/. Embryol. exp. Morph. Vol. 29, 3, pp. 697-712, 1973
697
Printed in Great Britain
Influence of retino-tectal innervation
on cell proliferation and cell migration in the
embryonic teleost tectum
By ERNEST SCHMATOLLA 1 AND
GEORG ERDMANN 1
From the Neuropathology Department and Neurochemistry Group,
Max-Planck Institute for Brain Research,
Frankfurt, West Germany
SUMMARY
The influence of retino-tectal innervation on cell proliferation and cell migration in the
optic tectum of the embryonic teleosts, Brachydanio rerio and Astyanax mexicamis, was
investigated in a combined autoradiographic and experimental embryological study.
Unilateral eye Anlage removal was the method used to compare development of the affected
non-optic-innervated tectum with that of its normal opposite innervated tectum.
The amount of mitosis as estimated by [3H]thymidine incorporation was not stimulated
by retino-tectal innervation. Likewise no increase in cell number was found in the innervated
tectum.
Cell migration and differentiation in the marginal zone was enhanced by retino-tectal
innervation.
Conflicting results concerning proliferation, migration and differentiation in different
species are explained by differences in time of ingrowth of the optic nerves and their depth
of penetration into the tectum.
INTRODUCTION
The optic tectum of the teleost offers an excellent model in which to study the
effect of afferent innervation on the development of a nervous system centre.
Retino-tectal axons from the retinal ganglion neurons of the developing eye
grow into the mesencephalon very early in development. In less than 72 h of
development, optic endings have reached the deepest layer of the subventricular
zone of the developing optic tectum before significant tectal cell proliferation,
differentiation and morphogenesis (cell migration and layer formation) have
taken place (Schmatolla & Fischer, 1972). In a previous study it was shown that
cell size and differentiation of neurons in the teleost tectum are dependent upon
optic innervation (Schmatolla, 1972). This present study is concerned with the
influence oi letino-tectal innervation on cell proliferation and migration in this
model.
1
Authors' address: Max-Planck-Institut fur Hirnforschung, Deutschordenstrasse 46,
6 Frankfurt a. M.-Niederrad, West Germany.
45-2
698
E. SCHMATOLLA AND G. ERDMANN
Early removal of the optic Anlage, before retino-tectal connexions are established, enables one to compare development of the affected contralateral nonoptic-innervated tectum with that of its normal opposite innervated tectum.
This method has been used by a number of investigators to examine the influence of optic innervation on tectal development in a variety of species with
conflicting results. In fish Pflugfelder (1952), studying Xiphophorus and Lebistes,
found no change in tectal cell number, but reported an increase in nuclear size
in the innervated tectal side. White (1948) in Fundulus, however, found an
increase in cell number in the innervated side, but made no comment as to
change in cell size. Neither investigator made observations as to differences in
mitotic activity or cell migration. In amphibians, Kollros (1953) in Rana
pipiens and Larsell (1929, 1931) in Hyla regilla reported an increase in cell
number in the superficial tectal layers of the innervated side, but they made no
attempt to count cell numbers in the deep tectal layers. Kollros reported an
increase in mitosis in the innervated side. Eichler (1971) in Rana pipiens found
no difference in cell size in the innervated side, whereas Larsell (1931) in Hyla
regilla observed an increase in cell size. Eichler reported increased mitosis in
the innervated side as well as increase in cell number in all tectal layers. In the
chick Filogamo (1950) described no differences in the development of the
tectum up to the 12th day of development in the affected side; after the 12th
day he reported a decrease in cell number on the affected side. Kelly & Cowan
(1972) found no effect of 'deafferentation' upon cell differentiation, cell proliferation or migration in the chick tectum up to the 14th day of development,
but thereafter report a considerable degree of cell loss in the affected side. Cowan,
Martin & Wenger (1968) found no effect of early eye removal on the mitotic
rate in the chick tectum.
During our previous study in the teleost (Schmatolla, 1972) we observed that
even though the affected optic lobe was smaller than the control side, largely
due to the poor dendrite development and absence of optic fibres, its cells
were smaller and more closely packed, suggesting the possibility that there
may be no difference or perhaps even an increase in cell number. In view of the
conflicting conclusions of the reports of Pflugfelder (1952) and of White (1948),
and differences observed in other species, we felt that the influence of retinotectal innervation on cell proliferation in the teleost required further
clarification.
The early works of Larsell and Kollros cited above are partly responsible for
the prevalent idea that innervation of the tectum by optic fibres stimulates proliferation. Since they only counted cells in the peripheral layers of the tectum,
the noted increase in cell number may have been a reflexion of an increase in
cell migration into the marginal zone on the innervated side, with total cell number
being unaffected. For this reason, in addition to evaluating the effect of innervation on cell number in the sub ventricular zone, we also report on its effect on
cell number in the marginal zone of the embryonic tectum. Our results indicate
Retino-tectal innervation
699
that in the teleost, optic innervation does not promote cell proliferation, and
on the contrary may even inhibit it, but innervation does appear to increase cell
migration into the marginal zone.
METHODS
The amount of mitosis was considered to be represented by the number of
cells labelled with [3H]thymidine in autoradiographs. For this purpose, eye
Anlage were removed unilaterally from 24 Brachydanio rerio embryos, 30-36 h
old, as previously described (Schmatolla, 1972). Each 24 h, from 48 h to the
10th day of development, at least two of the eye-extirpated embryos were
injected into the cardiac region with 0-05 /.iC\ of [3H]thymidine methyl, sp. act.
6-7 Ci/mM in 0001 jid of aqueous solution. After 24 h following injections, each
group of embryos was killed and fixed in Carnoy's solution for 4h. After
fixation the specimens were embedded in paraffin, and transverse sections at
5 /im thickness were made serially through the entire optic tectum. Following
routine processing of the paraffin sections, autoradiographs were made with
Uford L4 emulsion employing the dipping technique of Kopriwa & Leblond
(1962). Exposure time varied from 8 to 12 days. The autoradiographs were
developed with Kodak D 19b at 20 °C for 6 min, fixed, and permanently
mounted with Eukitt (O. Kindler, Freiburg, West Germany). Labelled cells
in the serial sections were counted in unstained autoradiographs with the aid
of phase-contrast microscopy.
For the evaluation of cell number in the subventricular zones of 1-week-old
embryos, one Astyanax mexicanus and two Brachydanio rerio embryos, each
with one eye extirpated, were used. These were routinely embedded in paraffin,
stained with haematoxylin and eosin, and cut at 7 pirn thickness. Photomicrographs were taken at x 400 of each serial section comprising the total tectal
length of each animal - a total of 111 photographs. These were enlarged to an
18 x 24 cm format, and with the aid of the automatic marker and counter of the
Zeiss TGZ 3 Particle Size Analyser, all cells in the innervated and affected tectal
sides were counted.
Marginal cell numbers were obtained by counting all cells in the tectal peripheral white matter (marginal zone) in serial sections of embryos whose optic
Anlage were removed at 30-36 h of development. The counts were done from
photomicrographs taken at x 400 of the serial sections in specimens 4, 7, 20,
14, 28 and 42 days old.
Since the actual cell counts included cells and fragments of cells, and because
cells in the non-innervated tecta were smaller than those in the innervated tecta,
the numbers of cells reported in the results represent cell counts corrected by
the application of the formula of Abercrombie (1946). The differing cell sizes
used in the calculations were derived from the previously published study on
cell differentiation in these fishes (Schmatolla, 1972).
The specimens reported on for evaluation of the cell number in the subven-
700
E. SCHMATOLLA AND G. ERDMANN
Table 1. Comparison of number of cells labelled with [3H]thymidine in matrixzones of optic tecta following unilateral eye Anlage extirpation in Brachydanio
rerio embryos*
No. of
labelled
cells in
control side
No. of
labelled
cells in
affected side
399
201
275
141
154
150
132
405
219
302
160
193
191
145
Animal
Age when
killed
(days)
Til
T3
T4
T2
T18
T8
T7
T20
3
4
4
5
5
6
6
6
T6
7
94
76
144
101
T5
T13
T22
7
124
109
8
9
70
92
12
2073
6
Totals 1822
Change in
affected
side
(%)
+ 1-5
+ 8-9
+ 9-8
+ 13-5
+ 25-3
+ 27-3
+ 9-8
+ 53-2
+ 32-9
-121
+ 31-4
+ 1000
+ 13-78
3
* Each embryo given [ H]thymidine injection 24 h before killing for autoradiography.
tricular zone, and the cell number in the marginal zone, were from the same
groups reported in the above study on cell differentiation.
RESULTS
Effect of innervation on mitosis
In the teleost optic tectum mitosis is confined to well-demarcated areas of the
embryonic grey matter. As described by Kirsche (1960) and by Richter &
Kranz (1970), and confirmed by our observations in both haematoxylin stained
specimens and in [3H]-thymidine-labelled preparations, these regions, the socalled 'matrix zones', consist of (1) a dorsal matrix zone in the medial tectal
corner, (2) a basal matrix zone in the lateral tectal corner, and (3) a caudal
matrix zone which represents the union of the above two zones in the caudal
region of the tectum (Fig. 1). The overall ependymal zone does not produce
new cells, and mitosis was not seen in the subventricular zone outside the matrix
zones. No mitosis was seen in the cells that had migrated out of the subventricular zone into the marginal zone.
Following [3H]thymidine injections from the 2nd to the 8th day of development, the number of labelled cells at sacrifice 24 h later are shown in Table 1
and Fig. 2. The total number of labelled cells seen at each day of development
represents the sum seen in serial sections of the entire tecta in the three matrix
zones. Generally the most obvious difference in the number of labelled cells
Retino-tectal innervation
701
US*
FIGURE 1
(A) Autoradiograph of transverse section through mid-tectum of 3-day-old
Brachydanio rerio embryo showing [3H]thymidine-labelled dorsal matrix zone
(dim) and basal matrix zone {bmz). [3H]Thymidine, 005 /*Ci given at 48 h of age,
embryo killed 24 h later.
(B) Caudal section through same tectum showing caudal matrix zone (cmz).
Unstained, phase-contrast.
702
E. SCHMATOLLA AND G. ERDMANN
700
r
Fig. 2. Effect of early eye removal on number of [3H]thymidine-labelled cells in
matrix zones in embryonic tecta of Brachydanio rerio. Note slight increase in mitosis
in non-innervated side. The data graphically represented here are derived from
Table 1. The dots for days 4, 5, 6 and 7 represent mean figures. # — # , Absent
retino-tectal innervation. O - - O , Control contralateral side.
between the control and affected side was seen in the basal matrix zone and in
the rostral portion of the caudal matrix zone.
Inspection of the curves in Fig. 2 shows that by the 9th day of development
very few labelled cells were present. From observations on older embryos in
material stained with haematoxylin and eosin, mitosis was never seen in the
matrix zones of Brachydanio after the 10th day of development.
Of the 12 experimental animals, 11 showed more labelled cells in the noninnervated side, with an over-all increase in the entire series of 13-8% o n the
affected side. The 3-day-old animal might be expected to show little effect of
optic innervation, since at 72 h the optic nerve has been in the tectum for less
than 24 h. It is difficult to determine how significant a 13-8 % increase in labelled
cells should be considered, but these data certainly suggest that mitosis is not
greater on the innervated side, and may actually be less.
Effect of innervation on cell number in the subventricular zone
Tectal cell number was evaluated in 1-week-old Brachydanio rerio and
Astyanax mexicanus embryos following unilateral eye removal. In the embryonic
tectum at this age the vast majority of cells are confined to the so-called sub-
Retino-tectal innervation
703
Fig. 3. One-week-old Astyanax mexicanus, transverse section through mid-tectum.
Non-innervated tectum (non-inn) is overall smaller than the innervated (inn), its cells
in the subventricular zone (subv) are smaller and more numerous, and there are
fewer cells (at arrows) in the marginal zone (marg). H and E.
ventricular zone. Only a small percentage of cells (less than 10 %) have migrated
out into the marginal zone (Fig. 3). In three embryos whose eye An/age were
removed unilaterally before optic nerve development, all cells were counted in
serial sections of the entire tecta on both sides. Casual inspection of a mid-tectal
section of one of these embryos was enough to distinguish the affected side
from the control side (Fig. 3). In addition to the reduction in overall size of the
affected side, its cells appeared smaller, more tightly packed and more numerous. This difference could be seen as early as the 3rd day of development,
and persisted well into the post-embryonic period at 6 weeks, at which age our
observations were terminated (Figs. 4, 5). However, inspection of the caudal
ends of the tecta did not reveal this consistently distinct difference in appearance.
The counts of subventricular cells up to the caudal end of the tectum (Table 2)
showed that there were between 18 % and 20% more cells in the affected side
at 1 week of age. But when counts of the subventricular cells of the caudal ends,
and of the marginal cells, were added in, the difference in cell number was not
so striking, varying from 4-7 % to 11-1 %. The chief point to emerge from these
data is that one can say with some certainty that the number of cells in the
innervated side was not increased, but actually may have been slightly decreased.
704
E. SCHMATOLLA AND G. ERDMANN
Retino-tectal innervation
705
Effect of innervation on marginal cells
In the embryonic stages cells migrate out of the subventricular zone to populate the marginal zone. The resultant teleost adult tectum consists of a densely
cellular stratum griseum periventriculare containing the residual majority of
cells that did not migrate out of the embryonic subventricular zone, and a number of rather sparsely cellular and poorly demarcated layers formed by these
migrated cells (Leghissa, 1955; Schmatolla, 1972). Our observations show that
in Brachydanio, as early as the 4th day of development, there is an increase in
the number of marginal cells in the innervated side as compared to the noninnervated side. This difference is clearly maintained through the embryonic
and early juvenile period of development (Table 3 A). Mitosis was never seen
in either side in the cells in the marginal zone. In addition to an increase in the
number of marginal cells in the innervated side, there was also noted a distinct
increase in their fibre development and cell and nuclear size (Fig. 4B, C),
indicating better differentiation of these cells over their counterparts in the noninnervated tectum. In general, the marginal cells in the innervated tectum
tended to migrate in greater numbers to the very superficial margin of the
tectum. In the affected side the marginal cells remained more restricted to the
inner two-thirds of the marginal zone.
In Table 3B is demonstrated the effect of the absence of retino-tectal innervation on marginal cell number in the 1-week-old Astyanax mexicanus embryo.
The percentage increase in marginal cell number due to innervation is of the
same magnitude as in Brachydanio. The blind cave Astyanax hubbsi, the
naturally occurring closely related Astyanax whose tectum contains no optic
fibres (Schmatolla, 1972), offers an opportunity to compare the effects of the
natural absence of innervation to that of extirpation of the eye in Astyanax
mexicanus. Here too, as seen in Table 3B, innervation in the A. mexicanus
tectum results in an increase in marginal cell number over the non-innervated
tectum of the cave fish.
FIGURE 4
Six-week-old Brachydanio rerio; transverse section through mid-tectum. Bodian
stain.
(A) Innervated side (inn) is larger than non-innervated side (non-inn), its marginal
zone contains more cells (at arrows) than affected side.
(B) Higher magnification of marginal zone of innervated side.
(C) Higher magnification of marginal zone of non-innervated side. Note larger
and more numerous marginal cells in innervated side.
706
E. SCHMATOLLA AND G. ERDMANN
Fig. 5. Cells in subventricular zones of same animal as in Fig. 4. Cells in innervated
side (A) are larger and more loosely packed than in the subventricular zone of the
non-innervated side (B), where they are smaller and more numerous. Bodian stain.
4 104
3 393
2 671
8 903
6 094
5 437
+ 20-3
+ 17-8
+ 18-5
CS
AS
4 054
3 581
2 342
AS
-1-2
+5-5
-12-3
Change
in AS
(%)
Cell no. in
caudal end
11 504
8 564
7 7.57
CS
r
12 958
9 675
7 779
AS
+ 12-6
+ 130
+ 7-2
Change
in AS
Cell no. in
entire tectum
^
1451
289
401
CS
950
162
238
AS
+ 52-7
+ 78-4
+ 68-5
Change
inCS
Cell no. in
entire tectum
i
Marginal zone
12 955
8 853
7 658
CS
r
13 908
9837
8 017
AS
+7-4
+11-1
+4-7
Change
in AS
(%)
Total marginal and
subventricular zone
* Eye Anlage removed before optic nerve outgrowth; stage 13 (Cahn, 1958) in Astyanax, and stage 21 (Hisaoka & Battle, 1958) in Brachydanio.
CS = control side. AS = affected side. Change in AS (%) = % change in affected side. Change in CS (%) = % change in control side.
Astyanax mexicanus 7 399
5 171
Brachydanio rerio
4 586
Brachydanio rerio
CS
Change
in AS
Cell no . to beginning of
caudal end
Subventricular zone
Table 2. Comparison of cell number in affected and control optic tecta in 1-week-old teleost embryos following
unilateral eye Anlage extirpation*
708
E. SCHMATOLLA AND G. ERDMANN
Table 3. Effect of retino-tectal innervation on marginal cell number
Specimen
no.
Age in
(days)
T3
T4
054
305
256
257
024
025
023/4
023/1
4
4
7
7
10
10
14
14
28
42
259
(A. mexicanus)
288
(A. mexicanus)*
016A
(A. hubbsi)*
No. of
sections
counted
(entire
tectum)
Total no. of marginal cells
per tectum
Innervated
side
A. Brachydanio ren'o
20
169
18
120
24
289
401
19
27
525
432
26
32
768
901
34
52
5460
8230
60
Noninnervated
side
Change in
innervated
side
(%)
93
75
162
238
254
251
645
766
4270
6659
+ 81-7
+ 600
+ 78-4
+ 68-5
+ 106-7
+ 72-1
+ 191
+ 17-6
+ 27-9
+ 23-6
B. Astyanax mexicanus and Astyanax hubbsi (blind cave)
7
42
1451
950
7
48
2042
—
7
46
—
996
+ 52-7
+ 1050
* Embryos 288 and 016A are normal controls. The blind A. hubbsi is treated as a noninnervated side.
DISCUSSION
A conservative interpretation of our data regarding the effect of retino-tectal
innervation in the teleost tectum on mitosis would indicate that innervation does
not stimulate mitosis. Actually the data suggest that mitosis is slightly retarded
in the matrix zones of the innervated tecta. From our previous study in which
we demonstrated that the neurons in the innervated tectum of the teleost are
better differentiated than those in the non-innervated tectum, one could speculate that innervation stimulates differentiation, which in turn inhibits mitosis.
This is consistent with the observation that differentiated neurons loss their
capacity to undergo mitosis. However, it may also be plausible that innervation
inhibits mitosis, which then allows the no-longer-dividing cell to differentiate.
The experiments of Kirsche & Kirsche (1961) and Richter (1968) on regeneration of the optic tectum in teleost fish are of interest in relation to the effect of
tectal innervation on matrix-zone activity. These investigators destroyed large
portions of the optic tecta of juvenile and adult Carassius and Lebistes teleosts,
and showed that an increase in the mitotic activity of the matrix zones
The authors wish to thank Lotte Ostertag for her assistance in the preparation of the
histological material.
Retino-tectal innervation
709
occurred which was followed by tectal regeneration. In effect, the destruction
of the tectum resulted in an interruption of retino-tectal innervation, resulting
in deafferentation and concomitant increase in the mitotic activity of the
matrix zones, which is in some respects similar to our observation of increase
in mitotic activity in the matrix zones in our non-innervated tecta. It is of course
unclear whether retino-tectal innervation has a direct effect through contact
with cells in the matrix zones, or whether some other mechanism is involved.
Richter (1968) found mitotic activity in the matrix zones of adult Lebistes,
whereas Rahmann (1968) found no mitotic activity in the matrix zones of
juvenile and adult Brachydanio rerio. Our findings in Brachydanio also indicate
that mitotic activity ceases in this species in the late embryonic period. It would
be of interest to see if regeneration, and the activity of mitosis in the matrix
zones would be stimulated by tectal resection in Brachydanio.
In our experimental material it may be of significance that the basal matrix
zone, in the lateral corner of the embryonic tectum, was observed to be most
affected by tetino-tectal innervation, since it is through this area that the optic
fibres enter in the greatest portion of the teleost tectum.
In the chick, in a study of the effects of early eye removal (2nd day of development), Cowan et al. (1968) conclude that innervation has no effect on
tectal mitosis. Here species differences play an important role. This result was
to have been expected since optic-fibre penetration of the stratum opticum of
the chick tectum does not take place until after the 10th day of development
(Kelly & Cowan, 1972), by which time mitosis has virtually ceased as shown
by the [3H]thymidine incorporation studies of Fujita (1964). Therefore the conclusion that early eye removal has no effect on tectal mitosis in the chick tectum
is irrelevant, for comparing mitosis in a normal side to an 'affected' side has no
meaning since both sides have no retino-tectal innervation until after mitosis
has almost ceased.
Cell numbers in the subventricular zones in control versus affected tecta
parallel the effects on mitosis. With a good deal of certainty one can say that
there is no increase in cell number on the innervated side; actually the data
suggest that cell number may be increased in the non-innervated side. This
would be expected if the mitotic activity is also greater.
The clear increase in number of cells seen in the rostral portion of the
rectum up to the caudal end in the affected side, as contrasted to the inconsistent findings in the caudal end, suggests a regionalized effect. Optic fibres
in the fish embryo enter the tectum in the lateral-rostral portion of the optic
lobes, and spread medially and caudally, with the caudal end last to be innervated. It is possible that much of the caudal ends on both sides never develops
retino-tectal innervation. This remains to be clarified.
The fact that proliferation takes place at least equally as well in the noninnervated side demonstrates that tectal cell proliferation in the teleost is
independent of retino-tectal innervation. Our cell counts were made in early
710
E. SCHMATOLLA AND G. ERDMANN
embryos for we were interested in the effect of innervation on early cell proliferation. It is possible that observations on older animals following deafferentation would show more cells in the innervated side due to failure of trophic
maintenance in the non-innervated side.
The most plausible conclusion to be drawn from the clear increase in marginal
cell number in the innervated tectum would be that the presence of retinotectal innervation promotes migration out of the subventricular zone. That
mitotic figures or incorporation of [3H]thymidine was never seen in these cells
in the marginal zone is evidence against the possibility that the increase in marginal cell number was due to local proliferation after the cells had migrated out.
Cell death as seen in the later stages of development in the chick deafferentated
tectum (Filogamo, 1950; Kelly & Cowan, 1972) is not a likely explanation for
the differences in marginal cell numbers reported here, since these differences
seen in the teleost occur very early in development, and are greater in the
younger than in the older specimens (Table 3). The marginal cells in the
innervated side are distinctly larger and have better fibre development than
those in the non-innervated side, paralleling our previous findings on the
promotion of differentiation by innervation within the subventricular zone
(Schmatolla, 1972).
How retino-tectal innervation stimulates the outward migration is unknown,
but several possibilities exist. Since the retino-tectal fibres themselves penetrate
the entire marginal zone to enter the subventricular zone perpendicularly, they
offer a scaffold of fibres which can guide migrating cells and support their
movement. Also, the subventricular cells in the innervated side differentiate
more completely and send apical dendrites perpendicularly into the marginal
zone which could also aid in migration. A somewhat similar mechanism of the
reliance of migration on orientation of fibres has been proposed by DeLong
& Sidman (1970) in discussing the failure of granule cell migration through
the disorganized malaligned Purkinje cell layer in the reeler mutant mouse. And
lastly, it may be that these better-differentiated cells have intrinsically a better
mobility.
Species differences appear to play a great role in explaining the conflicting
findings in the literature concerning the relationship of optic innervation to
tectal development. For example, Larsell (1931) in Hyla found an increase in
cell size in the innervated tectum, whereas Eichler (1971) in Rana found no
difference. Both report finding an increase in cell numbers in innervated tecta.
In the teleost we have found an increase in cell size in the innervated tectum
(Schmatolla, 1972), but no increase in cell number, only an increase in cell
migration. Kollros (1968) comments, in relation to these points of species
differences, and limiting his discussion to anurans: ' . . . [upon] the potential
variability of the regulating mechanisms [between species]. In the optic tectum,
for example, the regulation of tectal size in response to the absence of eye is
dependent upon control of mitotic rate in some species, upon regulation of
Retino-tectal innervation
111
cell size in another, upon production and then later degeneration of cells in
a third, while in a fourth type cell production is unaffected but only a fraction
of the usual cell number undergoes maturation. The findings of inconsistencies
between different anuran species need not be unexpected.' Certainly one may
consider these points to be even more applicable to the differences seen between
anurans and teleosts.
In the chick, from the recent work of Kelly & Cowan (1972), it is apparent
that cell proliferation, differentiation, cell migration and stratification of the
tectum is rather well completed by the 12th day of development when the optic
fibres penetrate the tectum, so that one can clearly say that these phenomena are
not dependent upon retino-tectal innervation. But in the teleost, whose tectum
is innervated very early in development, during the 3rd day, differentiation and
cell migration are dependent upon retino-tectal innervation, but cell proliferation, as in the chick, appears to be an independent process, possibly even
hindered by innervation and differentiation.
Between the chick, and the teleosts we have investigated, the important
variable accounting for the differences in findings is relatable to the developmental behaviour of the optic nerves. In the teleost the optic fibres grow into
the tectum early and penetrate to its depths, while in the chick they grow in
late and end in the superficial regions. For this reason we feel that the teleost
offers advantages with which to analyse the dependence of tectal development
on optic innervation. The chick, while a poor model for this purpose, raises
other questions, such as what are the driving mechanisms behind its tectal
development if not optic innervation?
The authors wish to thank Lotte Ostertag for her assistance in the preparation of the
histological material.
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