/ . Embryo!, exp. Morph. Vol. 55, pp. 143-165, 1980
Printed in Great Britain © Company of Biologists Limited 1980
143
Stable programming for map orientation in
disarranged embryonic eyes in Xenopus
By R. M. GAZE 1 AND C. STRAZNICKY 2
From the National Institute for Medical Research, London
SUMMARY
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 four 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 reprogramnting of one eye fragment by another.
INTRODUCTION
The retinotectal fibre projection in Xenopus has three main properties. (1)
it is continuously ordered, in that neighbourhood relationships between retinal
ganglion cells are preserved in the distribution of their fibre terminals on the
tectum; (2) it is oriented in a particular fashion, such that nasal retina projects
caudally, temporal retina rostrally, dorsal retina laterally and ventral retina
medially on the tectum; and (3) it extends over the whole of the tectal surface
in the adult.
The retinotectal projection can thus be characterized in terms of its order, its
orientation and its extension on the tectum. The orientation of the retinotectal
map which will eventually develop is determined by stage 21/22 (Nieuwkoop
& Faber, 1956) of embryonic life (Gaze, Feldman, Cooke & Chung, 1979).
The existence of an ordered and oriented map of the retina on the tectum implies
the existence of relative positional differences between retinal ganglion cells
such that ganglion cells from one part of the retina are different from those of
1
Author's address: National Institute for Medical Research, London NW7 1AA. U.K.
Wellcome Research Fellow, Author's address: School of Medicine, The Flinders University of South Australia, Bedford Park, South Australia 5042.
2
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R. M. GAZE AND C. STRAZNICKY
other parts, in a way that is causally related to the formation of the map. It is
commonly thought that these position-dependent cellular characteristics are in
the form of cytochemical specificities, or cell markers (Sperry, 1943, 1944,
1945, 1951).
The programme leading to the development of the map-related cellular
specificities in the retina is remarkably stable in the face of various forms of
operative interference to the eye anlage. Thus conventional 'compound' eyes
in Xenopus, formed by the surgical apposition of two nasal halves (NN), two
temporal halves (TT) or two ventral halves (VV) of the eye at stage 32, later
give visuotectal maps the orientation of which reflects accurately the orientations
of the individual retinal fragments making up the eye (Gaze, Jacobson & Szekely,
1963; Straznicky, Gaze & Keating, 1974). Each half of such a compound eye
develops autonomously as far as concerns the orientation of its retinotectal
projections.
In the previous paper of this series (Straznicky & Gaze, 1979) we showed that
the nature of the compound eye projection is not due to the initiation by the
operation of mirror-image pattern reduplication in the host retinal fragment.
When compound eyes are made with the grafted fragment rotated 180° with
respect to the host fragment (by using two similar fragments of the same handedness) the map that results reflects the nature of the operation in that the projection from the grafted fragment is rotated by 180° in relation to that from the
host fragment.
Conventional NN, TT and VV eyes, as well as the varieties described in the
previous paper (Straznicky & Gaze, 1979), are all incomplete in the sense that
they have a restricted range of positional values. Each such eye is composed of
two similar half-eyes (of different handedness) put together and thus lacks half
the positional values of a normal eye (Fig. 1).
In a normal animal each half of the retina projects over its appropriate half
of the tectum. With conventional compound eyes of the types mentioned above,
each half of the retina projects over a greater than normal area of the tectum;
indeed, with NN and TT eyes, the projection from each retinal half extends over
the entire tectum. The extent of the tectal distribution of the compound eye
projections will be discussed in detail elsewhere (Gaze & Straznicky, in preparation; Straznicky, Gaze and Keating, in preparation); in the present
paper we are mainly concerned with the orientation of the visuotectal map.
This paper describes experiments in which further varieties of eye recombination were formed, each with a complete range of positional values. This
was done by replacing the nasal, temporal, dorsal or ventral half of the right
eye anlage with the corresponding half of a left eye anlage (Fig. 2).
The result was to produce an eye which was internally disarranged in that
half of it was reversed along one axis. The visuotectal maps from such eyes
provide further evidence for the stability of the programme leading to the acquisition by the eye of its map-related cellular specificities. The maps obtained
Map orientation in disarranged Xenopus eyes
145
u
Fig. 1. Clockface diagrams illustrating circumferential positional values, arbitrarily
assigned, for a normal right eye, NN, VV and TT conventional compound eyes and
for the four varieties of compound eye discussed in this paper.
had orientations which reflected faithfully the anatomy of the disarranged retina
and, in keeping with the possession by the whole eye of a complete set of pesitional values, each half of the retina tended to restrict its projection to the appropriate part of the tectum.
Much confusion surrounds the events occurring in early eye development,
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R. M. GAZE AND C. STRAZNICKY
.
VD
Fig. 2. Types of eye operation performed. In each case Ihe hatched half-eye had
been taken from a left eye and is thus inverted dorsoventrally (NRTL and TRNL)
or reversed nasotemporally (DBVL and VRDL).
when the map related properties of the anlage are being determined. Thus it has
been reported:
(1) That the Xenopus eye becomes polarized along the nasotemporal axis at
stage 28 and along the dorsoventral axis a few hours later (Jacobson, 1967).
Map orientation in disarranged Xenopus eyes
147
(2) That the eye anlage is already polarized before stage 28 but that the
system is not yet finally determined (Hunt & Jacobson, 1973 a).
(3) That even though eye polarization is determined beforehand, one half of
an eye may repolarize another half when combinations such as N R T L are made
at stage 32 (Hunt & Jacobson, 1973 b; Hunt & Frank, 1975).
(4) That pattern reduplication resembling that seen with NN or VV eyes may
follow simple vertical or horizontal midline lesions through the eye at stage 32
(Hunt & Jacobson, 1974).
Jn relation to all this, we have recently shown that the eventual orientation
of the visuotectal map in Xenopus is already determined by stage 21/22 and
we have attempted to explain how the previous views could have developed
(Gaze et ah 1979). Jn the present paper, as well as in the previous one of this
series (Straznicky & Gaze, 1979) we show that, when various types of half eyes
are put together to form compound eyes, the visuotopic maps that result are
oriented in accord with the orientations of the fragments making up the
compound eye.
The experiments described in this paper were started in 1974. The results,
which have been consistent throughout the series, and which differ from the
previous work of Hunt and Jacobson, were not published previously because we
did not wish to add to the general confusion. Following our recent study of
early eye rotations (Gaze et ah 1979), in which we suggested that the technique
of operation, together with the particular operating solutions used, might
provide an explanation for the different results obtained, we have extended the
present series of compound eye studies. Specifically, we have performed operations to produce N R T L eyes, using a low ionic strength operating medium, as
used by Hunt and his collaborators. We find that, if the operation is performed
in solution of low ionic strength, such that the tissues appear damaged and
healing is delayed, we can replicate the results of Hunt & Jacobson (1973) in
that it is possible to induce mirror reduplication rather than to produce a
map which reflects the anatomical nature of the operation. We consider these
matters further in the discussion and suggest a new and simpler interpretation,
accounting for all the reported results, and dispensing with the notion that any
group of developing eye cells of the embryonic stages used, can reprogramme the
array of positional markers within any other group to which it is joined. A
brief report of some of this work has appeared elsewhere (Gaze & Straznicky,
1979).
METHODS
The microsurgery (performed in full strength Niu-Twitty solution), electrophysiological mapping and histology have been described in detail in a previous paper (Straznicky & Gaze, 1979). Xenopus embryos at stages 32/33
(Nieuwkoop and Faber, 1956) were operated upon in order to obtain the following eye recombinations (Figs. 1 and 2).
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R. M. GAZE AND C. STRAZNICKY
Type la. The temporal half of the right eye was substituted by a left temporal
half with reversed dorsoventral polarity (N R T L ).
Type lb. The nasal half of the right eye was substituted by a left nasal half
with reversed dorsoventral polarity (T R N L ).
Type 2a. The ventral half of the right eye was substituted by a left ventral
half with reversed nasotemporal polarity (D R V L ).
Type 2b. The dorsal half of the right eye was substituted by a left dorsal half
with reversed nasotemporal polarity (V R D L ).
At the end of recording, animals were prepared for histological analysis using
Holmes' silver stain.
In one group of embryos, operations to produce N R T L eyes were performed
in an operating solution comprising 15% Holtfreter's solution with 5%
Steinberg's solution ('low ionic medium').
RESULTS
The results of the main series of operations, performed in full strength NiuTwitty solution, are described first and are followed by the results of operations
performed in low ionic medium.
(A) Operations in full strength solution
The growth and differentiation of the eyes with successful operations, determined and selected 48 h after surgery, were normal except for the distribution of
eye pigmentation and the existence of secondary ' ventral' fissures in eyes where
the nasal/temporal half had been inverted dorsoventrally. In a few such animals
belonging to type T R N L and N R T L , a second ventral fissure was found at the
time of recording, though the presence or absence of the second fissure could
not be correlated with the normal or partly reversed polarity of the obtained
visuotectal map.
Altogether 51 animals with successful eye operations were reared to metamorphosis and beyond. Visuotectal mappings were performed from stage 56 to
four years after metamorphosis. The present study is based on 36 animals on
which the mapping was successfully completed (Table 1) In nine further D R V L
animals the optic nerve failed to develop, in accordance with the possible importance of the ventral fissure for the formation of an optic nerve. In a further six
animals, belonging to types N R T L , T R N L and V R D L , the visuotectal mapping
was not completed for technical reasons.
(a) Type-la operations (N R T L )
In these animals a right nasal and a left temporal retinal fragment were fused,
involving an inversion of the DV axis of the transplanted temporal fragment.
Four out of five such animals showed a nasal field projection from the operated
eye which was partly or wholly inverted along the dorsoventral axis (Fig. 3).
Map orientation in disarranged Xenopus eyes
149
Table 1. Summary of results
WAM, weeks after metamorphosis; YAM, years after metamorphosis
Type of
operation
1 a, NRTL
Time of
recording
(1)1 YAM
(2) 4 YAM
(3) 4 YAM
(4) 4 YAM
(5) 4 YAM
I b, TRNL
(1) St. 61
(2) St. 57
(3) St. 57
(4) St. 57
(5) St. 56
(6) 1 YAM
(7) 4 YAM
(8) 4 YAM
(9) 4 YAM
(10) 4 YAM
Nature of map
Messy normal projection from host
fragment. Few points from graft - inverted
dorsoventrally
Nasal field inverted dorsoventrall>
Nasal field inverted dorsoventrally
Nasal field inverted dorsoventrally
Distorted normally oriented map from TS
field, with doubled points ventrally
Normal map
Temporal field inverted dorsoventrally
Uninterpretable: two groups, NS and TI
Temporal field inverted dorsoventrally, with
reduplicated points
Temporal field inverted dorsoventrally, with
partial reduplication
Uninterpretable. Temporal half field only
Partial inversion of temporal field with
reduplicated points
Temporal field inverted dorsoventrally, with
reduplicated points
Normally oriented map with temporal
field disarranged
Temporal field inverted dorsoventrally
2 a,
(1)4 WAM
(2) 5 WAM
(3) 4 WAM
(4) 4 WAM
(5) 5 WAM
(6) 3 WAM
(7) 5 WAM
Complete nasotemporal reversal
Dorsal fisld reversed nasoteraporally
Dorsal field reversed nasotemporally
Dorsal field reversed nasotemporally
Dorsal field reversed nasotemporally
Dorsal field reversed nasotemporally
Dorsal field reversed nasotemporally
2 b, VKDL
(1) St. 66
Ventral fi;ld reversed nasotemporally, with
partial reduplication
Ventral fteld reversed nasotemporally
Ventral field reversed nasotemporally
(partial)
Ventral field reversed nasotemporally
Normal map
Partial reversal in ventronasal segment only
(2) 4 YAM
(3) St. 66
(4) St. 66
(5) 3 WAM
(6) 3 WAM
(7) 3 WAM
(8) 4 WAM
(9) 4 WAM
(10) 7 WAM
(11) 7 WAM
(12) 7 WAM
(13) 9 WAM
04) 9 WAM
Partial nasotemporal reversal of ventral
field
Ventral field reversed nasotemporally
Partial reversal in ventronasal segment only
Ventral fi^ld reversed nasotemporally
Ventral field reversed nasotemporally
Normal map with few reversed points,
ventral field
Normal map with few reversed points,
ventral field
Normal map
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R. M. GAZE AND C. STRAZNICKY
Fig. 3. Visuotectal projection, mapped 4 years after metamorphosis, from an animal
wilh an NRTL eye. The large upper diagram represents the left optic tectum, with the
heavy black arrow pointing rostrally along the midline. Rows of electrode positions
are indicated on the tectum. The large open arrow is to indicate the overall orientation of the map(s) and the broken line across the tectum shows the division between
the projections from the host and graft retinal fragments. The shaded area represents tectal positions receiving input from both retinal fragments. The large lower
diagram represents the right visualfield.The animal's eye is to be considered as being
behind the chart and looking out at the observer through its centre. Numbered rows
of stim alus positions in the field correspond to the rows of tectal positions. Rows
presumed to relate to the grafted retinal fragment are drawn with interrupted
lines. The large open arrows indicate orientation in relation to the open arrow on
the tectum. The inset diagram shows the nature of the operation to the right eye, with
the grafted fragment in black. N, Nasal; D, dorsal; T, temporal; V, ventral. The conventions in the other maps to be shown are similar to those in this figure. In this
projection, nasal field (grafted temporal retina) is inverted dorsoventrally.
Map orientation in disarranged Xenopus eyes
151
Fig. 4. Visuotectal projection from an NRTL eye mapped 4 years after metamorphosis. The projection from nasal field (grafted temporal retina) is inverted dorsoventrally. The projection from temporal field (host nasal retina) is considerably
distorted.
The remaining animal showed a projection mainly from the visual field corresponding to the host (nasal) retinal fragment, and this projection extended over
the greater part of the tectum, with essentially correct orientation. In four of the
five animals the projection from the host (nasal) retina was extensively distorted
(Figs. 3 and 4), and in one of these a complex form of partial reduplication was
found in the projection from the host fragment (Fig. 4). In this case the projection from the graft was completely inverted dorsoventrally while the greater
part of the projection from the host fragment was oriented in an approximately
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R. M. GAZE AND C. STRAZNICKY
Fig. 5. Visuotectal projection from a TKNL eye, mapped 4 years after metamorphosis.
The projection from temporal field (grafted nasal retina) is inverted dorsoventrally,
while the projection from nasal field (host temporal retina) is normally oriented.
correct fashion. Some of the reduplicated field positions were organised with
normal orientation (1, 2, 3) while others were inverted (14, 15, 16; 20, 21, 22).
In cases where the nasal field was inverted, host retina (temporal field)
projected alone to caudal tectum, grafted retina (nasal field) projected to rostral
tectum and there were some islands of rostral tectum which received input from
both retinal fragments (Figs. 3 and 4).
Map orientation in disarranged Xenopus eyes
153
Fig. 6. Visuotectal projection from a TRNL eye, mapped in a tadpole of stage 56.
Thefieldprojection inverts as it passes from nasal (host temporal retina) to temporal
(grafted nasal retina). In addition a small segment of temporodorsal field (positions
.1, 3,4, 5, 6 and 7) shows mirror reduplication resembling that expected from a somewhat rotated double-temporal compound eye.
(b) Type-lb operations (T R N L )
In these animals a right temporal and a left nasal retinal fragment were fused,
involving an inversion of the DV axis of the transplanted nasal fragment. Six
out of 10 such animals gave visuotectal maps where the temporal field projecjection was partly or wholly inverted along the dorsoventral axis (Fig. 5).
In two animals the operated eye gave an approximately normal projection and
in two others we were unable to interpret the maps, which were chaotic.
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R. M. GAZE AND C. STRAZNICKY
Fig. 7. Visuotectal projection from a DBVL eye, mapped 5 weeks after metamorphosis. The projection from dorsal field (grafted ventral retina) is reversed nasotemporally, while that from ventral field (host dorsal retina) is normally oriented.
Apart from the inversion of the temporal field projection, several of these
animals showed tectal positions which received input each from two field
positions. Characteristically the reduplicated field positions differed from those
of conventional compound eyes (Gaze et ah 1943; Straznicky et al. 1974) in
that they tended to be distributed diagonally across the visual field, suggesting
that one field position represented host retina and the other graft retina (Figs. 3
and 4).
In one case the map combined elements of both a T R N L inverted map and
a conventional double-temporal map (Fig. 6).
In those animals showing inversion of the projection from the temporal
Map orientation in disarranged Xenopus eyes
155
Fig. 8. Visuotectal projection from a DRVL eye, mapped 4 weeks after metamorphosis.
The entire projection is reversed nasotemporally.
field, host retina (nasal field) projected to rostral tectum, graft retina projected
to caudal tectum and there were some islands of tectal tissue which received
input from both retinal fragments (Figs. 5 and 6).
(c) Type-2a operations (DRVL)
In these animals a right dorsal and a left ventral retinal fragment were fused,
involving a reversal of the NT axis of the transplanted ventral fragment. Six
out of seven such animals gave maps in which the dorsal visual field projection
was reversed along the nasotemporal axis (Fig. 7). One animal showed a projection which was completely reversed nasotemporally (Fig. 8). Three of the
half-reversed maps showed completely separate tectal projections from each
retinal segment so that the tectum was neatly divided into a region receiving
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R. M. GAZE AND C. STRAZNICKY
Fig. 9. Visuotectal projection from a DRVL eye, mapped 4 weeks after metamorphosis. The projection from dorsal field (grafted ventral retina) is reversed nasotemporally, while that from ventral field (host dorsal retina) is normally oriented.
from host retina and one receiving from graft retina (Fig. 7). In the other three
half-reversed maps there were islands of tectal tissue that received input from
both retinal fragments (Fig. 9).
In all cases the grafted ventral retina projected medially on the tectum and the
partial map from this part of the retina included more of the visual field and
occupied more of the tectum, than did the partial map from the host (dorsal)
retinal fragment. This difference may however be illusory, since some of the
host retinal fragment would have projected round the inaccessible lateral
edge of the tectum.
Map orientation in disarranged Xenopus eyes
157
Fig. 10. Visuotectal projection from a VRDL eye, mapped 7 weeks after metamorphosis. The projection from ventral field (grafted dorsal retina) is reversed nasotemporally, while that from dorsal field (host ventral retina) is normally oriented.
(d) Type-2b operations (VRDL)
In these animals a right ventral and a left dorsal retinal fragment were fused,
involving a reversal of the NT axis of the transplanted dorsal fragment. Ten
out of 14 animals gave maps reflecting, in various degrees, the nature of the
operation. Two animals gave essentially normal maps with a couple of double
field positions in each and two gave normal maps.
Of the ten cases showing signs of reversal of the ventral field projection, four
gave a substantial projection from the grafted dorsal retina (Fig. 10); four
gave maps in which the reversed projection was less obvious and two showed
projections where the reversal was confined to the nasoventral sector of the
field (Fig. 11).
EMB 55
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R. M. GAZE AND C. S T R A Z N I C K Y
Fig. 11. Visuotectal projection from a VBDL eye, mapped 3 weeks after metamorphosis. The projection from dorsal field (host ventral retina) is normally oriented
while the projection from grafted retina is represented only by two rows of positions
in the ventronasal field.
(e) Overlap of the partial maps of the two retinal fragments
One of the most remarkable phenomena to emerge from this study is the
degree to which the projections from the component fragments of an eye
overlap each other. This is well shown in Fig. 12, where ventral field positions
12, 21, 22 and 29 are deeply interdigitated with positions belonging to the dorsal
field map. Also noticeable, though perhaps not so remarkable, in view of the
difficulty of estimating an exact half of an embryonic eye, is the degree of
doubling of the visual field positions projecting to one tectal position. Commonly, the most dorsal one or two ventral visual field positions are reduplicated (with
a nasotemporal discrepancy associated with the nature of the operation) in
Map orientation in disarranged Xenopus eyes
159
Fig. 12. Visuotectal projection from a VRDL eye, mapped at metamorphic climax
(stage 66). The projection from ventral field (grafted dorsal retina) is reversed
nasotemporally, while that from dorsal field (host ventral retina) is normally
oriented.
the most ventral one or two dorsal field positions (Fig. 12). This reduplication
of visual field projection to one tectal position is much more marked in animals
with dorsal retinal grafts (VRDL) than in animals with ventral grafts (D R V L ).
(B) Operations in low ionic medium
Operations to produce N R T L eyes were performed on embryos of stage 32,
in low ionic medium. Operations in this medium are more difficult than comparable operations in normal operating solution, as the embryonic tissue
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R. M. GAZE AND C. S T R A Z N I C K Y
VD
Fig. 13. Visuotectal projection from an NETL eye, operated in low ionic medium
and mapped at stage 60. In this case the operation has led to mirror reduplication
similar to that expected from a conventional NN compound eye. The projection is
rotated clockwise by some 30°, as is the eye.
becomes very sticky. Furthermore, following any minor unintentional damage
to the pigment layer, this may virtually disintegrate.
Animals were kept in the operating solution for 48 h and then transferred to
normal solution. Embryos were examined 12, 24 and 48 h after operation. At
12 and 24 h after the operation it was noticed that the implanted eye fragments
were considerably smaller than would be expected following operation in normal
medium and the junctional scar between the two fragments was very obvious.
In contrast, eyes operated in normal solution healed very quickly and by 24 h
it was difficult to detect any sign of the operation.
Two embryos died by 48 h after operation and six more had to be discarded
because of the failure of the optic nerve to develop. The remaining 16 animals
were mapped in late larval life or around metamorphosis. Eight gave reduplicated maps resembling those obtained from conventional double-nasal eyes
Map orientation in disarranged Xenopus eyes
161
(Fig. 13). Five animals gave normal maps and three gave maps deriving largely
from the host retinal fragment. In no case was a map obtained showing dorsoventral inversion of the nasal field.
DISCUSSION
The results of the present main series of experiments support the idea (Straznicky & Gaze, 1979) that the developmental programme within the eye anlage,
leading to the eventual acquisition of map-related cell specificities, is stable in
the face of operations of the type used. Thus if the eye fragments develop autonomously in respect to map orientation, type-la operations (N R T L ) should
give projections in which the nasal field is inverted dorsoventrally, type-lb
operations (TRNL) should give a dorsoventrally inverted temporal field,
type-2a operations (DKVL) should give a nasotemporally reversed dorsal
field and type-2b operations (VRDL) should give a nasotemporally reversed
ventral field (Fig. 14). The results are in accord with these predictions for all
four types of operation (Table 1).
Reasonable arguments can be made to account for the cases in which results
were found other than those predicted. Thus the four animals that gave normal
maps could represent cases in which the grafted fragment was small and failed
to thrive. In previous experiments it has been shown that half-eyes, if made at
comparable stages to those of the present operations, and if left with cut edges
at the periphery of the line of section through the eye unopposed, will usually
go on to regenerate a complete normal eye (Berman & Hunt, 1975; Feldman &
Gaze, 1975). In the present circumstances we would suggest that, in these four
cases, the host eye fragment behaved in this fashion. Such regulation to normal
is also a standard result in a minority of operations made to produce conventional compound eyes (Gaze et al. 1963).
In the four animals whichproduced essentially normal maps with some doubled
field positions, it is likely that the residual grafted retinal fragment made up
only a small fraction of the eye. The doubled field positions occurred in parts
of the visual field where we would expect them to be if they came from the residual parts of the originally grafted tissue; and the points may show relative
orientations consonant with the nature of the operation.
The animal (D R V L ) which produced a map that was completely reversed nasotemporally presumably represents a situation in which the grafted left ventral
fragment went on to regenerate a whole eye. In this case, because of the left/
right translocation the resulting map would be expected to be NT reversed.
No explanation need be attempted for the two cases in which the maps were
chaotic. This result presumably represents the 'slop' in the system and the
surprising thing is that untypical maps were so few.
Many of the maps in the present series are messy and these experiments cannot
exclude the possibility that some form of interactive modulation or influence,
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R. M. GAZE AND C. STRAZNICKY
T
N
D
TRNL
Fig. 14. Summary of the expected projection orientations from a normal eye and
from the four types of compound eye discussed in this paper, based on the assumption of stability of the developmental programme for map orientation.
between the eye fragments, may have contributed to the degree of disorder
found. On the other hand, any form of surgical interference with the developing
eye may be expected to lead to diminished order in the final map, for reasons that
may be quite general; and the overall result of the present experiments shows
clearly that the mapping orientation is preserved when these compound eyes
are formed.
In. the present experiments, although the eyes are 'compound' in the sense
that they were surgically compounded from individual half-eyes, each eye
contains a full set of positional markers (Fig. 1). Each eye has nasal, temporal,
Map orientation in disarranged Xenopus eyes
163
dorsal and ventral poles in terms of embryological origin. These have merely
been rearranged within the eye.
This at once destroys one of the main characteristics of the normal eye, which
is the continuity of neighbourhood relationships. There is now a line of dislocation through the eye such that ganglion cells near the geographical centre
of the retina bear an almost normal relationship to each other, whereas cells
facing each other across the line of fusion come to be more and morepositionally
disparate, the further they are from the centre of the retina.
The position of the ventral fissure and optic stalk in relation to the eye fragments may well be relevant to the nature of the maps obtained, in particular
the degree of order they show. For instance the eyes producing the most orderly
maps were those where the graft fragment was ventral and thus had immediate
access to the optic stalk. In all these cases (Figs. 7, 8, 9) the greater part of the
map came from the grafted fragment and was well ordered. Maps from grafted
dorsal fragments tended to be the smaller components of the total maps and to
be less well ordered (Figs. 10,11,12). Maps from eyes where the line of operation
went through the ventral fissure (nasal or temporal grafts) tended to show some
disorganisation in both part maps (Figs. 3 and 4). Thus it seems likely that access
to the optic stalk and the integrity of the ventral fissure are relevant to the orderliness of the map that is later formed.
If the graft and host fragments were accurate halves, one might expect, since
the eye contains a complete set of positional values, that the tectum would
become subdivided equally accurately into two areas, each receiving from one
retinal fragment. In this case the map would be complete, not reduplicated, and
show a line of dislocation separating a normally oriented part map (host
fragment) from a part map (graft fragment) which was reversed along one axis.
Some of the maps in the present series show this arrangement (Fig. 7). More
frequently, however, the present maps show reduplication of field positions at
some tectal points, particularly with dorsal grafts. The very noticeable difference
here between the dorsal and ventral grafts may again relate to a role played by
the ventral fissure in the establishment of fibre pathways (Scholes, 1979).
The existence of reduplicated field positions may also be attributable to systematic variations in the relative sizes of the fragments comprising an eye.
In general, each retinal fragment projects predominantly to its appropriate
part of the tectum. In addition, several animals showed islands of tectum that
responded to one or the other retinal fragment, or to both. It seems likely that
the number and distribution of these islands may reflect differences in growth
rates and times of arrival of fibre groups from different regions of the eye.
The general result of the present series of experiments has been to show that
the developmental programmes relating to the map-forming properties of the
eye fragments remain stable in the face of disarrangment of the geometry of the
eye. We must here comment on the differences between our results and those of
other workers.
164
R. M. GAZE AND C. STRAZNICKY
In contrast to the large body of work showing stable programming in the
component fragments of conventional NN, TT and VV eyes, Hunt and his
collaborators (Hunt & Jacobson, 1973 a; Hunt & Frank, 1975) have obtained
visuotectal maps which appeared to show extensive modifiability of the developmental programme of the eye fragments under various experimental conditions. In one particular situation, where a right nasal and left temporal fragment
were fused, instead of a map with dorsoventral inversion of the nasal field, in
accordance with the reversed dorsoventral polarity of the transplanted half of
the retina, systematically NN maps were obtained, with normal dorsoventral
polarity. These observations were taken to mean that the transplanted temporal
retinal fragment had been reprogrammed by the host nasal fragment.
The present results, in particular the operations performed in low ionic
medium, show that the reason for these different findings (in what seem to be
anatomically similar operations) lies at least partly in the nature of the operating
solutions used. In our main series of experiments we have used standard
embryological operating medium such as full strength Niu-Twitty solution;
Hunt and his collaborators, on the other hand, tended to use operating solutions
of a very low ionic strength ('rearing medium'). It is known that, in either
type of operating medium, certain operations, such as removal of half an eye
anlage, may result in mirror reduplication of the remaining fragment (Berman
& Hunt, 1975; Feldman & Gaze, 1975), particularly if rather more than half of
the eye is removed (Macdonald, in Feldman & Gaze, 1975; Macdonald. 1975).
Simple midline lesions through the eye anlage, made in full strength operating
medium, give in all cases normal eyes and normal maps (Macdonald and Gaze,
unpublished; Macdonald, 1975). If, on the other hand, midline lesions are
made in the low ionic strength medium, a proportion of the eyes go on tto
produce mirror reduplication (Hunt & Jacobson, 1974; Macdonald, 1975).
The mirror reduplication phenomenon may thus be seen to be at least partly
a response to the particular conditions of operation. It may follow the removal
of part of the eye and it may follow other injuries, such as midline lesions (which,
in normal operating conditions, will heal up perfectly) provided that the lesion
is made in the low ionic medium.
If, as suggested here, mirror-reduplication may be seen as a form of response
to injury even when (as with half-eyes) there is no eye fragment to re-programme,
or when (as with midline lesions) both the normal halves of the eye are present
initially, and thus no re-programming is called for by positional disparity, it
seems likely that many of the other mirror-reduplication results of Hunt
and his collaborators may be accounted for in the same way. We would suggest,
therefore, that these results should be interpreted, not as illustrating particular
programming activities of one eye fragment on another but as bearing on a
much more general and little understood phenomenon, the mechanism of mirrorreduplication.
When compound eyes are constructed using more conventional techniques,
Map orientation in disarranged Xenopus eyes
165
and including the varieties described in the previous paper (Straznicky & Gaze,
1979) and in the present paper, the standard result is to produce maps which
reflect the nature of the operation.
We would like to thank Mrs June Colville for expert histological assistance.
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(Received 18 July 1979, revised 2 October 1979)