Download Intraocular pressure-dependent and

Investigative Ophthalmology & Visual Science, Vol. 32, No. 9, August 1991
Copyright © Association for Research in Vision and Ophthalmology
Intraocular Pressure-Dependent and -Independent
Phases of Growth of the Embryonic
Chick Eye and Cornea
Prue Nearh, Sean M. Roche, and James A. Bee
The pattern and relative rates of diametric growth of the avian eye and cornea are described throughout
embryonic development. The effect of reduced intraocular pressure on eye and corneal diametric
growth also was investigated. Between embryonic day 4 (E4) and 1 day posthatching, the eye undergoes
two distinct phases of linear growth. The first phase (E4-10) is very rapid (1.193 mm/day). The second
phase, after E10, is significantly slower (0.346 mm/day). By contrast, over the same developmental
period, the cornea undergoes three distinct and sequential phases of linear growth. The second phase of
corneal growth (E7-10) is the most rapid (0.429 mm/day) and separates two periods of slow growth
(0.211 mm/day during E4-7 and 0.128 mm/day after E10). After the sustained release of intraocular
pressure by intubation on E4, growth of both the eye and cornea is reduced significantly. Operated eyes
grow at a rate of 0.356 mm/day (E4-10) and 0.155 mm/day (E10-16). Intubation reduces corneal
growth to a single phase of 0.125 mm/day (E7-16). Thus, from E4-10 both the eye and cornea possess
intrinsic growth potentials that are elevated significantly by intraocular pressure. After E10, the rate of
growth of both the eye and the cornea is independent of intraocular pressure. Because both control and
intubated eyes change their growth rate on E10, this transition also is independent of intraocular
pressure. This contrasts with the cornea which, after intubation, shows no detectable variation in
growth rate. Correlation of eye with corneal growth demonstrates an exponential relationship in the
presence of intraocular pressure and an almost linear relationship after intubation. Invest Ophthalmol
Vis Sci 32:2483-2491,1991
Eye development commences with the outgrowth
of a vesicle from the lateral walls of the diencephalon
that, on encountering the lateral head ectoderm, undergoes complex invagination to form the optic cup.
The establishment of the optic cup subdivides the
neurectoderm into an inner prospective neural retina
and outer prospective retinal pigmented epithelium
and is associated temporally and causally with lens
induction from the overlying ectoderm.1"4 The lens
resides within the tips of the optic cup and stimulates
the overlying ectoderm to synthesize and assemble
the primary corneal stroma5 that provides the framework onto which neural crest-derived cells migrate to
establish the corneal endothelium.6 After its formation, the corneal endothelium secretes hyaluronate
into the primary stroma7'8 which then is invaded by a
second population of neural crest-derived cells that
differentiate as corneal fibroblasts6 and synthesize
and assemble the secondary corneal stroma.9""
During avian embryogenesis, the major components and overall structure of the eye, including the
cornea, are established during thefirst7 days of in ovo
development. The newly formed eye primordium
grows rapidly to become the most prominent cranial
structure during the first half of avian development.
The enormous increase in size of the developing eye
primarily reflects an increase in size of the vitreous
chamber to accommodate elevated synthesis, and
thus an increasing volume, of vitreous humor.1213
Concomitant with the enlargement of the vitreous
chamber, the lens and cornea also increase in size,
resulting in overall coordinated eye growth. The initial phase of rapid ocular growth probably serves three
important roles fundamental to ultimate eye function. First, it permits the maturation of the neural
retina and early establishment of the central pathways
of the visual system.14 Second, it establishes a framework on which the appropriate intrinsic and extrinsic
musculature together with their peripheral innervation can be established.15 Third, the eye achieves an
appropriate size and position, relative to other cranial
From the Department of Veterinary Basic Sciences, The Royal
Veterinary College, London, England.
Supported by a project grant from the Medical Research Council
to JAB.
Submitted for publication: December 6, 1990; accepted March
29, 1991.
Reprint requests: James A. Bee, Department of Veterinary Basic
Sciences, The Royal Veterinary College, Royal College Street, London, NW1 OTU, England.
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INVESTIGATIVE OPHTHALMOLOGY 6 VISUAL SCIENCE / August 1991
structures, before a potential restriction of growth imposed by intramembranous ossification of the skull,
particularly the orbit.16
The development and biochemistry of the avian
eye were described in detail, and because of their obvious significance to the mature visual system, the cornea, lens, and neural retina received considerable attention. Although it constitutes a major component
of their development, growth of neither the eye nor
the cornea were analyzed comprehensively. It has
long been recognized that growth of the embryonic
chick eye is related to sustained intraocular pressure.17 The release of intraocular pressure during the
early stages of eye morphogenesis causes the scleral
cartilage to be substantially thicker17 and the neural
retina, but not the pigmented epithelium, to be highly
folded.18 Although cell differentiation apparently is
unaffected, the atypical morphology of these tissues
indicates that tension forces are instrumental in their
normal development.
The most dramatic effect of intubation on embryonic day 4 (E4) is a severe reduction of eye growth, a
largely reversible effect that is not an artifact of surgical manipulation.18 A similar reduction of corneal
growth after intubation shows that this is associated
with overall eye growth and thus indirectly related to
intraocular pressure.19 We extended these previous
studies, restricted to the eye and cornea during the
early stages of development, and we describe a comprehensive analysis of the avian eye and corneal
growth both in the presence (E4 to 1 day posthatching) and absence (E4-16) of intraocular pressure.
Materials and Methods
All procedures on embryos used in this study conformed to the ARVO Resolution on the Use of Animals in Research. The embryos were derived from
White Leghorn (Gallus domesticus) eggs obtained locally. The eggs were incubated in a forced-draft incubator at a temperature of 38 ± 0.5 °C. The embryos
were aged by comparison with the normal developmental tables of Hamburger and Hamilton.20
Measurement of Eye and Corneal Growth
The embryos were dissected free of extraembryonic
membranes in cold calcium- and magnesium-free
Tyrode's solution, pH 7.4. After their transfer to fresh
cold calcium- and magnesium-free Tyrode's solution,
the intact eyeball was removed and oriented to be positioned with the anterior segment facing upward. Eye
growth was determined from the average equatorial
diameter of a minimum of three eyeballs at each of
the developmental ages examined. The retinal pigmented epithelium provided a distinct boundary to
Vol. 02
the eyeball and enabled consistent measurements to
be made. Using the choroidfissureas the ventral reference point, both the dorsoventral and nasotemporal
diameters of the eyes were measured on a calibrated
Wild dissecting microscope (Wild Heerbrugg, Herrbrugg, Switzerland). These two axes of measurement,
which were never significantly different from one another, provided an average eye diameter directly proportional to overall eye size.18
Corneal diameter was used as an index of overall
corneal growth. For each of the developmental ages
examined, average corneal size was determined from
at least three eyes. Each of the corneas was derived
from previously measured eyeballs, enabling direct
correlation between overall eye and corneal growth to
be made. After the measurement of eyeball diameter,
the cornea was dissected free of adherent tissues and
flat mounted. Using the distinct limbal region as the
corneal boundary, the average diameter of a cornea
was determined from two measurements made at
right angles to one another. Because in situ orientation of corneas cannot be determined after their isolation, these axes of measurement necessarily were random. Initial measurements showed that isolated
corneas are circular structures at all stages of their
development.
Reduction of Intraocular Pressure
A circular window, approximately 1.5-cm diameter, was cut into the shell of all eggs on incubation day
4, corresponding to E3. Approximately 50 ^1 of sterile
Hanks' buffered saline, pH 7.4, containing penicillin
and streptomycin (Flow, Irvine, Scotland) was injected into each egg and the window sealed with adhesive tape. On the following day (E4), the adhesive tape
was removed, and the embryos underwent in ovo microsurgery under fiberoptic illumination. Access to
the right eye, naturally facing upward during this period of development, was generated by tearing the
overlying extraembryonic membranes with watchmaker's forceps. Using fine iridectomy scissors, a
small incision was made into the dorsal aspect of the
eyeball at a point midway between the edge of the
cornea and the equator of the eye. A glass tube was
inserted through this incision to establish continuous
release of intraocular pressure by drainage from the
vitreous chamber. The glass tubes used in this study
were hand pulled from capillary tubing and measured
under a calibrated microscope. The pieces of glass
tubing inserted into the eyes were selected to measure
approximately 3.0-mm long, 0.268 ± 0.025-mm internal diameter, and 0.412 ± 0.037-mm external diameter. Approximately 50 /x\ of sterile Hanks' buffered saline, pH 7.4, containing antibiotics was injected into the egg and the window resealed with
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EYE AND CORNEAL GROWTH / Neorh er QI
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adhesive tape. After in ovo surgery, the eggs were returned to the incubator to permit continued development. After appropriate periods of postoperative incubation, the embryos were killed, and the diameters
of intubated eyes and corneas were determined as described.
Results
Commencing on E4 and extending until 1 day
posthatching, the diametric size of right and left eyes ,
and right and left corneas was determined at 2-day
14"
EMBRYONIC
AGE [DAYS].
Fig. 2. The effect of intubation on E4 on the morphology of the
E8 chick embryo head. A glass tube (arrowheads) inserted into the
dorsal quadrant of the right eye causes it to be significantly smaller
than the unoperated left eye. Despite the reduction in eye growth,
intubation does not appear to disrupt or delay the development of
surrounding craniofacial structures. Scale bar = 2.0 mm.
(b)
S" 41
1.
intervals. The data presented in Figure 1 demonstrate
that over this developmental period both the eye and
the cornea grew rapidly and extensively. At each of
the embryonic ages examined, there was no significant difference in size between right and left eyes (Fig.
1 A). A parallel analysis also showed that there was no
significant difference in size between right and left
corneas (Fig. IB).
CC
o
CC 5J DC ^ DC
CC
EMBRYONIC
AGE [DAYS].
Fig. 1. Commencing on E4, average equatorial diameter of right
and left eyes (a) and average limbal diameter of right and left corneas (b) were determined at 2-day intervals until 1 day post-hatching. Columns are plotted to the average of the data and the solid
caps represent the positive standard deviation. The data demonstrate that over this period both the eye and cornea increase their
size dramatically. At each embryonic age there is no statistical difference in size between right and left eye or right and left cornea. L
= left (diagonal shading); R = right (horizontal shading). Numbers
on the horizontal axis refer to embryonic age in days.
The Effect of Intubation on Eye Growth
Glass tubing was inserted into the dorsal aspect of
the right eye on E4 to prevent the accumulation of
intraocular pressure. Figure 2 shows an E8 embryo,
killed 4 days after in ovo surgery. In addition to demonstrating the tube inserted into the right eye, the
operated eye was significantly smaller than its fellow
control (left) eye. This difference was accentuated further on El2, by which time the control (left) eye was
substantially larger than the operated right eye (Fig.
3). As previously described, insertion of a glass tube
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991
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Fig. 3. The effect of intubation on E4 on the morphology of the E12 chick embryo head. One week after the sustained release of intraocular
pressure the right eye (b) is significantly smaller than the control left eye (a). The operated eye is typically covered by well developed eyelids and
demonstrates rupture of the neural retina through the point of insertion of the glass tube into the vitreous chamber (arrow). While the lower
jaw is not affected, the upper beak is always crossed away from the midline toward the operated side of the head. Scale bar = 2.0 mm.
into the eye on E4 causes rupture of the neural retina
in addition to a reduction of its growth.18 In contrast
to the control eye, the El2 operated eye was covered
partially by thickened eyelids, and the upper beak was
crossed characteristically toward the operated side
(Fig. 3).
During embryonic development, control (left) eyes
underwent two distinct phases of growth (Fig. 4Ai).
The first phase of eye growth extended from E4-10
and was both linear and very rapid. During this 6-day
period, the eye grew in diameter from an average of
1.070 mm on E4 to an average of 7.983 mm on E10,
an increase of approximately 750%, The second phase
of eye growth was shown after E10 and, although only
data up to and including El6 is presented in the figure, extended until at least 1 day posthatching. Although the second phase of eye growth also was linear,
it was much slower, and over the 6-day period (E1016), the average eye diameter increased to 9.910 mm,
an increase in size of approximately 125%.
Eye growth was reduced significantly and permanently after intubation on E4 (Fig. 4Aii). By E10, the
average diameter of intubated eyes (3.434 mm) had
increased by only 320%. Between E10 and El6, intubated eyes continued to grow slowly and increased in
diameter by only 180%. The data presented in the
figure suggest that, despite a severe reduction in size,
intubated eyes showed two phases of growth with a
reduction in rate occurring on E10. Both the initial
rapid and subsequent slow phases of intubated eye
growth tended toward linearity although there was
fluctuation in growth rate between 24 and 48 hr after
the release of intraocular pressure.
An overall description of the pattern of diametric
growth of the embryonic chick eye was extended to a
detailed analysis of the relative rates of the two phases
of control and intubated eye growth (Figs. 4B-C).
From the data presented in Figure 4A, both control
and intubated eyes showed a sharp change in diametric growth rate on E10. The relative rate of the first
phase of eye growth (E4-10) was determined by normalizing the mean size of both E4 control and intubated eyes to unity. Values obtained from E5, E6, E7,
E8, E9, and E10 eyes were then corrected to the normalized E4 eye diameter. Similarly, the relative rate
of the second phase of eye growth, commencing on
and extending beyond E10, was determined by normalizing the mean diametric size of both E10 control
and intubated eyes to unity. Values obtained from
El2, El4, and E16 control and intubated eyes, in addition to El8, E20, and 1 day posthatching control
eyes, then were corrected to the normalized E10 eye
diameter. The resultant data demonstrated that control eyes increased in diameter at a rate of 1.193 mm/
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EYE AND CORNEAL GROWTH / Nearh er ol
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10
10
12
14
16
EMBRYONIC AGE [DAYS).
DAYS OF DEVELOPMENT.
DAYS OF DEVELOPMENT.
Fig. 4. Between E4 and El 6, the average equatorial diameter of control eyes increases in two distinct and linear phases (a, [i]). Control eye
diameter increases at a rate of 1.193 mm per day until E10 (b, [i]) and 0.346 mm per day after E10 (b, [ii]). Over the same developmental period
the average equatorial diameter of intubated eyes is significantly reduced (a, [ii]). Following the release of intraocular pressure on E4, eye
growth is reduced to 0.356 mm per day until E10 (c, [i]) and 0.155 mm per day between E10andE16(c, [ii]). Solid and vertically shaded areas
(a) and open circles and squares (b, c) indicate the standard deviation of the data. Average values are represented by the interface (a) and solid
circles and squares (b, c).
day from E4-10 (Fig. 4Bi) and 0.346 mm/day from
E10 to 1 day posthatching (Fig. 4Bii). After intubation
(E4-10), the eye diameter increased at a rate of 0.356
mm/day (Fig. 4Ci), and this decreased to 0.155 mm/
day from El0-16 (Fig. 4Cii).
Control Versus Intubated Corneal Growth
Analysis of control corneas showed a sigmoidal increase in diameter from E4-16, with changes in diametric growth rate occurring on E7 and E10 (Fig.
5Ai). From E4-7, average corneal diameter increased
by approximately 230% from 0.455-1.059 mm. By
E10, the average corneal diameter was 2.320 mm, representing an increase in size of almost 220% since E7.
The average diameter of the E16 cornea was 3.147
mm, representing an increase in size of 135% since
E10 and 690% since E4. Thus, the measurement of
corneal diameter demonstrated three phases of diametric growth of the developing cornea. The first
phase (E4-7) and the second phase (E7-10) were both
relatively rapid. The third phase, demonstrated after
E10 and continuing uniformly until 1 day posthatching, was considerably slower.
After intubation on E4, corneal growth was reduced significantly and permanently (Fig. 5Aii). On
E6, although the average diameter of corneas from
intubated eyes increased, it was significantly less than
control. From E6-7, the average diameter of corneas
from intubated eyes decreased and remained rela-
tively unchanged on E8 (Fig. 5 Aii). After E8 and until
El6, the maximum age we examined, the average diameter of corneas from intubated eyes increased gradually and linearly. Over the period from E4-16, the
average diameter of corneas from intubated eyes increased by approximately 380%. However, corneas
from E16 intubated eyes were approximately 55%
smaller than controls.
The relative rates of control corneal growth was determined over the developmental periods E4-7, E710, and E10 to 1 day posthatching. For thefirstperiod
of growth, the average diameter of E4 corneas was
normalized to unity and corneal size on E5, E6, and
E7 corrected to this starting value. For the second
phase of growth, the average diameter of the E7 cornea was normalized to unity and corneal size on E8,
E9, and E10 corrected to this starting value. Similarly,
for the third phase of growth, the average diameter of
the E10 cornea was normalized to unity and corneal
size on E12, E14, E16, E18, E20, and 1 day posthatching corrected to this value. For corneas from intubated eyes, the relative rate of growth was determined
over the complete developmental period from E4-16
by normalizing the E4 value to unity and correcting
corneal diameter on E5, E6, E7, E8, E9, E10, El2,
El4, and E16 to this initial value. These data show
that control corneal diameter increased at a rate of
0.211 mm/day from E4-7 (Fig. 5Bi), 0.429 mm/day
from E7-10 (Fig. 5Bii), and 0.128 mm/day from E10
to 1 day posthatching (Fig. 5Biii). From E4-16, cor-
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991
10
12
Vol. 32
14
EMBRYONIC AGE (DAYS).
DAYS OF DEVELOPMENT.
DAYS OF DEVELOPMENT.
Fig. 5. Between E4 and E16, the average limbal diameter of control corneas increases sigmoidally (a, [i]). Control corneal diameter increases
at a rate of 0.211 mm per day between E4 and E7 (b, [i]), 0.429 mm per day between E7 and ElO (b, [ii]), and 0.128 mm per day after ElO (b,
[Hi]). Following the release of intraocular pressure on E4, corneal growth is significantly reduced (a, [ii]) to a single linear phase of 0.125 mm
per day (c, [i]). Solid and vertically shaded areas (a) and open circles and squares (b, c) indicate the standard deviation of the data. Average
values are represented by the interface (a) and solid circles and squares (b, c).
neas from intubated eyes increased in diameter at a
constant rate of 0.125 mm/day (Fig. 5Ci).
The Relationship Between Eye and Corneal Growth
in Control and Intubated Eyes
The average diameters of eyes and corneas from
either control or intubated eyes were arranged in
order of increasing size regardless of embryonic age.
Eye diameter then was plotted against corneal diameter for either control or intubated eyes. For the developmental period from E4 to 1 day posthatching, the
increase in average diameter of control eyes and their
corneas showed an extremely close and exponential
association (Fig. 6A). Thus, although both structures
grew rapidly and necessarily in conjunction with one
another, the eye increased in size at a greater rate than
the cornea. Over the developmental period from E416, correlation of the average diameter of intubated
eyes and their corneas was more linear (Fig. 6B). By
contrast with the disproportionate growth of the control eye and cornea, these data therefore demonstrated that, in the absence of intraocular pressure,
corneal growth was more directly proportional to
overall eye growth.
Discussion
Together with the developing brain, the eye serves
as a template on which the major processes associated
with facial morphogenesis are established.21'22 In ad-
dition to the obvious importance of a precisely developed visual system, eye development and growth thus
constitutes a significant and substantial component of
craniofacial development. The sustained release of intraocular pressure by intubation severely reduces
growth of the embryonic chick eye17 and cornea19
without disrupting their histogenesis.l8 More recently,
a major interest in growth of the hatchling chick eye
arose from the demonstration of its ability to grow
into focus and thereby compensate for experimentally induced changes in focal length.23 However,
growth of the embryonic chick eye and cornea were
not analyzed in detail. Employing diameter as an index of overall size,1819 we described a comprehensive
analysis of growth of the embryonic chick eye and
cornea in both the presence and absence of intraocular pressure.
During the initial stages of in ovo development, the
avian embryo transiently is oriented with its right eye
naturally facing upward. Although unlikely, we considered the possibility that during these early stages
the right eye and cornea could be growing at different
rates than the left eye and cornea. Our data demonstrated that there was no significant difference in size
between either right and left eyes or right and left corneas at any of the developmental stages examined.
Thus, any difference in eye or corneal size resulting
from subsequent surgical manipulation cannot be an
artifact of growth differences intrinsic to the operated eye.
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EYE AND CORNEAL GROWTH / Neorh er ol
No. 9
Fig. 6. Average values for
eye and corneal diameter
were size ranked regardless
of embryonic age. Correlation between control eye
and control corneal diameter demonstrates an exponential relationship indicating a more rapid growth of
the eye relative to the cornea
(a). Similar correlation of
intubated eyes and corneas
(b) demonstrates a shift in
the relationship between eye
and corneal size towards linearity. Thus, in the absence
of intraocular pressure the
cornea grows in direct proportion to the eye.
o.o
o
2489
.0
1
EYE DIAMETER [MM].
Between E4 and 1 day posthatching, control eye
size increased dramatically in two sequential phases
of linear growth. The first phase, before El 0, was very
rapid; the second phase, after E10, was considerably
slower. The elimination of intraocular pressure by intubation on E4 substantially reduced both phases of
eye growth. Nevertheless, intubated eyes continued to
grow, and like control eyes, this growth occurred in
two sequential phases which both tended toward linearity. The reduction of eye growth after intubation
was most pronounced from E4-10, and consequently
this phase primarily was dependent on sustained intraocular pressure. Intubation also revealed that, during
this period, the eye was capable of a slow rate of
growth in the absence of intraocular pressure. It remains to be determined whether the rate of eye
growth normally achieved from E4-10 was the combination of intraocular pressure independent and dependent elements.
Before E10, the eye expands without restriction by
surrounding tissues. Because it is the most prominent
cranial structure, this phase of very rapid growth must
facilitate the precise positioning of topographically associated tissues around the eye during the early stages
of development.1624 This period of growth coincides
with the enhanced accumulation of vitreous fluid1213
which, in turn, must be accommodated by additional
growth of the neural retina and retinal pigmented epithelium18 together with the periocular mesenchyme.
The growth of each of these tissues was initiated by
cell proliferation, but subsequently, the surface area
of both the neural retina and retinal pigmented epithelium increased primarily by lateral spreading and flattening of these two cell sheets. However, because intubation causes the neural retina to be highly folded,18
the lateral expansion of this tissue was independent of
eye size and did not promote eye growth.
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
EYE DIAMETER [MM].
The rate of growth of both control and intubated
eyes decreased sharply on E10, and therefore, this
transition must be independent of intraocular pressure. This transition in growth rate coincided temporally with the establishment of the scleral cartilage
from the periocular mesenchyme.2526 Because the
scleral cartilage encapsulates the eye, it may restrict
growth indirectly by either suppressing the synthesis
of vitreous humor or stimulating its rate of turnover.
Preliminary studies indicated that intraocular pressure per se does not change during avian development
(D. Beebe, personal communication), and consequently the term "pressure" may be misleading.
Thus, alterations in vitreous humor volume must be
accommodated directly by corresponding changes in
overall eye size and vice versa. The relationship between scleral cartilage and eye growth was highlighted
by the demonstration that experimentally induced alterations in eye dimension elicited corresponding
changes in both the proliferative and biosynthetic activity of scleral chondrocytes.27"29
After E10, eye growth was restricted potentially by
both the scleral cartilage and the establishment of the
bones of the orbit. Although the eye continues to
grow, this is in conjunction and coordinated with a
general increase in head size. We therefore concluded
that eye growth after E10 was independent of intraocular pressure, and this was reinforced by the demonstration that, after intubation, the rate of eye growth from
E4-10 was identical to the rate of growth of control
eyes from E10 to 1 day posthatching. Intubated eyes
reduced their rate of growth on E10 because of an
overall perturbation of facial development around the
operated eye (Fig. 3). This disruption must result in a
much smaller orbit in addition to substantially
thicker scleral cartilage.17 Thus, commencing on E10,
eye growth was constrained extrinsically, and the
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1991
growth rate of intubated eyes was reduced additionally because they already were significantly smaller.
Growth of the eye closely correlated with a corresponding increase in diametric size of the cornea.19
Analysis of corneal size with respect to age revealed
three phases of diametric growth from E4 to 1 day
posthatching. Initial corneal growth (E4-7) was relatively rapid and increased further from E7-10. The
subtle transition in growth rate exhibited around E7
was not an artifact; we made many measurements of
E6, E7, and E8 corneas, and these reinforced our initial and unexpected observation. The supplementary
elevation of growth rate from E7-10 reflected an increase in corneal diameter in addition to the establishment of corneal curvature that began on E8 and also
was related to intraocular pressure.30'31 Thus, during
this phase of its growth, the surface area of the cornea
increased more rapidly than its circumference. We
did not analyze corneal curvature in our study, but
our measurements of flat-mounted corneas combine
both parameters. After E10 corneal growth was reduced, and this effect persisted at a constant rate until
at least 1 day posthatching. Intubation on E4 reduced
subsequent corneal growth to a single slow phase.
Thus, the transitions in corneal growth rate demonstrated on E7 and E10 were both a function of intraocular pressure. The rate of corneal growth in intubated
eyes was identical to the slow phase of corneal growth
normally demonstrated by control corneas after E10.
We therefore concluded that corneal growth was dependent on intraocular pressure before E10 and independent of this after E10.
Throughout their development, control eye and
corneal growth were correlated exponentially. The
control eye grew extensively, and this, in turn, promoted the growth of the contiguous cornea. Because
eye growth from E4-10 was dependent on intraocular
pressure, intubation should result in a concomitant
reduction in corneal growth, as we demonstrated and
as has been shown elsewhere.19 Although our data
showed that corneal growth from E4-10 was dependent on intraocular pressure, this dependency was indirect. Intraocular pressure promoted the growth of
the eye which then facilitated corresponding corneal
growth. This was shown by the linear correlation between eye and corneal size after intubation.
After surgery the rate of diametric growth of the
cornea from E4-16 was identical to that of control
corneas from E10 to 1 day posthatching. Thus, this
rate of corneal growth was indirectly independent of
intraocular pressure and related to overall growth of
the eye which, under both conditions, was directly
independent of intraocular pressure. From El0-16
the cornea acquires many of the physical properties
characteristic of its normal function, most notably its
Vol. 32
transition from opacity to transparency,32 in addition
to being extensively innervated.33 In a subsequent
paper (Bee and Roche, manuscript in preparation),
we demonstrate that intubation resulted in precocious development and innervation of the embryonic chick cornea. Thus, this novel system provided
the unique opportunity to investigate the mechanisms by which the development and innervation of
the cornea is controlled, and transparency is achieved.
Key words: eye, cornea, development, growth, pressure
Acknowledgments
The authors thank Amata Hornbruch for her invaluable
guidance and advice on the microsurgical procedures used.
References
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