Download A morphological analysis of experimental myopia in young

A Morphological Analysis of Experimental
Myopia in Young Chickens
D. P. Hayes, F. W. Firzke, W. Hodos, and A. L. Holden
Devices that degrade vision were applied to the left eyes of 3-day old chicks. The dome device affected
the entire visual field, and the arch device, only the lateral field. Control chicks wearing a circumorbital
ring and untreated chicks were also examined. The dome device produced — 15D and the arch device
—4D of mean refractive error, while the ring and untreated chicks were emmetropic.1 Morphological
measurements were made from macrophotographs of the intact and hemisected eyes fixed as for electron
microscopy. The effects of the devices were analysed from the mean differences between the left (treated)
and right (control) eyes. Nearly linear growth of the normal eye was found during the period in which
measurements were taken (age 20-55 days). The ring device did not affect eye growth. The arch device
significantly increased the dorsoventral equatorial diameter of the eye. The dome device had the greatest
effect, and resulted in increases in both axial length and equatorial diameter during the treatment period.
Dome eyes had a bulging cornea, increased anterior chamber depth, more open angle, and greater corneal
diameter than controls. The axial length and equatorial diameter of the posterior segment also were
increased. Two inflammatory responses of the eye were found, particularly in dome eyes; about 50% of
treated eyes exhibited choroidal swelling, and vitreal clouding was found less frequently. The association
between inflammation and excessive accommodation in producing the observed changes is discussed.
Invest Ophthalmol Vis Sci 27:981-991, 1986
Myopia in humans may be caused by a number of
different structural abnormalities, such as elongation
of the eye or changes in the curvatures of its refracting
surfaces. Animal models have been developed to investigate these structural abnormalities and how they
arise, by altering the visual input or suturing the eyelids.2
This study is the third part of an investigation of
myopia produced by visual deprivation in young
chickens. In the first part of the investigation, Hodos
and Kuenzel3 showed that ocular enlargement was
produced by visual deprivation with plastic goggles.
The second part of the investigation was the electroretinographic measurement of the induced myopia. 14
In this paper we present a morphological analysis of
the external and internal dimensions of the myopic
eyes. In future papers, we intend to examine the fine
structure, intraocular pressure, eye temperature, and
optics of this animal mode of myopia.
Hodos and Kuenzel3 produced ocular enlargement
in young chickens by visual degradation with dome-
like plastic goggles. Differently shaped devices resulted
in either equatorial or both axial and equatorial enlargement. Electroretinographic refraction has since
confirmed that the plastic goggles produce substantial
myopia. 14 In their original anatomical study, Hodos
and Kuenzel3 measured only two parameters: axial and
equatorial diameter of the eye. However, other investigations of experimental myopia in young chickens
have shown that the depth of the anterior chamber,
corneal diameter and curvature, axial length of the
posterior segment, and thicknesses of the lens, cornea,
retina, choroid, and sclera may be altered.2
In this study, we have, therefore, carried out a morphological analysis of the external and internal dimensions of myopic and normal chicken eyes in an attempt
to answer two main questions: 1) What structural
changes accompany the altered refractive state of the
eye? 2) Do the observed changes provide any explanation for the way in which the myopia develops? The
eyes were made myopic by the dome and arch shaped
devices used by Hodos et al.1 The refractive states of
the majonty of the eyes in this study were measured
by Fitzke et al4 and Hodos et al.1 They found that
domes produce - 1 5 and arches —4 dioptres of axial
myopia, and that untreated eyes and those treated with
a circumorbital ring were emmetropic.
Measurements were made from macrophotographs
of intact and hemisected eyes taken in known orientations. The results were subjected to a statistical anal-
From the Department of Visual Science, Institute of Ophthalmology, London, United Kingdom.
Supported by grants from Moorfields Eye Hospital, The Royal
Society, the Smith, Kline and French Foundation, the National Retinitis Pigmentosa Foundation Inc., and the National Eye Institute,
National Institutes of Health, Bethesda, Maryland.
Reprint requests: Dr. B. P. Hayes, Department of Visual Science,
Institute of Ophthalmology, Judd Street, London WC1H 9QS.
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / June 1986
Vol. 27
Table 1. Treatment of the animal groups
Normals
age
ref.
Rings
age
d.on
Domes
Arches
d.off
ref.
age
d.on
d.off
21
21
22
23
24
27
18
18
18
20
20
21
0
0
1
0
1
3
28
21
30
26
ref.
age
d.on
21
18
27
27
28
21
24
25
29
25
31
34
35
35
36
36
36
28
31
31
25
25
32
30
0
0
1
7
8
38
41
42
42
43
43
33
31
39
37
40
34
2
2
0
6
45
40
2
48
32
14
55
48
d.off
ref
20
20
23
28
28
28
31
34
28
25
31
28
35
32
35
35
36
37
38
41
35
34
0
4
42
43
43
39
1
44
44
37
4
31
31
31
37
38
33
35
42
32
1
1
2
1
0
45
25
17
49
28
18
3
0
0
*
*
*
*
*
2
3
7
*
•
2
*
*
•
47
51
52
49
50
51
39
39
45
7
8
3
Age (in days), days the device was worn (d.on), days between removal of the
ysis, showing that domes increase axial and equatorial
dimensions, both externally and within the eye, and
that arches stimulate equatorial growth alone.
Materials and Methods
Three-day-old domestic chicks (White Rock, Rhode
Island Red crosses), both males and females, were
treated as follows: 15 were untreated normals, 21 wore
transparent plastic hemispheres over the left eye
(domes) (the domes were functionally translucent for
most of the experiment; see discussion and Hodos et
al1), 16 wore transparent plastic devices affecting the
lateral, upper and lower visual fields (arches), and 10
wore 1 mm thick acrylic plastic rings as a control for
mechanical impediments to growth (rings). Details of
the time course of treatment are given in Table 1; devices that fell off during the treatment were not replaced. The chicks were housed in cages that were
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*
device and sacrifice (d. off) and the occurrence of left eye refractions (*) for
the four groups of animals.
warmed for the first 4 weeks by thermostatically controlled, indirect infrared radiation. Ambient illumination was a combination offluorescentand natural
light. During the winter months, the light-dark cycle
was 9L/15D; during the remainder of the year the light
cycle was determined by seasonal variations in day
length. During the entire year, the intensity of illumination was affected by daily weather conditions.
Additional details of the rearing conditions and application of these devices and their optical properties are
given in Hodos et al.' The experiments were conducted
in accordance with the ARVO Resolution on the Use
of Animals in Research. After 3 to 8 weeks, the refractive state of the left eye of 32 of the chicks was
measured with an electroretinographic optometer in
the living animal1 (Table 1); all devices were removed
before refraction and not replaced. Mean refractive
states for the four groups were: untreated —0.2D, dome
- 14.88D, arch -4.1 ID, and ring -0.19D. The external
No. 6
MORPHOLOGY OF MYOPIC CHICK EYE / Hayes er ol.
and internal dimensions and macroscopic anatomy of
the left and right eyes were examined in all 62 chicks.
Zero to 18 days after the removal of the device and
immediately after refraction, the eyes of both refracted
and nonrefracted chicks were prepared as follows: the
animals (age 20-55 days) were killed with an excess of
ether anaesthetic, both eyes were excised, and all extraocular tissues removed. Eyes were fixed for 30 min
in phosphate buffered 3% glutaraldehyde.5 Macrophotographs were taken of the front of the globe with the
nerve head orientated at about 70° to the horizontal
(its position in the alert, unrestrained bird) (Fig. 1),
and of the side of the globe at right angles to this orientation (Fig. 2). Eyes were then partly hemisected in
the horizontal plane to aid fixative penetration. Fixation was continued overnight at 4°C. Eyes were then
transferred to sucrose buffer and hemisection was
completed. Macrophotographs were made at 8-10
times calibrated magnification and measured to O.I.
mm accuracy.
The following measurements were made for the intact eye: anterior corneal surface to posterior scleral
surface on the approximate geometrical axis of the
globe (A/P axis), maximum equatorial diameter of the
globe in the naso-temporal (N/T equator) and dorsoventral (D/V equator) planes (with the nerve head at
70° to the horizontal in the ventral third of the eye,
crossing the dorsoventral axis; Hayes and Holden, un-
Fig. 1. Macrophotograph of the front of a normal left eye, age 44
days. The equatorial diameters of the eye are 15.64 mm (naso-temporal) and 15.89 mm (dorso-ventral). Asymmetry can be seen in the
white scleral ossicles (o) and the displacement of the pupil towards
the nasal pole of the eye. The orientation, but not the exact position,
of the pecten within the eye is shown (p). N = nasal, T = temporal,
D = dorsal, V = ventral. Scale line I mm (X3.8).
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983
Fig. 2. Macrophotograph of a dorsal view of a normal right eye
from the same animal as Figure I. The axial length of the eye is 12.9
mm. The comea and pupil appear tilted towards the nasal pole and
the scleral ossicles (o) are larger temporally. The line shows the approximate geometrical axis of the cornea and pupil, on which axial
measurements were made. Corneal radius is 3.6 mm. on: optic nerve,
s: specimen support, N = nasal, T = temporal. Scale line 1 mm
(X3.8).
published), and radius of curvature of the anterior surface of the cornea found geometrically over its central
1 mm. Difficulty was experienced in orientating the
globe for the nasal and temporal views of the eye, but
control experiments showed that these slight inaccuracies did not affect the apparent axial length or corneal
radius. The horizontal plane of hemisection was found
to pass through the centres of the cornea, pupil, and
lens, and the maximum diameter of the ciliary body
(Fig. 3). It intersected the retina approximately 2.3 mm
dorsal to the tip of the pecten and 1.3 mm dorsal to
the area centralis, which was seen as a shallow depression of the posterior retina in 65% of eyes (Fig. 3).
Certain of the measurements of the hemisected eye
were made on its approximate optical axis, which was
taken as a line through the geometrical centers of the
corneal and lens curvatures and the center of the pupil
(Fig. 3). This line intersected the retina about 1.3 mm
dorsal and 0.2 mm nasal to the area centralis. Measurements of the hemisected eye were: corneal thickness
in the centre of the cornea, pupil diameter, anterior
chamber depth on axis, lens thickness on axis, lens
diameter at the lens equator, posterior lens radius
measured by geometrical location of the centre of curvature of the central 1 mm of the posterior lens surface,
posterior lens surface to distal tip of the photoreceptor
cells (lens to photoreceptors), retinal thickness on axis
984
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / June 1986
Fig. 3. The ventral half of a hemisected left eye from a normal
(untreated) chick, age 28 days. The line shows the approximate geometrical axis, which intercepts the retina dorsal and nasal to the area
centralis (arrow). Corneal diameter is 6.10 mm and corneal thickness
0.28 mm. Anterior chamber depth is 1.80 mm, and the drainage
apparatus (d) is well-defined temporally. The anterior surface of the
lens appears concave (this is probably a fixation artifact). The different
curvatures at the nasal and temporal lens equator can be seen. Lens
diameter is 4.98 mm, lens thickness 2.19 mm, and radius of the
posterior lens surface is 1.10 mm. The scleral ossicles (o) and ciliary
body (c) are much larger temporally. The ossicle angle (a) is 32°.
Lens to photoreceptor distance is 6.21 mm, and the retina, pigment
epithelium/choroid and sclera are 0.27, 0.15 and 0.19 mm thick,
respectively, p = pecten. N = nasal, T = temporal. Scale line 1 mm
(X6.5).
from the inner surface of the retina to the distal tip of
the photoreceptors, combined thickness of the pigment
epithelium and choroid on axis, and thickness of the
sclera on axis. The ossicle angle (Fig. 3) was an ap-
Vol. 27
proximate measurement of the angle between the anterior and posterior ends of the deep scleral ossicle on
the temporal side of the hemisected globe. In addition,
the age, number of treatment days (days on), days between removal of the device and sacrifice (days off),
and refractive state (where measured) of each animal
were recorded.
A statistical analysis was made of the raw data and
the left (treated) minus right (untreated) eye differences
for all of the above measurements. Analysis of variance,
t-tests, and Tukey (a) tests, were made with the BMDP
package and purpose written software on the University
College London EUCLID system. The effect of each
device was tested by two-tailed t-tests between the mean
L-R differences and zero, and Tukey (a) tests for significant differences between the mean L-R differences
for different treatment groups.
Shrinkage of the eye was measured from its external
dimensions before and after fixation, and was found
to be negligible (0.5%). Shrinkage of the internal tissues
of the eye was not measured.
Results
Morphology of the Normal, Untreated Eye
Table 2 shows the mean measurements of the normal
eye of age 20-55 days. In a view of the front of the
globe (Fig. 1), the equator appears approximately circular in outline and no significant difference is found
between the mean nasotemporal and dorsoventral
equatorial diameters (two-tailed t-test, P = 0.261). The
pupil is slightly displaced towards the nasal pole of the
eye and both the scleral ossicles and iris are wider tern-
Table 2. Mean eye measurements
L-R Difference5
Dimensions
Variable
AP axis
NT equator
DV equator
Corneal curv.
Corneal diam.
Corneal thk.
Pupil diam.
A.C. depth
Lens thk.
Lens diam.
Post lens rad.
Lens to photo.
Retinal thk.
Ch/pe thk.
Scleral thk.
Ossicle ang.
Normal
(n = 14-30)
S.D.
Mean
11.08
14.65
14.61
3.22
6.55
0.26
3.61
1.78
2.54
5.23
1.86
6.39
0.26
0.21
0.18
30.61
1.04
1.39
1.40
0.52
0.70
0.03
0.48
0.28
0.20
0.53
0.46
0.64
0.02
0.07
0.03
2.27
Arch
Dome
(n- = 13-16)
(n = 14-21)
Normal
(n = 12-15)
S.D.
Mean
Ring
(n = 9-W)
Mean
S.D.
Mean
S.D.
0.04
-0.06
0.02
0.07
0.03
0.01
0.10
0.05
0.00
0.00
0.36
0.01
0.00
0.01
0.01
1.21
-0.09
0.20
0.24
0.17
0.13
0.02
0.09
0.04
0.02
0.11
0.00
-0.06
0.00
0.03
0.00
0.70
0.15
0.05
0.44
0.16
-0.01
0.01
0.04
-0.06
0.02
0.05
0.13
0.07
0.00
0.10
0.00
1.87
0.37
0.14
0.33*
0.31
0.22
0.03
0.19
0.17
0.18
0.17
0.75
0.44
0.02
0.10
0.02
3.96
0.16
0.18
0.16
0.33
0.14
0.03
0.20
0.11
0.13
0.05
0.55
0.21
0.02
0.06
0.01
2.58
Mean dimensions of the normal (untreated) eye (left and right eyes included,
n = 14-30), and mean left-right differences (n = 9-21) for the four groups;
normal, ring, arch, and dome. Corneal curvature refers to the radius of curvature
of the cornea. All measurements are in millimetres except for ossicle angle
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0.33
0.18
0.30
0.63
0.15
0.04
0.20
0.23
0.12
0.04
0.42
0.27
0.01
0.08
0.01
2.50
(degrees).
S.D., standard deviation.
• P < 0.05.
** P < 0.01, (Tukey (a) test probabilities).
Mean
1.72
1.11
0.90
-0.26
0.34
0.00
0.15
0.62
0.02
0.10
0.16
0.86
-0.01
0.15
0.00
7.57
S.D.
0.87**
0.63**
0.61**
0.75
0.29**
0.04
0.19
0.40**
0.12
0.15
0.52
0.58**
0.03
0.18*
0.03
5.84**
MORPHOLOGY OF MYOPIC CHICK EYE / Hoyes er ol.
No. 6
porally. A side view of the eye shows that the pupil,
and to a lesser degree the cornea, are tilted towards the
nasal pole. The posterior segment curves more sharply
temporally than nasally. All these irregularities are approximately symmetrical about the eyes horizontal
meridian, which was the plane of hemisection used in
this study.
In the hemisected eye (Fig. 3) both the lens and pupil
appear tilted with respect to the posterior segment. This
appears to be a result of the shape of the ciliary body,
which is about 1.5 times wider temporally than nasally;
therefore, the nasal retina approaches the lens and cornea more closely than the temporal retina. The trabecular meshwork and Schlemm's canal are better developed temporally, and the scleral ossicles are larger.
The anterior curvature of the lens was often found to
be concave in the hemisected eye, and this measurement was not, therefore, included in the morphological
analysis. The curvature at the equator of the lens, ie
in the annular pad region, mimics that of the posterior
segment and is more sharply curved temporally.
Growth curves showed that the measured parameters
increased nearly linearly with age (Fig. 4), except for
retinal thickness and ossicle angle. Regression analysis
of the data (left and right eyes combined) gave correlation coefficients better than 0.57, with associated
chance-occurrence probabilities of 0.01 or less, except
for ossicle angle (r = -0.112, P = 0.57) where the data
was scattered between 26 and 35°. Retinal thickness
decreased with age (r = -0.626, P = 0.001) from about
0.28 mm at 20 days to 0.24 mm at 55 days.
Morphological Analysis of the Experimental Eye
The effects of the different devices (domes, arches,
and rings) on eye growth were examined by an analysis
985
/
12.8-
12.4-
12.0
/
1 1.6
AP a x i s
o
o
1 1.2
(mm)
10.8/
10.4-
:A
•
10.0-
25
35
40
Age(days)
30
45
50
Fig. 4. Axial length of the eye (AP axis) plotted against age (days
after hatching) for normal (untreated) chicks. A regression line is
shown (correlation coefficient, r = 0.93, P < 0.001). Axial length
increases linearly from about 10.2 mm at age 20 days to about 13
mm at age 55 days. Both left and right eye measurements are included
in the graph.
of variance (Table 3). No significant differences were
found between the mean ages of the animals in the
four groups (domes, arches, rings, and normals) (F
= 2.131, P = 0.106). There were also no significant
differences between the mean measurements for any
of the right eyes, including the normals (F values 0.5142.593, P values 0.062-0.675), showing that the application of a device to the left eye did not affect the growth
of the right eye. The effects of the devices were further
analysed by the left (treated) minus right (control) eye
Table 3. Results of the analysis of variance
Left eye
L-R differences
Right eye
Variable
F
(n)
P
F
(n)
P
F
(n)
P
AP axis
NT equator
DV equator
Corneal curv.
Corneal diam.
Corneal thk.
Pupil diam.
A.C. depth
Lens thk.
Lens diam.
Post lens rad.
Lens to photo.
Retinal thk.
Ch/pe thk.
Scleral thk.
Ossicle ang.
11.402
4.995
2.457
2.696
1.788
1.073
2.856
14.093
1.532
1.728
0.734
6.475
2.568
3.792
1.235
14.454
(59)
(62)
(62)
(59)
(62)
(61)
(58)
(58)
(60)
(61)
(43)
(60)
(59)
(61)
(60)
(60)
<0.001
0.004
0.072
0.055
0.160
0.368
0.045
<0.001
0.216
0.171
0.538
0.001
0.064
0.015
0.306
<0.001
.242
.173
.159
.309
.124
0.940
:>.593
0.514
.643
0.960
0.779
0.819
.728
.161
.215
.427
(61)
(62)
(62)
(59)
(62)
(61)
(62)
(57)
(60)
(60)
(47)
(60)
(57)
(61)
(61)
(61)
0.303
0.328
0.333
0.281
0.347
0.428
0.062
0.675
0.190
0.418
0.512
0.489
0.172
0.332
0.313
0.244
37.622
35.312
14.174
2.144
9.293
0.418
0.897
20.748
0.095
1.996
0.324
16.578
0.983
4.650
0.604
9.655
(58)
(62)
(62)
(56)
(62)
(61)
(58)
(54)
(59)
(60)
(37)
(59)
(56)
(61)
(60)
(60)
<0.00l
<0.001
<0.001
0.106
<0.001
0.741
0.449
<0.00l
0.962
0.125
0.808
<0.001
0.408
0.006
0.615
<0.001
The F value for each dependent variable, sample size (n) and the associated
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P value are shown. Corneal curvature refers to the radius of curvature of the
cornea.
986
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / June 1986
Fig. S. Frontal view of a left eye treated with the arch device for
32 days, age 42 days. The dorsoventral diameter (15.98 mm) is greater
than the nasotemporal diameter (15.28 mm) but the eye is otherwise
morphologically similar to untreated controls. The refractive state of
the eye was — 1.95D. N = nasal, T = temporal. Scale line 1 mm
(X3.8).
differences (L-R differences) for each measurement
(Table 2). Analysis of variance of the mean L-R differences showed significant differences between the
Vol. 27
treatment groups for axial length, equatorial diameter,
corneal diameter, anterior chamber depth, lens to photoreceptor distance, pigment epithelium/choroid
thickness, and ossicle angle (F values 9.655-37.622, P
values 0.000-0.006). This analysis demonstrated that
one or more of the devices had an effect on these parameters.
Ring device: This device appeared to produce a small
increase in the equatorial diameter of the eye, but had
no effect on axial length or the other measurements
(Table 2). Although both nasotemporal and dorsoventral mean diameters were apparently increased by about
0,2 mm (1 %) over the control eye, only the mean nasotemporal L-R difference was significantly different from
zero (two-tailed t-test, P = 0.006). A Tukey (a) test
showed that this mean L-R difference was not significantly different from the mean L-R difference in normal eyes (P > 0.05). The ring device, therefore, had
no effect on eye growth. No change in the internal
structure of the eye was found for the ring device.
Arch device: This device produced an increase in
equatorial diameter of the eye without any significant
effect on axial growth (Table 2). Mean equatorial diameter was only increased in the dorsoventral axis, by
0.44 mm (3%). Two-tailed t-tests showed that this mean
L-R difference was significantly different from zero and
from the nasotemporal equator (P values < 0.001). A
Tukey (a) test showed a significant difference between
the arch and normal mean L-R differences (P = 0.05).
The effect of the arch device was to produce an eye
which was elliptical in frontal view with the major axis
of the ellipse orientated approximately dorso-ventrally
(Fig. 5). No equatorial changes were found within the
eye; however, these would not be detected if they were
only in the dorso-ventral axis. The only change found
within the arch eye was a significant increase over the
control eye (P = 0.002) in the mean thickness of the
choroid/pigment epithelium of about 0.1 mm (50%);
however, a Tukey (a) test showed that this was not
Fig. 6. Nasal view of a left eye treated with the dome device for
48 days, age 55 days. The eye is enlarged when compared with the
control eye from the same animal (Fig. 7). Bulging of the anterior
segment can be seen. Refractive state of the eye was - 16.35D. Axial
length is 14.93 mm (ie 3 mm larger than the control eye), and corneal
radius is 3.43 mm. s = specimen support, D = dorsal, V = ventral,
on = optic nerve. The optic nerve appears on the nasal side of the
dorsoventral axis due to slight misalignment of the globe. Scale line
I mm (X3.8).
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significantly different from the normal eye. Choroidal
thickness was doubled in 44% of arch eyes. We believe
that this is an inflammatory response to wearing the
device.
Dome device: Prominent bulging of the cornea of
the dome eye was observed, both in vivo after removal
of the device, and when the eye was isolated (Figs. 6,
and 7). No corneal clouding was found in domes or
any other group before refraction, although such
clouding did occur during the period of electroretinographic refraction (up to 2 hr).
Both axial and equatorial dimensions of the eye were
increased by wearing the dome device (Table 2), and
the dome also produced more scatter in the measurements than the other devices (Fig. 8). The frequency
distribution histograms (Fig. 8) suggest the presence of
987
MORPHOLOGY OF MYOPIC CHICK EYE / Hoyes er QI.
No. 6
two populations of dome eyes; one with modest increases in axial (+1 mm) and equatorial (+0.5 mm)
length, and a further population with increases of about
three times these values. Mean axial length of the globe
was increased by 1.7 mm (15%) (Figs. 6, 7); a L-R
difference that was significantly different from zero and
the other treatment groups (Tukey [a] test P values
< 0.01). Both the mean nasotemporal and dorsoventral
equatorial diameters were increased by 7% over the
control eyes and the other treatment groups (P values
< 0.01, except arch P < 0.05). This resulted in an enlarged eye which was slightly elliptical in face view (Figs.
9, 10) with its naso-temporal equatorial diameter significantly larger than the dorso-ventral equatorial diameter (t-test, P = 0.007).
The depth of the anterior chamber was significantly
increased (L-R difference 0.62 mm, ie 35% larger than
controls and significantly different to zero and the other
treatment groups, lvalues < 0.01). This was accompanied by an apparent increase in the curvature of the
cornea (Figs. 11, 12); however, the reduction in mean
corneal radius was not significant (P = 0.130). Qualitatively, an increase in the angle between the iris and
cornea was seen (Fig. 11), and the trabecular meshwork
appeared more stretched and open in dome than other
eyes (this has been verified by scanning electron mi-
Fig. 7. Temporal view of the right (untreated) eye from the same
animal as Figure 6. Axial length is 11.84 mm, and corneal radius
5.22 mm s = specimen support, D = dorsal, V = ventral, on = optic
nerve. Scale line 1 mm (X3.8).
croscopy). The growth of the scleral ossicles was also
affected, increasing the mean angle between the anterior and posterior part of the ossicle from 30 to 38°
(L-R difference significantly different to zero and the
AP a x i s
DV equator
NT e q u a t o r
10
Normal
5
It
rf
n
O
1O
Ring
5
0
No.
10
Arch
5
O
10
Dome
5
O
0
0.8
1.6
mri
n
Wm
-0.8
2.4
3.2
-0.8
0
L-R d i f f e r e n c e
0.8
1.6
2.4
O.8
1.6
2.4
(mm)
Fig. 8. Frequency distribution histograms of L-R differences for axial length and equatorial diameter {nasotemporal and dorsoventral). The
four groups, normals, rings, arches, and domes, are shown. Domes show the greatest scatter, although all the devices introduce more scatter in
L-R differences than is found in normal eyes.
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988
INVESTIGATIVE OPHTHALMOLOGY G VISUAL SCIENCE / June 1986
Fig. 9. Frontal view of the dome-treated (left) eye shown in Figures
6 and 11. The dorsoventral equatorial diameter of the eye is 18.52
mm and the nasotemporal equatorial diameter 18.41 mm (the nasotemporal diameter was usually larger than the dorsovenlral diameter
in domes); these measurements are about 2 mm larger than the control
eye (Fig. 10). The increase in corneal and pupillary diameters can
also be seen when compared with the control. Clouding of the cornea
occurred during electroretinographic refraction. N = nasal, T = temporal, D = dorsal, V = ventral. Scale line I mm (X3.8).
other treatment groups, P < 0.01). In spite of the greater
volume of the anterior chamber, no reduction in corneal thickness was found. Mean corneal diameter was
Fig. 10. Frontal view of the right (untreated) eye shown in Figure
7. The eye has a normal morphology and the equatorial diameters
(16.59 mm nasotemporal and 16.34 mm dorsoventral) are similar
to those of a normal eye of the same age. N = nasal, T = temporal,
D - dorsal, V = ventral. Scale line I mm (X3.8).
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Vol. 27
Fig. 11. Ventral half of a hemisected dome-treated eye; the specimen is the same as Figures 6 and 9. The size of the anterior segment
is increased when compared with the control eye (Fig. 12), and the
cornea is more sharply curved. Corneal diameter is 0.81 mm greater,
and anterior chamber depth 1.29 mm greater than the control eye.
The drainage angle appears more open and the ossicle angle (a) is
increased by 14°. The dimensions of the lens are approximately the
same as the control. Cloudy vitreal material extends from the ciliary
body to the pecten (p). The posterior segment is more rounded than
in the control eye, and the lens to photoreceptor distance is increased
by 1.44 mm over the control eye, Choroidal swelling is present and
is greatest towards the temporal pole. N = nasal, T = temporal. Scale
line 1 mm (X6.5).
increased by 0.34 mm (significantly different to zero
and the other treatment groups (P < 0.01) except arch
( J P>0.05).
Dome eyes showed a large increase in the size of the
posterior segment; lens to photoreceptor distance was
lug. IZ. uorsai nan oi me cumroi ingm; eye snuwn m n g m u i
and 10. The dimensions of the eye are similar to those of a normal
eye at the same age (55 days). Anterior chamber depth is 1.95 mm
and corneal diameter 7,34 mm. Ossicle angle is 30°. The vitreous is
clear, as in untreated chicks. Lens to photoreceptor distance is 6.76
mm and the posterior layers of the eye are normal. N = nasal, T
= temporal. Scale line I mm (X6.5).
No. 6
989
MORPHOLOGY OF MYOPIC CHICK EYE / Hoyes er ol.
increased by 13%, and equatorial diameter by 7%. The
increase in the lens to photoreceptor distance was significantly different to zero and the other treatment
groups (P < 0.01). The posterior segment of the chick
eye normally curves sharply at the equator (Fig. 11),
but in dome eyes this curvature decreased, to give a
more spherical chamber, bulging postero-nasally
(Fig. 12).
Thinning of the retina to 75-85% of its control value
was found in four dome eyes, and did not seem to
relate to any other factors, such as the time for which
the device was worn. The largest increase in the combined thickness of pigment epithelium and choroid was
found in domes (mean L-R difference 0.15 mm, significantly different to zero and normal eyes, (P < 0.05).
Pigment epithelium/choroidal thickness was doubled
in 62% of domes eyes. No effect was found on the
thickness of the sclera.
Development and decay of the effects of the devices:
This was investigated by plotting graphs of the L-R
differences against time for which the device was worn
(days on) and time between the removal of the device
and sacrifice (days off). The graphs were then fitted
with regression lines. The ring and arch devices showed
no correlation between L-R differences and days on.
The dome device showed significant correlation for
nasotemporal equator L-R differences and days on (r
= 0.573, P = 0.007), and also for dorsoventral equator
L-R differences and days on (r = 0.598, P = 0.004)
(Fig. 13). The equatorial diameter of the eye therefore
increases with the time for which the dome device is
worn. Axial length (r = 0.537, P = 0.012) and pupillary
diameter (r = 0.544, P = 0.013) may also increase with
days on.
No correlation was found for any of the L-R differences with days off and no trends were apparent from
the graphs.
Inflammation of the eye: A thickened choroid was
found in the left eyes of 1 normal, 2 ring, 7 arch, and
13 dome chicks. Choroidal thickness was increased 1.6
to 3.8 times (mean 2.26 ± 0.63) (Fig. 11). All but two
of these eyes had been refracted by electroretinography.
Clouding of the anterior vitreous was found in the left
eyes of single normal, ring, and arch chicks and in five
domes, usually in combination with increased choroidal thickness. The cloudy vitreous extended from the
ciliary body to the pecten (Fig. 11). In addition to vitreal
clouding and choroidal swelling, a single dome eye exhibited retinal folds and detachment of the inner limiting membrane of the retina.
Discussion
The anterior segment of the normal chick eye shows
a marked morphological asymmetry: the ciliary body,
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2. 1
1 .8
1 .5
yS
1 .2
L-R
dilf.
0.9
y<
(mm)
O.6
°0
yS
O.3
O
15
20
25
30
35
Days on
Fig. 13. Left minus right difference (L-R diff.) in dorsoventral
equatorial diameter plotted against days of wearing the dome device
(days on). The graph suggests a linear relationship between the two
variables.
scleral ossicles, and other associated structures are wider
temporally than nasally, as first reported by Walls.6
This may be an adaptation to extend the temporal visual field,6 to tilt the lens and cornea relative to the
retina, or to apply an asymmetrical accommodative
force to the lens. The ciliary muscle could not clearly
be seen in macrophotographs of the hemisected eye,
but, assuming that its size varies with that of the ciliary
body, it might be able to exert more effect on the temporal than the nasal equator of the lens. This would
provide a mechanism for selective accommodation for
frontal vision; the existence of such a mechanism in
birds was first suggested by Hodos and Kuenzel.3 Studies of the fine structure of the accommodative apparatus
and the effect on image formation of the unusual eye
shape are under way. The approximate geometrical axis
of the eye, as determined from the corneal and lens
curvatures, intercepts the retina just dorsal and nasal
to the area centralis, and this axis has been chosen for
many of our measurements. In the breed of chicken
used, the area centralis is relatively well-developed, and
appears as a shallow depression of the posterior retina,
although in other breeds no foveal pit is found. 78
Nearly linear growth of all the eye dimensions measured, except retinal thickness and ossicle angle, was
found from 20-55 days after hatching. Retinal thickness decreased linearly with age over this period, although, just after hatching, the retina is still increasing
in thickness.9 The results suggest that passive stretching
is the main factor increasing retinal area in young
chicks.
Study of the effects of the devices on eye growth by
an analysis of variance shows that the application of a
device to the left eye does not affect the growth of the
990
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / June 1986
right eye. In the left eye, axial length, equatorial diameter, corneal diameter, anterior chamber depth, lens
to photoreceptor distance, pigment epithelium/choroid
thickness, and ossicle angle may be affected by wearing
a device.
The ring device did not affect the visual field and
was designed to control for mechanical impediments
to growth of the circumorbital tissue. This device has
no effect on the refractive state of the eye,1'4 and did
not significantly affect eye growth.
The second type of device was the arch, designed to
degrade vision in the lateral, upper, and lower visual
fields. This device was developed to test the hypothesis3
that degradation of lateral vision alone could produce
an axial myopia without equatorial enlargement.
Arches produce a mean axial myopia o f - 4 diopters,1'4
but have no significant effect on the axial length of the
eye. Perhaps the arch myopia is a result of alterations
to the curvatures of the refracting surfaces; it is hoped
that ray-tracing will explain this. The dorso-ventral
equatorial diameter of arch eyes is significantly increased (+3%). This is different from the effect of the
crescent devices used by Hodos and Kuenzel.3 Crescents degrade frontal vision and increased equatorial
diameter in both axes, an effect thought to be caused
by excessive accommodation for frontal vision.3 The
effect of the arch device might be explained by increased
growth of the eye at the dorsal and ventral margins
where the device was attached. This could be an inflammatory response to the device; raised orbital temperature is one of the factors thought to cause myopia
in humans. 10 Cell generation at the ora serrata11 might
be stimulated by a local rise in temperature in the circumorbital tissues, increasing equatorial growth of
the eye.
Domes degrade vision over the entire visual field
and produce a high degree of myopia (mean — 15D). 14
They produce large increases in the axial length (+15%)
and equatorial diameter (+7%) of the eye. Domes are
comparable to the hemispherical goggles employed by
Hodos and Kuenzel,3 and we would have expected
them to have the same effect on eye growth. The increase in axial length of dome eyes reported here is,
however, five times that found by Hodos and Kuenzel
for hemispherical goggles. This may best be explained
by the different methods employed; the electron microscope fixative used here may provide better preservation of the anterior chamber, and Hodos and
Kuenzel did not puncture the eye during fixation.
The shape and internal structure of the dome eye
closely resembles that produced by lid suture.12 Both
procedures produce a bulging anterior segment, change
in corneal curvature, deeper anterior chamber, and increased posterior segment length and equatorial diameter. Deepening of the anterior chamber is also produced by translucent occluders.13 This is different from
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Vol. 27
the effect of continuous illumination14 or dark rearing,2
which result in an eye of increased axial length with a
shallow anterior chamber. Continuous illumination or
dark rearing affect both eyes, but our devices only alter
the dimensions of the left eye. The deeper anterior
chamber is not accompanied by a reduction in corneal
thickness, suggesting that the effect is not the same as
experimental buphthalmos. 15 For much of the experiment, the domes may have acted as translucent occluders, rather than specific refractive devices, because
of condensation, abrasion, and the accumulation of
food particles. Examination of the anterior chamber
of dome eyes shows that the drainage apparatus is more
open than usual, in contrast to chicks exposed to continuous illumination,14 where angle closure, raised intraocular pressure, and glaucoma develop. The open
angle in dome eyes would suggest that the drainage
mechanism is functioning normally, but this awaits
confirmation from physiological measurements.
The dome device produces a substantial axial myopia.14 The main structural change involved would appear to be the 13% increase in the length of the posterior
segment, although increased scatter of corneal radius
in dome eyes indicates that change in corneal curvature
may also occur. It is hoped that ray tracing of dome
eyes of known refractive state will explain the optical
effects of these changes. Preliminary ray tracing calculations (unpublished), based on the mean morphological measurements, but rejecting eyes with an abnormal lens curvature, show that the increased posterior segment length in domes introduces about 18D
of myopia. This would be partly offset by the increased
depth of the anterior chamber, which would reduce
the refractive error to about — 15 dioptres. The mechanism of increase of the axial length in dome eyes is
not clear. Results from other animal models, reviewed
by Yinon, 2 suggest that the myopic changes are a result
of excessive accommodation, and this is also supported
by clinical findings. If excessive accommodation is involved, we might expect to see morphological changes
in the accommodatory apparatus, but in our experiments the devices had no effect on the dimensions of
the lens or the macroscopic appearance of this apparatus. Increased intraocular pressure, perhaps itself a
consequence of excessive accommodation, inflammation, and rise in the eye temperature, may also be
involved. It is hoped that the role of these mechanisms
will be resolved by work in progress.
The nasotemporal equatorial diameter of dome eyes
is significantly larger than the dorsoventral diameter;
an effect that might be explained by excessive frontal
accommodation or by local variations in inflammatory
response to the device. We would expect that the increased growth of the anterior chamber in domes might
also be a consequence of a rise in temperature within
the eye.
No. 6
MORPHOLOGY OF MYOPIC CHICK EYE / Hoyes er ol.
Thinning of the retina, when compared to the control eye, was only observed in four dome eyes (work
in progress shows that microscopic measurements of
retinal thickness agree with macroscopic measurements
to within 8%). Retinal thinning has also been described
after continuous illumination14 and lid suture16 in
chicks, although no quantitative data are given in these
papers. In lid-suture chicks,16 thinning of the retina
involves histological changes of the retinal layers, and
we are now investigating the possible histological and
ultrastructural effects of the devices we have used. No
thinning of the choroid and sclera was found in this
study. Choroidal and scleral thinning are features of
human degenerative myopia,17 and these might occur
in chicks exposed to visual degradation for longer periods than we have used. Study of the changes in thickness of the eye layers of normal chicks between age 20
and 55 days shows that the retina increases its area in
a different way to the choroid and sclera, ie the retina
thins by about 0.04 mm, but the choroid (+0.16 mm)
and sclera (+0.09 mm) both become thicker. Therefore,
in dome eyes, accelerated active growth in these layers
must compensate for the increased size of the posterior
segment in myopic chick eyes. Choroidal thickness was
increased in dome and arch eyes, and occasionally in
rings and normals. We believe this to be an inflammatory response to wearing the device, although this
effect also occurs in response to low intensity blue
light.18
Two types of inflammatory response were found in
the left (treated) eyes of chicks. The most frequent (23
eyes) was choroidal thickening, and clouding of the
anterior vitreous was less frequently observed (8 eyes).
Both occurred most frequently in dome-treated eyes,
although they were also found in the other device types
and in a single normal eye. Swelling of the choroid and
vitreal clouding both occur in uveitis of the human
eye.19 Clouding of the vitreous and choroidal edema
have also been described in chronic endophthalmitis
of chickens.20 In such birds, lesions of the iris, ciliary
body, and retina were usually found, and histological
studies in progress will show whether these also occur
in experimental myopia.
The equatorial diameter of the eye tends to increase
with the time for which the dome device is worn, and
this may also be true for axial length and pupillary
diameter. Increased myopia with time of exposure to
a similar myopia-producing device has been described
by Wallman et al.21 Progressive reversal of the effect
after removal of the device has also been reported,21
but we have found no correlation between the time
after removal and our morphological measurements.
This may be because the animals were sacrificed only
a few days after removal of the device.
Key words: chicks, myopia, ocular enlargement, morphology,
inflammation
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991
Acknowledgments
The authors thank F. Sheen, P. West, J. Low, and D.
Goulding for technical assistance and P. K. Clark for statistical
assistance.
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