Download Growth of Fish Retinas1

AMER. ZOOL., 21:447-458 (1981)
Growth of Fish Retinas 1
PAMELA RAYMOND JOHNS
Department of Neurobiology, Harvard Medical School,
Boston, Massachusetts 02115
SYNOPSIS. This review discusses development and growth of the retina. A geometric model
of retinal differentiation is proposed in which four phases are recognized; the first three
are common to all vertebrate embryos. The last, post-embryonic growth phase has two
alternate routes, one followed by birds and mammals and the other by fish and amphibians. All retinas grow by expansion, and retinal cells are spread apart as the retina enlarges.
In fish (and larval amphibians), the retina not only expands but also adds cells. In these
retinas a marginal germinal zone persists and continues to produce neurons, which are
added appositionally in concentric annuh around the perimeter. The genesis of one class
of photoreceptor cell, the retinal rods, is different from all other retinal neurons in that
the proportion of rods increases with growth in central retinal regions far from the germinal zone. The source of these centrally-added rods is not yet established; several hypotheses are discussed. Alterations of synaptic connectivity within the retina and between
retina and brain are suggested by the pattern of growth and cell addition. The capacity
of adult fishes to generate new neurons and to form new synapses is a remarkable property, one which most animals abandon much earlier in life.
INTRODUCTION
A beginning student of embryology
when watching a developing embryo for
the first time is immediately impressed by
the eyes—they appear so early and quickly
grow so large. Perhaps because they are
such a prominent feature of the developing embryo, but certainly also because the
eyes are such complex and exquisitely specialized organs, the study of their development has always fascinated and challenged embryologists. Surprisingly little
attention has been paid to the growth and
development of eyes beyond embryonic
stages. Eyes do grow post-embryonically,
and in some animals, such as fish, the eyes
increase enormously in size during juvenile and adult life. My work and that of
others has shown that in the retinas of fish,
growth results in part from the addition of
new neurons. A mature, functioning piece
of nervous tissue that adds neurons as it
grows is remarkable because neural function is to a large extent determined by the
precision of connections between neurons.
How then are new neuronal elements inserted into the circuit without interfering
with ongoing activity? I cannot yet answer
that question, but I do have some clues as
to how and where new cells are added.
The work reviewed here mainly concerns the morphology of growth, but the
underlying motive for looking at structure
is to understand function. At the end, I
will mention what little we do know about
how vision is affected by retinal growth.
Before considering growth I must first
briefly describe the structure of the adult
retina. More details specific to fish retinas
and the problems of growth will be added
in subsequent sections.
STRUCTURE OF THE RETINA
The vertebrate retina is a thin sheet of
neural tissue, approximately hemispherical in shape. Figure 1 is a photomicrograph of a fish retina; all retinas have a
similar laminar organization (Polyak, 1957;
Walls, 1967; Ramon y Cajal, 1973). At the
outer (convex) surface are the photoreceptor cells, specialized sensory neurons that
respond to light. Most retinas have two
kinds: rods, which function at low levels of
illumination, and cones, which are active
in brighter light (Rodieck, 1973). The nuclei of photoreceptor cells are in the outer
nuclear layer (ONL); those of the cones
1
From the Symposium on Developmental Biology of
are at the level of the outer limiting memFishes presented at the Annual Meeting of the American Society of Zoologists, 27—30 December 1979, at brane and those of the rods are closer to
the internal retinal layers.
Tampa, Florida.
447
448
PAMELA RAYMOND JOHNS
OFL
FIG. 1. This photomicrograph is of the retina of a goldfish. The inside of the eyeball is downwards. Photoreceptor cells (PC), surrounded by finger-like cytoplasmic processes of pigmented epithelial cells, are at the
top. The cones are large, pale and ovoid; the rods are long and slender and closer to the pigmented epithelium.
Their nuclei in the outer nuclear layer (OXL) are at different levels; the cones are indicated by the small open
arrow, and the rods by the filled arrow. The inner nuclear layer (INL) contains several types of cells. The ganglion cells (GC) and their axons in the optic fiber layer (OFL) are at the inner surface. The internal and external
limiting membranes are indicated by the arrows on the right. The calibration bar is 50 pm.
The next layer of cells is the inner nuclear layer (INL), which contains the nuclei
of several types of neurons (bipolar, araacrine and horizontal cells) as well as those
of the primary retinal glia (Miiller cells).
In the cellular layer nearest the inner surface of the retina are the nuclei of ganglion
cells (GC). Separating these three nuclear
layers are two plexiform layers in which
cytoplasmic processes of retinal neurons
GROWTH OF FISH RETINAS
make synaptic connections with one
another. The axons of ganglion cells, the
optic fibers, form a discrete layer at the
inner retinal surface (OFL). A vascular layer is interposed between them and the vitr e o u s humor, the gel-like substance that
Tills the eyeball.
Next I will describe the ways in which
retinas grow. It is instructive to look at earlier, embryonic stages, before considering
post-embryonic growth, because later
growth of fish retinas is in many ways
merely an extension of patterns and activities begun in the embryo.
EMBRYONIC DEVELOPMENT OF RETINAS
Steps in the formation of eye and retina
are similar in all vertebrates (O'Rahilly and
Meyer, 1959; Mann, 1969; O'Rahilly,
1975). The basic pattern of retinal histogenesis was deduced from early studies,
particularly those of Ramon y Cajal (1959)
who used classical histological methods.
Recent electron microscopic studies (Hattori and Fujita, 1974; Hinds and Hinds,
1974; Smelser et al., 1974, and many others) have provided additional information,
as have autoradiographical studies that
use [3H]thymidine, a specific precursor of
DNA, as a marker for dividing cells (Sidman, 1960, 1970; Fujita and Horii, 1963;
Hollyfield, 1972; Kahn, 1974; and again
many others). At the earliest stages the
presumptive neural retina is composed of
a hemispherical sheet of undifferentiated
neuroepithelial germinal cells. These are
elongated, spindle-shaped cells, which
span the width of the neuroepithelium.
The germinal cells have two critical roles:
i) they generate the retinal cells and ii) they
provide a vertical scaffolding around
which neuronal connections are organized.
Differentiation of retinal neurons does
not occur simultaneously throughout the
embryonic retina, but begins in the center.
Figure 2 is a general model of the spatiotemporal pattern of retinal differentiation.
Phase I represents the undifferentiated
neuroepithelium; dividing germinal cells
are indicated by "x's." Mitotic activity ceases and differentiation begins in the center
(the "o's" in Fig. 2, phase II). Retina is con-
449
structed concentrically as more cells differentiate within an ever-widening circle
(phase III) until finally the perimeter of
the neuroepithelium is reached. Up to this
point, which occurs before or shortly after
the end of the embryonic period, all vertebrate retinas develop similarly. In the
last phase (IV) there are two paths, and a
choice must be made. The choice is to stop
or to continue producing retinal cells.
Birds and mammals choose the former,
and all of the neuroepithelial germinal
cells are consumed when the circular wave
of differentiation reaches the retinal margin (the upper diagram in Fig. 2, phase
IV). Cell production ceases and the retina
then contains its full complement of neurons (Johns et al., 1979). In fish and amphibians, by contrast, the germinal cells
are not exhausted; a few remain to constitute a circumferential germinal zone
whose cells continue to proliferate
throughout larval stages into adult life (the
lower diagram in Fig. 2, phase IV). In
these retinas, new neurons are added and
new retina is formed for a prolonged period as the eye continues to grow (Straznicky and Gaze, 1971; Jacobson, 1976;
Johns and Easter, 1977; Johns, 1977;
Beach, 1979).
POSTEMBRYONIC GROWTH OF FlSH RETINAS
Unlike mammals, fish continue to grow
throughout the life of the animal (Brown,
1957). The rate of growth is extremely
variable and difficult to predict, however,
as it depends upon a complex interplay of
many factors, including the availablity and
quality of food, the temperature of the
water, and the density of fish in the population. Growth rate also changes with age;
older fish tend to grow more slowly.
The eyes grow as the body does, but not
quite as rapidly, so that larger fish have
relatively smaller eyes (Miiller, 1952; Lyall,
1957a; Ali, 1964; Johns and Easter, 1977).
Inside the eye the retina also enlarges. In
goldfish (Carassius auratus) between 1 and
4 yr of age, for example, the retinal surface area increases from 20 to 120 mm2
(Johns and Easter, 1977). In young animals this dramatic increase in retinal size
is due in part to cell addition.
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PAMELA RAYMOND J O H N S
EMBRYONIC
POST-EMBRYONIC
FIG. 2. Four phases of vertebrate retinal development are shown. The retina is represented as a circle,
which in phase I contains only undifferentiated neuroepithelial germinal cells (x). Cells that have ceased
dividing (o) appear in phase II and increase in number in phase III. The vertical dashed line approximately divides embryonic from post-embryonic developmental periods, though the time of birth or
hatch relative to the state of retinal development varies in different species. In phase IV the upper diagram represents retinas in which cell proliferation
has ceased; further enlargement of the retina results
from growth of individual cells (the "o's" are larger).
The lower diagram shows how other retinas grow by
adding new cells ("o" with a slash) from a circumferential germinal zone of dividing cells (x) In these
retinas, the cells already present enlarge as well, and
contribute to the overall growth of the retina.
The germinal zone
Cells are added to growing fish retinas
at the margin. Here mitotic figures are
seen (Muller, 1952; Lyall, 1957a; Blaxter
and Jones, 1967). The dividing cells are
also selectively labeled with [3H]thymidine,
and examples are shown in Figure 3A, B.
The labeled cells are distinctive—notice in
particular their spindle-shaped nuclei. In
this and in other cytological features they
are identical to the neuroepithelial germinal cells of the embryonic retina. From
their appearance and position at the retinal perimeter I conclude that these cells
are the descendents of embryonic neuroepithelial germinal cells that never relinquished their capacity to proliferate.
The germinal zone in larval and adult
fish surrounds the entire retina and adds
new cells at the margin (Johns, 1977; Meyer, 1978). Figure 2, phase IV illustrates
how concentric annuli of new retina are
produced by the germinal zone. Some of
the autoradiographical results from which
this scheme was developed are illustrated
in Figure 3C. This is a histological section
from the retina of a goldfish killed
months after injection of [3H]thymidine
Labeled cells in this section are distributed
in a vertical band, which includes all three
nuclear layers. These labeled neurons are
the progeny of germinal cells that incorporated the label and differentiated soon
after. The unlabeled segment of retina
now interposed between the labeled cells
and the margin was produced in the subsequent 6 month interval by germinal cells
whose label was diluted by multiple divisions in the absence of radioactive precursor.
Further evidence that the germinal zone
produces neurons is the immature appearance of the retina adjacent to it (Muller,
1952; Johns, 1977). The most obvious
signs of immaturity are the stubby, undeveloped cones in the photoreceptor layer
near the margin; toward the center they
grow longer and larger (Fig. 4).
Addition of cells
How many cells is the germinal zone
producing? To answer that question we
counted retinal cells in goldfish from 5 to
20 cm in length. In the goldfish retina, regional differences in cell density are minor
and inconsistent (Stell and Harosi, 1976;
Johns and Easter, 1977), so total number
of cells was calculated from average cell
density. Cell number, excluding vascular
cells and glia in the optic fiber layer, is plotted in Figure 5; it increases from 3,000,000
to 8,000,000 cells as the retina grows from
4 to 8 mm in length. Muller (1952) counted cells in guppy retinas and found an
analogous increase in total cell number
from 525,000 to 1,900,000 as the fish grew
during the first year. In both guppies and
goldfish, this increase in total cell number
results from the addition of all types of
retinal cells. Others have counted neurons
in growing fish retinas but have confined
their counts to either cones or ganglion
cells.
The cones in most teleost fish are ar-
GROWTH OF FISH RETINAS
431
A
Fie. 3. A and B. This autoradiograph shown in both darkfield (A) and brightfield (B) illumination was
prepared from the retina of a cichlid fish (Haplochromis burtoni) injected with [3H]thymidine. Dividing cells at
the margin of the retina are labeled with silver grains. The retina extends toward the lower left corner;
pigmented epithelium is across the top. The calibration bar is 20 pirn. C. This darkfield autoradiograph is the
retina from a goldfish injected with [3H]thymidine six months previously. Labeled retinal cells are in a vertical
band (larger arrow), now displaced about 150 /urn from the retinal margin, which is at the right. The smaller
arrow indicates a labeled cell in the outer nuclear layer displaced centrally from the band of other labeled
cells (see text for further discussion). The calibration bar is 100 /xm. D. This is the retinal margin of a large
goldfish, 23 cm in length. No germinal zone is present. The calibration bar is 100 //.m.
ranged in a regular mosaic pattern (Engstrom, 1960, 1963), which makes them
easy to count in tangential sections. The
size of the mosaic unit and the size of individual cones increases as the retina
grows, but not enough to account for the
entire increase in retinal area. The conclusion is that mosaic units must be added
with growth (Lyall, 1957a; O'Connell,
1963; Ahlbert, 1968).
Ganglion cells can be conveniently
counted in intact flat mounts of the whole
retina, though they must be distinguished
from surrounding glia and vascular cells.
In crucian carp (Carassius carassius), a
species closely related to goldfish, Kock
and Reuter (1978) found that ganglion
cells increased steadily in number as eyes
grew from 4 to 7 mm' in diameter, but with
further increase in eye diameter up to 10
mm the number of ganglion cells remained constant. Their results imply that
the germinal zone finally ceased producing
ganglion cells in these large fish, which
were up to 7 yr old. Recently we have examined retinas from goldfish larger than
those used in our previous study and
found that in these older fish retinal cells
are no longer being added (Johns and
Easter, unpublished observations). In sup-
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PAMELA RAYMOND J O H N S
Germinal Zone
Zone of Differentiation
Differentiated Retina
FIG. 4. This is the peripheral retina of a medium-sized goldfish (12 cm in length). The large short arrow
points to germinal cells, one of which is in mitosis (M). The most peripheral retina (in the zone of differentiation) contains undifferentiated cells; immature cones (C) are especially distinctive. In more central, differentiated retina, the cones are larger and more mature. The calibration bar is 50 jim.
port of this apparent cessation of ganglion
cell addition, the retinal margin from a
very large goldfish in Figure 3D shows
no evidence of a germinal zone. (We have
not yet injected such large fish with
[3H]thymidine.) Counts of optic nerve fibers in electron micrographic mosaics of
cross sections through whole optic nerves
of these large goldfish also suggest that the
number of ganglion cell axons reaches a
plateau (Easter et ai, 1979). Despite the
waning of cell production, the eye and retina in old fish continue to enlarge, though
very slowly. Without cell addition the only
mechanism for further growth is expansion.
Expansion of the retina
By "expansion" I mean that the retinal
surface area enlarges and the cells are
spread apart. As an aside, it is worth pointing out that all retinas expand as the eye
grows to adult size. Histological studies of
postnatal development of the retina in several mammals indicate that the cellular layers become thinner as if the retina were
being stretched (Mann, 1969; Vogel, 1969;
Braeckevelt and Hollenberg, 1970). The
mechanism of expansion is unclear. Cou-
lombre and co-workers (Coulombre, 1955;
Coulombre et al, 1973) suggest that increases in intraocular pressure serve to inflate the eyeball and so stretch the retina.
In 1-yr-old goldfish injected with
[3H]thymidine, expansion of the retina was
demonstrated directly: The retina present
at the time of injection, and enclosed by
the labeled annulus, doubled in area after
one year (Johns, 1977). This increase in
the size of the existing retina accounted for
80% of the total increase in retinal area;
new retina added at the margin contributed the remaining 20%.
In large goldfish, which are no longer
adding retinal cells, the retina becomes
thinner as it expands, as illustrated in Figure 6. These are photomicrographs of sections from mid-dorsal retina of three sizes
of goldfish, small (A), medium (B) and
large (C). In the retina from the mediumsized fish (B), the density of cells is less
than in the small fish (A), as it must be
since the retinal area is increased. (The
rods are an exception and are discussed
below.) The overall thickness of the retina
is slightly increased in (B), presumably because the cells are bigger and have more
processes in the larger retina. In the larg-
453
GROWTH OF FISH RETINAS
est fish (C) growth is due entirely to expansion, which now stretches the retina (it
is thinner by about 50%) as well as spreading apart the cells. When all three retinas
in Figure 6 are compared, the increased
•>ize and decreased density of the cones
from A to B to C are obvious, as is the
depletion of cells in the inner nuclear and
ganglion cell layers. It is equally obvious,
however, that the density of rod nuclei in
the outer nuclear layer does not decrease
at all; if anything it is slightly increased in
the medium-sized fish. Miiller (1952) saw
the same pattern in growing guppy retinas—the density of rods remains constant
as the eye enlarges.
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The dilemma of the rods
The simple story of how these retinas
grow, add cells at the edges and spread
apart those in the middle, has now become
10
4
6
8
more complicated. The dilemma to be exRETINAL
LENGTH
(mm)
plained is: How do the rods maintain a
constant density while the retina expands? Fie. 5. This shows the increase in retinal cell numWe might imagine that new rods are in- ber with growth in goldfish. Each point is one retina.
serted into the outer nuclear layer (The different symbols represent different experithroughout the retina, but if so, where do mental groups which are not important for the presdiscussion.) On the abscissa is plotted an index of
they come from? The germinal cells at the ent
retinal size—the linear distance along the retina from
retinal margin were implicated as the one margin to the other in a meridional section
source of new retinal cells in the above dis- through the center of the eye.
cussion, but I also showed that the cells
produced by the germinal zone differentiate where they were born, at the perimeter. Simple apposition of new cells at the already present in the inner nuclear layer,
margin, as shown in Figure 2, phase IV, which then migrate when needed into the
cannot, however, explain how the ratio of outer nuclear layer and there differenrods increases in central regions far from tiate. This is not a new idea, nor is it
the germinal zone. The problem is even unique to fish: Bernard (1900) and Gliicksmore perplexing if we consider the retinas mann (1940) reached similar conclusions
of larval fish. Blaxter and co-workers from histological studies of developing
(Blaxter and Jones, 1967; Blaxter and amphibian retinas. If movement of cells
Staines, 1970; Blaxter, 1975) have shown from inner to outer layers continues in the
that in ten different species of teleost fish, growing adult retina, it would account for
in which the adults have both rods and the observed increase in the ratio of rods.
cones, the larvae have only cones in an oth- A depletion of cells in the inner nuclear
erwise differentiated retina. At metamor- layer would be required, and there is some
phosis rods suddenly appear interspersed evidence that such a loss occurs with
among the cones. Similar observations growth in herring (Blaxter and Jones,
have been made by Ali (1959) and Lyall 1967) and in goldfish (Johns and Easter,
(1957a) in two other species. Because they 1977). In goldfish during the first four
saw no obvious evidence of mitotic activity years of life, however, the number of cells
in central retina, Blaxter and Jones (1967) lost from the inner nuclear layer is only
suggested that rods may develop from cells about one half the number acquired by the
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PAMELA RAYMOND J O H N S
FIG. 6. These three photomicrographs compare equivalent retinal regions in three goldfish, 6 cm (A), 12 cm
(B) and 23 cm (C) in length. The photomicrographs are aligned at the level of the external limiting membrane.
Bending of the photoreceptor cell processes is an artifact of the histological preparation. The calibration bar
is 100 /xm.
outer nuclear layer, so it is unlikely that
this is the complete explanation (Johns,
1977).
Another potential source of rods is suggested by the results from autoradiography of the growing retinas of juvenile and
adult fish. Following injections of
[3H]thymidine, labeled nuclei, presumably
those of rods, have been observed in the
outer nuclear layer displaced from the annular region that contains other labeled
nuclei. These cells spread centrally into the
outer nuclear layer; an example is shown
in Figure 3C. That they are rods is suggested, but not proven, by their location in
the outer nuclear layer, and by their persistence for months or years (Johns, 1977;
Meyer, 1978). Their origin is unclear.
Scholes (1976) suggested that proliferating
cells which generate rods in the Black Molly retina are not confined to the germinal
zone, but are found in the outer nuclear
layer adjacent to the margin. He saw occasional mitotic figures and sometimes labeled cells there. Meyer (1978) also saw labeled cells in the outer nuclear layer
displaced 200 /u,m or more from the labeled annulus, but not until eleven or
more days following injections in goldfish.
I saw a similar distribution of labeled cells,
again in goldfish retinas, at 80-336 days
(Johns, 1977). The apparent delay in the
appearance of these displaced labeled cells
suggests the possibility that they may have
been generated at the margin and then
migrated centrally. It is not easy to distinguish between these two possibilities—a
scattered population of proliferating rod
precursors or a migration of those progeny of the circumferential germinal cells
that are destined to become rods. The interpretation of retinal autoradiographs at
GROWTH OF FISH RETINAS
455
short times after injection is complicated
In summary, the mystery of where new
by the presence of dividing glia and rods come from is still unsolved. Several
phagocytes. Both of these cells are labeled suggestions have been made, and evidence
with [3H]thymidine, and they are scat- given to support them. It is certainly postered in and among the layers of retinal sible and even likely that more than one is
eurons, throughout the retina. They dis- the correct explanation. No matter what
appear after longer survival times when the answer is, it is certain that as the retina
further cycles of mitotic division have di- grows, the rods account for an increasing
luted their label. The histological tech- proportion of the neuronal population. So
niques used in the previous studies were even if there is no shear between layers,
inadequate to allow undifferentiated rod new synaptic relations must be formed in
precursors, if they existed, to be distin- central, differentiated retinal regions.
guished from these other kinds of prolif- From counts of synapses in the inner plexierating cells. To do so requires electron form layer in retinas from goldfish of difmicroscopy or specific histochemical mefh- ferent sizes, Fisher and Easter (1979) conods; these studies are in progress.
clude that new synapses are formed with
There is yet another way to account for growth. Though the synapses they countthe displacement of labeled cells in the out- ed did not include those of the rods, their
er nuclear layer and to explain the con- results provide quantitative evidence for
stant density of rods. This idea was first synaptogenesis in mature fish retinas.
suggested by Miiller (1952) in his compreUp to now I have only asked "how" and
hensive study of the guppy retina; the re- not "why" rod density is maintained with
sults of my autoradiographical study in growth. All I can offer is the speculation
goldfish were consistent with this theory that it may be important for visual sensi(Johns, 1977). Imagine that the rods resist tivity. At low levels of illumination where
the forces of expansion, and instead re- the rods function the goal is to catch all
main closely-packed while the ganglion available quanta of light, and that may recells, cones and inner nuclear layer cells quire a tightly-packed sheet of photorespread apart. This leads to a progressive ceptors no matter how large the eye.
lateral displacement of these cells away Growth of the eye has other predictable
from their contemporaries in the lay- effects on visual function, which are diser of rods. The observed displacement of cussed next.
labeled cells in the outer nuclear layer in
this scheme results not from rods migrat- Effects of retinal growth on vision
ing centrally but from other cells moving
Does a bigger eye see more of the world?
peripherally. If all new cells, including We (Easter et ai, 1977) measured the size
rods, are produced at the margin and and shape of the visual field in goldfish and
none penetrate into already differentiated found that it is 185° and spherically symretina, then a central circular region metric in eyes of all sizes. So a big eye sees
should remain free of labeled cells. This is the same world as does a small eye.
what I observed in the goldfish retina
Does a bigger eye see better? How well
(Johns, 1977). The consequence of shear an animal sees depends on the optical rebetween retinal layers is dramatic—syn- solving power of its eye as well as the efaptic connections must be altered. As the ficiency of its retinal detectors and procesretina expands, the post-synaptic partners sors. Acuity is said to be limited by the size
of each rod would move away and other and packing density of cones (Tamura,
cells, formerly in more central positions, 1957). The optical "grain" of the retina in
would assume their place. This theory pos- a big fish is finer than in a small fish betulates, then, a fundamental instability in cause there are more cones to divide up
the retinal circuity, requiring whole-scale the same amount of visual space, so acuity
shifts in synaptic connections, as a result of could potentially improve with growth
a normal growth process. Such an idea is (Muller, 1952; Tamura, 1957; Hester,
bizarre, but not impossible.
1968; Johns and Easter, 1977). A few be-
456
PAMELA RAYMOND JOHNS
havioral studies have shown that bigger
fish do have better acuity than smaller conspecifics (Baerends et ai, 1960; Hester,
1968; Northmore and Dvorak, 1979).
A bigger eye produces a bigger retinal
image, and this may affect the activity of
retinal neurons. Each neuron has a receptive field, an area on the retina and its corresponding region in visual space over
which photic stimuli will evoke a response.
The size of the receptive field of an individual cell, measured either in degrees of
visual angle or in retinal area, must change
as the retina enlarges. Electrophysiological
analysis of goldfish retinal ganglion cells
suggests that receptive fields change in size
by an amount predicted if the overlap between adjacent fields is held constant as the
eye grows (Macy and Easter, 1979). Thus
the number of retinal cells responsible for
a given point in visual space is the same in
small and large fish.
Finally, the geometry of cell addition has
a profound effect on the spatial properties
of the retinal neurons. A ganglion cell at
the peripheral margin in a small retina is
displaced centrally as new cells are added
at the perimeter. Likewise, the position of
its receptive field shifts centrally, and the
spatial information contained in the message it carries to the brain is altered. The
central neural link between visual input
and motor output must be continually reinterpreted and readjusted. How this is
accomplished is unknown.
CONCLUSION
Structures related to the eye also grow.
Bigger eye muscles are required to move
a bigger eye, and new muscle fibers are
added to the extraocular muscles (Easter,
1979). The brain of the fish also enlarges,
and like the retina, it contains germinal
zones which persistently generate new
neurons (Kirsche, 1967; Segaar, 1965;
Richter and Kranz, 1977). In the optic tectum, which receives most of the retinal input, new cells are added at the edges, but
not all the way around. The tectal germinal zone is not circular but U-shaped, with
a gap at the anterior pole (Kirsche, 1967;
Meyer, 1978; Johns and Easter, 1979).
Since the optic fibers synapse in topo-
graphic order across the tectum, and since
new ganglion cells are added at all points
along the retinal perimeter, some new optic fibers (those from the temporal margin)
innervate the anterior tectal margin where
no new cells are being generated. A simil
mismatch in the pattern of cell producti
in retina and tectum in larval amphibians
led Gaze and colleagues to propose that
retinotectal connections shift with growth
(Straznicky and Gaze, 1972; Gaze et al.,
1979). The synaptic remodeling implied
here is analogous to that proposed for the
retinal rods by the hypothesis of laminar
shear. Progressive and systematic movement of synapses in functional neural tissue as a normal consequence of growth is
not an established concept in neurobiology. Unfortunately we as yet have no direct
techniques to demonstrate the growth and
movement of neuronal processes in the living animal, but the fish visual system is a
good place to search for such a process.
What is astonishing about the retina of
fishes, and indeed about their entire nervous system, is that generation of neurons
and production of synaptic connections
continues into adult life. These embryoniclike activities perhaps compete with or
even hinder neural function, but certainly
must influence it.
ACKNOWLEDGMENTS
Dr. S. S. Easter, Dr. W. A. Harris, Dr.
D. H. Hubel and Dr. A. C. Rusoff commented on the manuscript, and my thanks
to them. Mr. M. Peloquin assisted with the
photography and Ms. O. Brum helped
with the typing. The autoradiograph in
Figure 3A and B was prepared with the
assistance of Dr. R. D. Fernald. P.R.J. is
supported by PHS grant EY-03301.
REFERENCES
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retinas of larval goldfish, rods are not present until 3
days after hatching. Production of rods is delayed until
larval stages, although all of the cones in central retina
Recent radioautographic studies in juvenile and
adult fish, goldfish (Johns, P. R., 1980, Neurosci. Abst. are post-mitotic at hatching. This is consistent with
6:639) and the cichlid, Haplochromus burtoni (Fernald, previous observations in other teleost species.
R. D. and P. R.Johns, 1981, Invest. Ophthal. Vis. Sci. Radioautography demonstrated that rods in larval
20:77), have demonstrated that new rods are pro- goldfish are generated by scattered, proliferating cells,
duced by dividing precursor cells which are scattered analogous to and, indeed, the progenitors of those
among mature rod nuclei in the outer nuclear layer. which continue to produce rods in the adult retina.
Most of the labeled precursors for rods were in The precursors for rods derive from clusters of what
peripheral (the youngest) regions of the adult retina, appear to be neuroepithelial germinal cells whose
but some labeled nuclei were found even in central nuclei are sequestered in the inner nuclear layer until
retina. New rods are thus continually inserted into the after about the third postembryonic day, when some of
mature retina, and this probably accounts for the their progeny move into the outer nuclear layer where
maintenance of a constant density of rods as the grow- they divide and generate rods. Thus another of the
ing retina expands. The hypothesis of laminar shear proposed hypotheses, migration of cells from the
with which my earlier study was consistent, is not sup- inner to the outer nuclear layer, is also supported by
ported by these new results, but neither do they dis- these results. The dilemma of the rods is now less
troublesome, but none the less intriguing as we realize
prove it.
I have also recently shown (Johns, P. R. and Y. that the ontogenesis of rods in the teleost retina follou s
Hwang. 1981, Invest. Ophthal. Vis. Sci. 20:150; Johns, a special course.
P.R.J.
P. R., 1981, Neurosci. Abst. 7, in press) that in the
NOTE ADDED IN PROOF