J. Embryol exp. Morph. Vol. 62, pp. 13-35, 1981
Printed in Great Britain © Company of Biologists Limited 1981
\ 3
The development of the retinotectal projections
from compound eyes in Xenopus
By C. STRAZNICKY, 1 R. M. GAZE 2
AND M. J. KEATING 2
SUMMARY
The retinotectal projections from double-nasal (NN), double-temporal (TT) and doubleventral (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.
INTRODUCTION
'Compound eyes' may be formed in Xenopus embryos by surgical operation
to fuse two eye fragments in the same orbit. Such an operation is usually
performed at stage 32 (Nieuwkoop & Faber, 1967); that is, at tail-bud stage,
after the eye anlage has become spatially determined in terms of the orientation
of the map it will eventually form, but before any optic fibres have yet passed
from the eye to the tectum. If the temporal half of an eye anlage is removed
and replaced by a nasal half, taken from an eye of opposite laterality, the
result is a double-nasal (NN) compound eye. In a comparable fashion doubletemporal (TT) and double-ventral (VV) eyes can be made.
If an animal with an NN, TT or VV eye is allowed to develop until after
metamorphosis and the retinotectal projection from the compound eye is then
examined electrophysiologically, it is found, as expected, that each such eye
gives a reduplicated projection to the tectum. The orientation of the tectal
projection from each half of the compound eye is appropriate to the embryonic
origin of that half-retina. However each (similar) half of the compound eye
spreads its connexions across most of the tectum, instead of confining its
projection to the appropriate half of the tectum, as would such a half-retina in
1
Authors address: Centre for Neuroscience, School of Medicine, The Flinders University
of South Australia, Bedford Park, South Australia 5042.
2
Authors'1 address: National Institute for Medical Research, The Ridgeway, Mill Hill,
London NW7 1AA, England.
14
C. STRAZNICKY, R. M. GAZE AND M. J. KEATING
a normal animal (Gaze, Jacobson & Szekely, 1963; Straznicky, Gaze & Keating,
1974). The orderly spreading of the projection from a half-retina (comprising
half of a compound eye) raises interesting questions about the developmental
relationships between the eye and the tectum.
In a normal animal the entire retina maps, in a continuous fashion, over
the entire tectal surface. The development of a normal retinotectal projection,
as well as its restoration when the optic nerve is cut and allowed to regenerate,
is usually ascribed to the existence of specific chemical affinities between
recognition markers on retinal and tectal cells (Sperry, 1943, 1944, 1945, 1951,
1963, 1965). The spreading of the projection from each half-retina of a compound eye presents problems for such a mechanism. Difficulties arise because
there are several possible interpretations of the compound-eye results.
One possibility would be for the compound-eye operation to be followed
by regulation of the specificity-structure, or distribution of markers in the
eye, such that each half of the compound eye acquires a full set of cell markers.
This could account for the spreading of the projection. We have recently
furnished strong evidence, however, that this is not the case. Each component
half of a compound eye, when allowed to regenerate fibres to the ipsilateral
tectum, behaves as if it possesses only the markers characteristic of that half-eye
(Gaze & Straznicky, 1979, 1980; Straznicky, Gaze & Horder, 1979).
Another possible explanation for the spreading of the compound eye projection, is that only that area of tectum develops which is innervated by optic
fibres (Sperry, 1965). Thus fibres from an NN eye may permit only caudal
tectum to develop, since this is where nasal fibres normally go; fibres from
a TT eye may permit only rostral tectum to develop, and fibres from a VV
eye may permit only medial tectum to develop. If this were to happen, we
might expect the resulting tectum to be approximately normal in size, even
though derived from only half the original tectal structure, since the optic
input comprises fibres from two (similar) half-retinae, rather than just one.
We could call this the 'overgrown half-tectum' hypothesis.
A further possibility is that recognition by optic axons of localized tectal
markers may not be involved in the initial establishment of the visual projection.
It could be that some other mechanism is responsible for the first setting up
of the retinotectal map (Hope, Hammond & Gaze, 1976; Willshaw & von der
Malsburg, 1979) and the tectal markers (which we know to exist in later life,
since they are involved in nerve regeneration, and which could either be cellular
or related to fibre debris) are then placed on the tectum by the optic nerve
fibres themselves.
Any adequate theory that seeks to account for the formation of topographically ordered retinotectal connexions must accommodate the orderly connectivity found in the projections from compound eyes. More information
might be provided about the elaboration of ordered maps from such eyes
by observing the development sequence through which the projection passes
Developing compound eye projections in Xenopus
15
on its way to the adult state. We have previously shown, in normal animals,
that because of the different patterns of growth of the retina and tectum
(Straznicky & Gaze, 1971, 1972), the normal development of the retinotectal
projection requires a continuous rearrangement of functional connexions as
the system grows (Gaze, Chung & Keating, 1972; Gaze, Keating & Chung,
1974; Gaze, Keating, Ostberg & Chung, 1979). Thus the parts of the tectum
with which the various regions of the retina connect are not necessarily the
same in the young larva as in the post-metamorphic animal. Throughout
development the general orientation of the retinotectal map remains the same
but the specific retinotectal connexions, which underlie the map, change. We
felt, therefore, that it would be useful to observe the way in which the projections
from compound eyes responded to this developmental situation.
The present results show that, contrary to the electrophysiological evidence
from older animals, during tadpole life the projection from each type of
compound eye tends to restrict itself to a different part of the available tectum.
Thus TT projections are found mainly rostrally on the tectum and VV projections mainly medially, while NN projections extend over the whole available
tectum but are best developed caudally.
In the discussion we consider the possible significance of these results for
theories of retinotectal connectivity. An abstract of some of this work has
been published elsewhere (Straznicky, Gaze &. Keating, 1979).
METHODS
Laboratory-bred Xenopus laevis at various developmental stages were used
in this study.
Microsurgery
The right eye of stage-32 to -33 embryos (Nieuwkoop & Faber, 1967) was
operated on according to previous descriptions (Gaze & Straznicky, 1980) to
obtain an NN eye. In other embryos right TT or W eyes were formed. Animals
with right NN, TT or VV eyes were reared separately. Between stage 50 and
8 weeks after metamorphosis the contralateral retinotectal projections from
the right operated and left intact eyes were assayed by injecting [3H]proline
(3HP) into both eyes. In some animals the visuotectal projection from the
operated eye was also mapped electrophysiologically.
Histology
Twenty-four hours before sacrifice 3 HP (Radio Chemical Centre, Amersham :
specific activity 24 Ci/m-mole) was injected into the posterior chamber of the
right compound eye and left intact eye, under anaesthesia with MS222 (Tricaine
Methane Sulphonate, Sandoz; 1:1500). One /*Ci of isotope was given to the
animals up to stage 54, 2 /*Ci from stage 54 to stage 60, 3 /tCi from stage 60 to
16
C. STRAZNICKY, R. M. GAZE AND M. J. KEATING
stage 66 (metamorphosis) and 5 jttCi in post-metamorphic animals. The head
of the animal was fixed in Bouin's solution, the dissected brain was embedded
in paraffin, serially sectioned at 10 ftm and mounted on slides. The slides were
coated with Ilford K2 emulsion, exposed in light-tight boxes at 4 °C for 14 days,
developed in Kodak Dektol and counterstained with Harris's hemotoxylin.
Reconstruction of the brains, based on camera-lucida drawings of every fifth
section, were made to assess the extent of the projection from the compound
and intact eye. The proline distributions were estimated by visual inspection
of the sections.
Electrophysiology
Several animals of varying age with right NN, TT or VV eyes were recorded
before sacrifice in order to ascertain that the eye operation was successful.
Visuotectal mapping procedures were similar to those described previously
(Straznicky, Gaze & Keating, 1971; Gaze et al. 1974) thus only a short account
is given here. Animals were anaesthetized with MS222. The tectum was exposed,
photographed and the animal was set up in a small Perspex globe filled with
oxygenated anaesthetic solution (approximately 0-01 % MS222) at the centre
of an Aimark projection perimeter with the right compound eye centred and
the left eye covered.
Visually evoked action potentials were recorded by a Wood's metal electrode
from pre-determined tectal positions and the corresponding receptive fields
located. The visuotectal projection from the operated eye was estimated by
sampling about 30 tectal recording points.
RESULTS
The results are based on study of 25 animals with one NN eye each, 25 animals
with one TT eye each and 18 animals with one VV eye each. The compound
eyes were the same size as the normal eyes at the time of autoradiographic
analysis, which was performed on all experimental animals. In many cases the
tectum contralateral to the compound eye was smaller than that contralateral
to the normal eye (Figs 1, 4 b, 5, 8 and 11) but this was not consistently so.
Electrophysiological recording of the visuotectal projection from the compound
eye was made in 5 animals with one NN eye, 16 animals with one TT eye
and 5 animals with one VV eye, to confirm electrophysiologically that the
compound eye construction had been satisfactory. A larger number of TT
animals was mapped because of the particular adjustments in retinotectal
relationships required by the development of the projection from this kind of
eye (see below).
Developing compound eye projections in Xenopus
17
(a) TT projections
In 22 of the 25 animals with one TT eye, the autoradiographic projections
from the eyes were studied at different developmental stages between stage 50
and 8 weeks after metamorphosis (Table 1). The projection from the normal
eye to its contralateral tectum occupied the greater part of the available
tectum, with a small deficit at the caudomedial margin during larval stages 50
to 59, by which time complete coverage of the tectum was reached. At all
developmental stages the contralateral projection from the TT eye occupied
less tectum than did that from the control eye. At the earliest developmental
stages examined, the projection from the TT eye was restricted to a relatively
small area of rostrolateral tectum. With development the projection spread
caudomedially across greater areas of the tectal surface but even by 8 weeks
after metamorphosis there was still no projection from a TT eye to the most
caudomedial tectal sector (Fig. 1). Representative dark-field autoradiographs
from an 8-week post-metamorphic animal with one TT eye are shown in
Fig. 2. It may be seen that, compared to the projection from the normal eye,
there is a relative deficit in the TT projection to caudomedial tectum.
Electrophysiological mapping of the projection through the TT eye was
carried out in 16 animals. Fifteen yielded projections characteristic of TT
eyes, while in the remaining animal only the host temporal retina had projected
to the tectum. A map obtained from a stage-59 animal, together with the
histological reconstruction of the autoradiographic projection, is shown in
Figure 3. The visuotectal projection shows the whole visual field of the TT
eye projecting, with the reduplication and orientation appropriate to a TT
eye, to rostrolateral tectum. The autoradiographic reconstruction (Fig. 3 b)
confirms that the TT projection is limited to rostrolateral tectum. The map
and autoradiography from a TT animal at 8 weeks after metamorphosis are
illustrated in Fig. 4. Both the map and the autoradiography demonstrate that
by this stage considerably larger tectal areas are covered than at stage 59, but
the autoradiography indicates that there is still a significant deficit in caudomedial tectum.
(b) VV projections
Successful autoradiography was obtained in 15 of the 18 animals with one
normal and one VV eye, the extent of the projection being examined at various
developmental stages between stage 50 of larval life and 2 weeks after metamorphosis (Table 1). In two animals the label from the compound eye was
restricted to the central region of the contralateral tectum, an exceptional
result. In the other 13 animals, label from the VV eye was concentrated medially
on the tectum with a relative deficit in the projection to lateral tectum. In
contrast, the label from the normal eye covered its own contralateral tectum
completely from stage 58. The projection deficit from the VV eye was more
50
50
50
50
52
52
54
54
59
59
59
60
60
61
61
61
62
66
66
66
WAM
WAM
WAM
WAM
WAM
NN2
NN3
NN4
NN5
NN6
NN7
NN8
NN9
NN 10
NNU
NNP
NN 13
NN 14
NN 15
NN16
NN 17
NN 18
NN19
NN 20
NN 21
NN 22
NN 23
NN 24
NN 25
1
2
2
3
6
Stage
Expt
NN
NN
NN
NN
NN
Map
8, 9
8
Label on contralateral tectum less dense rostrally
Label uniform across contralateral tectum
Label on contralateral tectum less dense rostrally and less extensive caudally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally (also on right tectum)
Label on contralateral tectum less dense rostrally both tracts heavily labelled
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally tracts very prominent
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label uniform across contralateral tectum
Label uniform across contralateral tectum. Tectum thinner caudally
Label on contralateral tectum less dense rostrally
Label uniform across contralateral tectum. Only medial tract seen
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally (also on right tectum)
Label on contralateral tectum less dense rostrally
Label on contralateral tectum less dense rostrally
8
8
11
8
8
10
8
8
8
Fig.
Autoradiography
WAM = weeks after metamorphosis; NN = Double-nasal; TT = Double-temporal; VV = Double-ventral; DV = Dorsoventral.
Table 1
o
Z
w
O
m
N
o
Z
o
N
on
H
p
oo
50
55
58
58
59
59
59
59
59
60
60
62
64
65
65
65
WAM
WAM
WAM
WAM
WAM
WAM
WAM
WAM
WAM
2
2
2
3
3
3
8
8
8
TT t
TT2
TT3
TT4
TT5
TT6
TT7
TT8
TT9
TT 10
TT1X
TT 12
TT 1 3
TT 14
TT 15
TT 16
TT 17
TT 18
TT 19
TT 20
TT 2 1
TT 22
TT 23
TT 24
TT 25
TT
TT
TT
TT
TT
TT
Temporal
ret. only
TT
TT
TT
TT
TT
TT
TT
TT
TT
label
label
label
label
label
label
label
label
label
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
restricted
restricted
restricted
restricted
restricted
restricted
restricted
restricted
restricted
to
to
to
to
to
to
to
to
to
rostrolateral
rostrolateral
rostrolateral
rostrolateral
rostrolateral
rostrolateral
rostrolateral
rostralateral
rostrolateral
tectum
tectum
tectum
tectum (labelled at st. 57)
tectum (labelled at st. 57)
tectum
tectum
tectum
tectum
Contralateral label restricted to rostrolateral tectum
Contralateral label restricted to rostrolateral tectum
Labelled at st. 50. Label rostrolateral
Contralateral label restricted to rostrolateral tectum
Label on contralateral tectum less dense rostrally, restricted caudally
Contralateral label restricted to rostrolateral tectum
Contralateral label restricted to rostrolateral tectum
Contralateral label restricted to rostrolateral tectum (also on right tectum)
Contralateral label restricted to rostrolateral tectum
Contralateral label restricted to rostrolateral tectum
Contralateral label restricted to rostrolateral tectum
Contralateral label restricted to rostralateral tectum
Contralateral label restricted to rostrolateral tectum
4
1,2
o
S3
X
Hi
C
a.
8
b
50
50
50
50
53
53
56
56
58
58
58
58
62
66
66
66
2 WAM
2WAM
W,
vv2
vv3
vv4
vv5
vv6
vv7
vv8
vv9
w 10
wu
wu
vv 13
w14
w15
vv 16
w17
vv 18
Stage
Expt
VV
Messy VV
DV
reduplication
Messy VV
DV
reduplication
Map
label restricted to medial tectum
label restricted to mid-tectal region
label restricted to medial tectum
label restricted to medial tectum
label confined to mid-dorsal region
label restricted
label restricted
label restricted
label restricted
label restricted
label restricted
label restricted
to
to
to
to
to
to
to
medial tectum
medial tectum
medial tectum
medial tectum
medial tectum
medial tectum
medial tectum
Contralateral label restricted to medial tectum
Contralateral label restricted to medial tectum
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral label restricted to medial tectum
Too poor
Too poor
Contralateral
Contralateral
Contralateral
Contralateral
Contralateral
z
o
H
>
o
Z
N
o
o
N
Z
Autoradiography
Fig.
H
Table 1 (contd)
C/3
n
Developing compound eye projections in Xenopus
Stage 50
8WAM
Fig. 1. Reconstruction of the retinotectal projections from the normal and the
TT compound eye in animals at various stages of development. Tritiated proline
was injected into each eye and later serial transverse sections through the tecta
were autoradiographed. The outline of every fifth section was drawn with the
aid of a camera lucida and the mediolateral extent of the tectum and of the labelled
region, as estimated by visual inspection, was measured with a map measurer.
The resulting measurements were marked on graph paper and the overall outlines
joined up. Thus each diagram represents a flattened view of the tecta seen from
dorsally. The mediolateral extent of each diagram is thus a measured dimension
(bar = 1 mm) while the rostrocaudal dimension is arbitrary.
In each case the right eye was compound and projected to the left tectum. The
left eye was normal and projected to the right tectum.
In each diagram rostral is indicated by the arrowhead. The left tectum, receiving
the projection from the compound eye, is to the right in each case.
It may be seen that the TT projection was initially restricted to the rostrolateral
tectum and that complete tectal coverage has not been reached even by 8 WAM.
21
22
C. S T R A Z N I C K Y , R. M. GAZE AND M. J. KEATING
• - <»
Fig. 2. Dark-field autoradiographs of transverse sections through rostral (top),
mid-tectal (middle) and caudal tectum (bottom) in an animal with a TT eye. The
animal was TT24, 8 weeks after metamorphosis. Bar = 400 fim. The left tectum,
contralateral to the TT eye, is to the right. It may be seen that the TT projection is
dense rostrally and falls off medially and caudally.
Developing compound eye projections in Xenopus
(a)
(b)
-N
Fig. 3. (a) Visuotectal map from an animal with a TT eye (TT6, stage 59). The map
shows the reduplication of field positions characteristic of TT eyes. In this and
each of the other visuotectal maps shown the upper diagram is of the dorsal
surface of the left tectum, with the heavy black arrow projecting rostrally. Numbered
recording positions and dots on the tectum correspond to positions in the chart
of the visual field below. This chart extends out from the centre of the field for
100° and the animal is to be thought of as sitting behind the chart with his right
eye looking out at the observer through the centre of the chart. N, nasal; T,
temporal; D, dorsal; V, ventral. The large open arrows are to indicate orientation
of the field map in relation to the tectum. (b) Reconstruction of the retinotectal
projections in this animal. Bar = 1 mm (mediolateral). The left tectum (right in
the Figure), which receives the projection from the TT eye, shows label restricted
to the rostrolateral region.
23
24
C. STRAZNICKY, R. M. GAZE AND M. J. KEATING
(a)
(b)
Fig. 4. (a) Visuotectal map from an animal with a TT eye (TT23, 8 WAM). Conventions as in Fig. 3. The map shows more extensive tectal coverage than that of
Fig. 3. The field positions show the reduplication characteristic of TT eyes, (b)
Reconstruction of the retinotectal projections in this animal. Bar = 1 mm (mediolateral). The left tectum (right in the Figure), which receives the projection from the
TT eye, still shows a caudomedial deficit at 8 WAM.
marked in the younger animals but was still present at 2 weeks after metamorphosis (Fig. 5), although in the older animals the lateral tectal deficit
showed only as a localized decrease in density of label towards the lateral
edge of the contralateral tectum (Fig. 6).
Electrophysiological mapping of the visuotectal projection through the VV
eye was carried out in five animals. All five maps displayed dorsoventrally
reduplicated field positions characteristic of VV eyes. The best VV map obtained
(Fig. 7) was from a stage-66 animal and shows the type of field reduplication
characteristic of VV eyes (Straznicky et al. 1974). Unfortunately in this case
Developing compound eye projections in Xenopus
25
Stage 50
2 WAM
Fig 5 Reconstructions from the retinotectal projections from the normal and
VV compound eye in animals at various stages of development. Conventions
are as in Fig. 1 except that interrupted lines on the tecta indicate diminished
density of projection. Bar = 1 mm (mediolateral).
the histology was not available. The other VV maps were compound but poorly
organized, for reasons that are not understood.
(c) NN projections
Autoradiographic analysis of the retinotectal projections was obtained in
25 animals with one normal and one NN eye, at various developmental stages
from stage 50 to 6 weeks after metamorphosis (Table 1).
The extent of tectal coverage by the projection from the NN eye, in contrast
to that by the projections from TT and VV eyes, was essentially similar to the
tectal coverage by the projection from the normal eye (Fig. 8). In 21 of the
25 animals, however, it was noted that the autoradiography density of the
projection from the NN eye was reduced in the rostral tectum. In the other
26
C. S T R A Z N I C K Y , R. M. GAZE AND M. J. KEATING
Fig. 6. Photomicrograph of a transverse section at the mid-tectal level in an
animal with a VV eye (VV14, stage 66). Bar = 300/*m. The left tectum, receiving
the projection from the VV eye, is on the right. It may be seen that this projection
is most dense towards the midline and tails off toward the lateral edge of the
tectum.
four animals the NN projection did not show this decreased density rostrally.
In contrast to the majority result from the NN eyes, the projection from the
normal eye showed, in all but two cases, no reduced density of label of tectum
rostrally. The reduction in the density of the rostral tectal label from NN eyes
was seen at all developmental stages studied and is illustrated for a stage-50
projection in Fig. 9, and for a stage-61 projection in Fig. 10.
The visuotectal projection through the NN eye was mapped electrophysiologically in five animals and all five yielded field maps characteristic of NN
eyes. One such map, from a stage-62 animal, is shown in Fig. l l a and the
autoradiographic reconstruction in Fig. 11 b.
DISCUSSION
These experiments were intended to reveal something of the mechanisms
by which ordered retinotectal projections are established. The results show
that TT projections initially innervate rostrolateral tectum, VV projections
initially innervate medial tectum and NN projections initially innervate the
more caudal tectal areas most densely. At first sight this differential distribution
of projections from different types of compound eye seems to provide support
for the chemoaffinity theory, which suggests that the normal adult projection
arises as a result of selective affinities of temporal retina for rostral tectum,
ventral retina for medial tectum, nasal retina for caudal tectum and dorsal
retina for lateral tectum. In fact, however, such an interpretation makes no
allowance for the changes that occur in the retinotectal projection with the
Developing compound eye projections in Xenopus
T-
27
-N
Fig. 7. Visuotectal map from an animal with a VV eye (VVi6, stage 66). Conventions
as in Fig. 3. The map shows reduplication of field positions characteristic of VV
eyes.
passage of time, either during the development of the normal projection or
of that from a compound eye. We consider first these changes and then return
to examine alternative mechanisms for the differential behaviour of the compound eye projections.
In normal animals the retina grows throughout larval life and into postmetamorphic stages by the accretion of cells at the ciliary margin (Straznicky
& Gaze, 1971; Jacobson, 1976; Gaze et ah 1979). Tectal histogenesis, on the
other hand, occurs in a curvilinear fashion from rostrolateral to caudomedial
tectum and appears essentially complete by stage 58 of larval life (Straznicky
& Gaze, 1972). Analysis of the way in which the normal retinotectal projection
develops indicates that while the general orientation of the growing map on
the growing tectum is preserved, the individual retinotectal connexions under-
28
C. STRAZNICKY, R. M. GAZE AND M. J. KEATING
(a)
Stage 50
Stage 52
I
Stage 60
Fig. 8. (a) For legend, see opposite.
lying the map undergo continuous changes (Gaze et al. 1972; Gaze et al.
1974; Gaze et al. 1979). Thus to account for the development of the normal
retinotectal projection we require a mechanism which will preserve the fourdimensional topology of the system while allowing the individual neural
elements to vary their interconnexions as development proceeds.
An even more remarkable shift occurs during maturation of the projections
from TT and VVeyes. This point may be illustrated, in relation to TT projections,
by considering for example Figs. 3 and 4. The topography of a TT map is
such that the vertical meridian of visual field, and hence of the retina, projects
to the most caudal part of the innervated tectum. Figures 1, 3 and 4 demonstrate, however, that the particular tectal area which constitutes the caudal
margin of the projection changes markedly with development. Since neurogenesis in compound eyes, as in normal eyes, occurs at the retinal ciliary margin
(Feldman&Gaze, 1972); Straznicky & Tay, 1977) central retina is the oldest part
of the retina. This portion of the retina in TT eyes projects during mid-larval life
to rostral tectum but by 8 weeks after metamorphosis the same retinal area is
Developing compound eye projections in Xenopus
29
(b)
6WAM
Fig. 8. (a, b) Reconstructions from retinotectal projections from the normal and
the NN compound eye in animals at various stages of development. Conventions
are as in Fig. 1. Interrupted lines on the tectum indicate decreased density of label.
It may be seen that, in most cases, the NN projection shows decreased density
rostrally. Bar = 1 mm (mediolateral).
projecting to caudal tectum, while the rostral tectal area previously receiving
input from central retina now receives from more peripheral retina. Similarly,
newly appearing neurons at the nasal and temporal retinal poles project to
the rostral tectal margin which earlier received input from older retinal neurons,
while the tectal connexions of these older neurons are displaced caudally on
the tectum. The maturation of the TT map thus involves fairly radical readjustment in individual synaptic relationships between retinal and tectal
elements, but these changes are of the same sense., although quantitatively
greater, than those that occur in the maturation of the normal map.
In fact, although we cannot make the point so strongly because our electrophysiological maps from VV eyes were poor, consideration of the information
about the development of the VV maps leads to similar conclusions. At the
earlier larval stages the projection from the retina, which in the adult will
form central retina, is to the medial aspect of the tectum. After metamorphosis
2
EMB 62
30
C. S T R A Z N I C K Y , R. M. GAZE AND M. J. KEATING
Fig. 9. Photomicrographs of transverse sections through rostral (upper) and
mid-tectal (lower) regions in an animal with an NN eye (NNx, stage 50). Bar = 100/«n.
The left tectum, receiving the projection from the NN eye, is to the right. It may
be seen that there is some diminution in the density of label rostrally on this tectum.
this retina projects to much more lateral tectum while newly grown dorsal and
ventral retina projects now to medial tectum.
With NN eyes the situation is somewhat different. The topography of the
NN projection is such that central (oldest) retina projects to rostral tectum
while the later appearing nasal and temporal retinal margins project to caudal
tectum, so that the growth patterns of retina and tectum in this particular
case are more congruent than normal.
With TT eyes, and probably with VV eyes, therefore, marked shifts in connectivity relations between retinal and tectal elements occur during the
maturation of the adult map, while the overall topological relations between
developing retina and tectum are preserved. The present results, particularly
those from TT projections, appear to dispose of the 'overgrown half-tectum'
hypothesis mentioned in the introduction. A large part of caudomedial tectum,
opposite a TT eye, develops a characteristic tectal structure in the absence of
optic innervation.
Developing compound eye projections in Xenopus
31
\v
_ - , - »•"»"»#"!Rifl^ V<£, '"*'•* **:.'
<**l--":*"T7^.wMpr-.v».»,__,Tr
*«*
•••i^^.^V^^.'Fig. 10. Photomicrographs of rostral (upper), mid-tectal (middle) and caudal (lower)
tectum in an animal with an NN compound eye (NN14, stage 61). Bar = 300 fim.
The left tectum, receiving the projection from the compound eye, is to the right.
It may be seen that this projection is less dense rostrally than caudally.
32
C. STRAZNICKY, R. M. GAZE AND M. J. KEATING
(a)
(b)
-N
//
/
/
///
/,
V\
\
\
\
\
\
//
// VX
\
\
\
\
\1\
Fig. 11. (a) Visuotectal map from an animal with an NN eye (NN17, stage 62),
showing the reduplication of field positions characteristic of NN projections.
Conventions as in Figure 3. (6) Reconstruction of the retinotectal projection in
this animal. Both tecta are completely covered by label and there is here no
decrease in density rostrally on the left tectum (right in Figure). Bar = 1 mm
(mediolateral). Arrowhead points rostrally.
The differential behaviour of the three types of 'compound eye' projection
requires comment. TT projections initially show a preference for rostrolateral
tectum during development, VV projections initially prefer medial tectum and
NN eyes initially innervate most densely the more caudal available tectal
area. Initially, fibres from TT eyes do not occupy the entire available tectal
surface; and both VV and NN eyes show a relative deficit in input to part of
the tectum. These clear selective preferences displayed by the different types
of compound eye for particular regions of the then-existing tectum appear to
mimic the nature of the mature normal adult projection.
In the developing Xenopus visual system we therefore seem to have two
Developing compound eye projections
in Xenopus
33
conflicting phenomena. The first is that the re-adjustment of connexions
required by the different modes of growth of retina and tectum argues against
a fixed system of tectal place-specific markers. The second is that the initial
differences between the projections from NN, TT and VV eyes indicate that
some distinguishing cues exist at the tectal end of the system. We suggest the
following mechanisms to account for the findings.
We assume that an optic fibre from any part of the retina can establish
connexion, in principle, with cells in any part of the tectum. The ordering of
fibres that exists within the optic tract (Steedman, Stirling & Gaze, 1979; and
unpublished) may be sufficient, from the earliest stages of development, to
deliver temporal retinal fibres preferentially to rostral tectum, ventral fibres
to medial tectum (Straznicky et ah 1979) and nasal fibres to the most caudal
regions of the available tectum. To account for the more limited spreading
of TT and VV projections, in comparison with NN projections, we propose
that newly arriving temporal and ventral fibres reach tectal areas already
innervated. To establish connexions in these areas they must disrupt established
connexions and displace the resident fibres more caudally (temporal fibres) or
laterally (ventral fibres) on the tectum. Newly arriving fibres from an NN
eye, on the other hand, are delivered from the tracts to newly generated regions
of the tectum and do not have to compete with a resident fibre population to
establish their connexions. It seems reasonable, therefore, that the covering
of available tectal space by fibres from an NN eye would take place more
easily, and hence more rapidly, than that by fibres from a TT or VV eye.
The present results, considered in conjunction with previous work in this
field, lead us to the view that several mechanisms co-operate in the establishment
of the retinotectal projection. It seems necessary to postulate that early developmental processes lead to the acquisition by retinal cells (and their fibres) of
place-related labels (Gaze, Feldman, Cooke & Chung, 1979; Sharma & Hollyfield, 1980). We think that the present evidence argues against the existence of
independently developed tectal positional markers. Even should further experiments (for instance, graft t r a n s l a t i o n on a virgin tectum) furnish evidence
in favour of independent tectal positional markers, it would be necessary to
postulate that such markers must be overridden by mechanisms such as fibre/
fibre interactions and competition among fibres for available tectal space.
Whilst interactions between labelled retinal fibres themselves, and between
the fibres and the tectum, seem sufficient to account for both the ordering and
the coverage of the projection, it does seem necessary to postulate in addition a
mechanism to orient the map on the tectal surface. This mechanism may
involve some relatively non-specific positional information on the tectum; but
we have drawn attention here to the possibility that orientation of the map
may be a consequence of fibre ordering in the tract.
We know little about the mechanisms leading to fibre ordering in the optic
tract; and until recently we did not even know that the tract showed retino-
34
C. STRAZNICKY, R. M. GAZE AND M. J. KEATING
topic order (Scalia & Fite, 1974; Steedman et ah 1979, and unpublished).
Attardi & Sperry (1963) showed that regenerating optic fibres in adult goldfish were able to select the correct branch .of the optic tract and they suggested
that pathways were ordered by selective affinities between fibres and their
substrates. This aspect of the problem has not received the further attention
it deserved until very recently.
In Xenopus, fibres are retinotopically grouped as they enter the optic nerve
but this ordering is progressively reduced as the fibres pass along the nerve
to the chiasma (Fawcett, 1980). Beyond the chiasma, however, the fibres again
show a retinotopic ordering, although the arrangement of the fibres is different
from that at the optic nerve head (Steedman, Stirling & Gaze, unpublished).
We do not know whether this tract order reflects specific fibre/substrate
interactions. An alternative possibility is that some rather low-grade positional
information influences the pathways selected by the earliest, pioneering, fibres,
and that thereafter order is built up by fibres selectively associating with closely
related fibres during growth. Clearly further information is required on the
mode of development of the optic pathway itself, to permit greater understanding of the way this factor interacts with others in the establishment of the
retinotectal projection.
We thank Mrs June Colville for expert histological assistance.
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JACOBSON,
(Received 6 August 1980, revised 15 October 1980)