye fragments in Xenopus. J. Embryol. exp. Morph. 55, 123-142.
STRAZNICKY, K., GAZE, R. M. & KEATING, M. J. (1971). The retinotectal projections after
uncrossing the optic chiasma in Xenopus with one compound eye. J. Embryol. exp. Morph.
26, 523-542.
STRAZNICKY, K., GAZE, R. M. & KEATING, M. J. (1974). The retinotectal projection from
a double-ventral compound eye in Xenopus laevis. J. Embryol. exp. Morph. 31, 123-137.
STRAZNICKY, K., TAY, D. & LUNAM, C. (1978). Changes in retinotectal projection in adult
Xenopus following partial retinal ablation. Neurosci. Letts. 8, 105-111.
STRAZNICKY, C, GAZE, R. M. & HORDER, T. J. (1979a). Selection of appropriate medial
branch of the optic tract by fibers of ventral retinal origin during development and in
regeneration. An autoradiographic study in Xenopus. J. Embryol. exp. Morph. 50,253-267.
STRAZNICKY, C, GAZE, R. M. & KEATING, M. J. (19796). Selective optic fibre projection
from compound eyes to the tectum during development in Xenopus. Neurosci. Abstr. 5,
p. 181.
STRAZNICKY, C, GAZE, R. M. & KEATING, M. J. (1981). The development of the retinotectal
projections from compound eyes in Xenopus. J. Embryol. exp. Morph. 62 (in the press).
TAY, D. & STRAZNICKY, K. (1978). The pattern of early optic nerve regeneration from
compound eyes to the tectum in Xenopus. Proc. Austr. Physiol. Pharmacol. Soc. 9, p. 65.
UDIN, S. B. (1977). Rearrangements of the retinotectal projections in Rana Pipiens after
unilateral caudal half-tectum ablation. /. comp. Neurol. 173, 561-582.
WILLSHAW, D. J. & VON DER MALSBURG, C. (1979). A marker induction mechanism for
the establishment of ordered neural mappings: its application to the retinotectal problem.
Phil. Trans. Roy. Soc. Ser. B 287, 203-243.
LEVINE,
(Received 10 June 1980, revised 30 July 1980)