The Correction of Myopia Evaluation Trial (COMET): design and general baseline characteristics.

The Correction of Myopia Evaluation Trial (COMET) is a multicenter, randomized, double-masked, controlled clinical trial evaluating whether there is a difference in the progression of myopia between children wearing progressive addition lenses (PALs) versus conventional single vision lenses (SVLs), as measured by cycloplegic autorefraction. Axial length, measured by A-scan ultrasonography, is an additional outcome measure. To meet the recruitment goal of 450 participants, eligible children ages 6-11 years (inclusive) with myopia in both eyes (spherical equivalent between -1.25 diopters (D) and -4.50 D, astigmatism < or = 1.50 D, and anisometropia < 1.00 D) were recruited at four clinical centers between September 1997 and September 1998. Children who participated were assigned to receive PALs (Varilux Comfort with a +2.00 D addition) or SVLs. Measures include standardized cycloplegic autorefraction (Nidek ARK700A autorefractor), axial length (Sonomed A2500 ultrasound), subjective refraction (Marco TRS system), visual acuity (modified Early Treatment Diabetic Retinopathy Study protocol), accommodation (Canon R-1), and phoria (cover test and Maddox rod). Outcome measures are collected annually; adherence is assessed and prescriptions updated semiannually. Participants are being followed for at least 3 years. COMET enrolled 469 children. Their mean age is 9.3 years (range 6-11 years); 52% are female. COMET children are ethnically diverse, according to a self-report with 46% White, 26% African American, 14% Hispanic, and 8% Asian. Best-corrected visual acuity is better than 20/32 in both eyes. Baseline mean (+/-SD) cycloplegic refractive correction is -2.38 D (+/-0.81) in the right eye and -2.40 D (+/-0.82) in the left eye; mean (+/-SD) axial length is 24.1 mm (+/-0.7) in both eyes. Follow-up of these children will provide a first step in answering the important question of whether there are effective means to slow myopia progression. Study results should be applicable to a large proportion of children with myopia. The study will also provide useful information on myopia progression in children wearing conventional single vision lenses.

Steady state mRNA levels in tree shrew sclera with form-deprivation myopia and during recovery.

PURPOSE: In tree shrews, visual form deprivation induces myopia and tissue remodeling in the sclera, characterized by decreased levels of collagen and glycosaminoglycans (GAGs) and increased levels of matrix metalloproteinases (MMPs). Removal of the visual deprivation allows recovery. This study investigated whether these changes are accompanied by changes in steady state mRNA levels in the sclera. METHODS: Quantitative competitive reverse transcription-polymerase chain reaction (RT-PCR) was used to measure steady state levels of mRNA for collagen (alpha1(I) chain), decorin (core protein), gelatinase-A (MMP-2), stromelysin-1 (MMP-3), and a tissue inhibitor of metalloproteinase (TIMP-1) in the scleras of tree shrews that received either 11 days of monocular form deprivation (MD) or 11 days of MD followed by 4 days of recovery. A group of age-matched normal animals was also measured. RESULTS: After 11 days of MD, alpha1(I) collagen mRNA levels were 34% lower, and MMP-2 mRNA levels were 66% higher in the deprived eyes than in the control eyes. After 4 days of recovery, collagen mRNA levels were 33% higher, MMP-2 levels were 20% lower, and TIMP-1 levels were 43% higher in the recovering eyes than in the control eyes. Decorin and MMP-3 mRNA levels were not significantly different between the treated and control eyes after MD or after recovery. CONCLUSIONS: The tissue remodeling in mammalian sclera induced by altering the visual environment is accompanied by modulation of mRNA levels in the sclera. The levels of collagen and MMP-2 mRNA were modulated in a pattern generally consistent with observed changes in protein levels, suggesting that visual regulation of the levels of these scleral proteins may involve modulation of gene expression at the transcriptional level.

Effect of interrupted lens wear on compensation for a minus lens in tree shrews.

BACKGROUND: When a young animal wears a monocular minus (concave) lens that shifts the focal plane away from the cornea, the vitreous chamber elongates over a period of days, shifting the retinal location to compensate for the altered focal plane. We examined the effect of removing the lens for a portion of each day on the amount of compensation in tree shrews. METHODS: Starting 24 days after natural eye opening, juvenile tree shrews wore a goggle frame that held a -5 D lens in front of one eye, with an open frame around the fellow control eye. The goggle was removed for 0, 0.5, 1, 2, or 7 h each day (N = 5, 5, 5, 5, and 3 animals per group, respectively), starting 0.5 h after the start of each 14 h light-on period. After 21 days of treatment, measures were made of the cycloplegic refractive state (streak retinoscopy) and the ocular component dimensions (A-scan ultrasound). Normal animals that experienced 14 h each day with no lens (N = 3) were also examined. RESULTS: The treated eyes of the 0 h group developed full refractive compensation for the lens (treated eye - control eye, mean +/- SEM = -5.8+/-1.1 D) and had increased vitreous chamber depth (0.13+/-0.02 mm) and axial length (0.12+/-0.02 mm) relative to the untreated control eye. The groups in which the lens was removed for 0.5 and 1 h each day showed partial compensation for the -5 D lens, both in refractive state (-4.2+/-0.4 D; -2.9+/-1.6 D) and in vitreous chamber depth (0.12+/-0.02 mm; 0.09+/-0.02 mm). The 2, 7, and 14 h (normal) groups showed no significant refractive or axial compensation. In the 0.5 and 1 h groups, A-scan ultrasound showed a thinning of the region between the front of the retina and back of the sclera. CONCLUSIONS: The eyes of tree shrews can tolerate altered monocular visual stimulation produced by a minus lens worn for 12 h of a 14-h light cycle without developing an induced myopia. However, when the lens is worn more than 12 of 14 h each day, compensation appears to increase linearly with decreased lens-off time. If the eyes of human children respond similarly to defocus from near work or other sources, it would seem that the defocus must be present almost all the time to induce myopia. If defocus contributes to human myopia through a compensation mechanism, then an increase in the amount of time that focused images are present should reduce myopic progression.

Regulation of the mechanical properties of tree shrew sclera by the visual environment.

Experiments in several species have shown that the axial elongation rate of the developing eye can be increased or decreased by manipulating the visual environment, indicating that a visually guided emmetropization mechanism controls the enlargement of the vertebrate eye during postnatal development. Previous studies in tree shrews (Tupaia glis belangeri) suggest that regulation of the mechanical properties of the sclera may be an important part of the mechanism that controls the axial elongation rate in this mammal. To learn whether the mechanical properties of the sclera change when the axial elongation rate is increased or decreased under visual control, uniaxial mechanical tests were performed on 3-mm wide strips of tree shrew sclera. The creep rate was measured under 1, 3, and 5 g of tension, maintained for 30 min at each level. The modulus of elasticity was calculated from the elastic extension that occurred when the force was increased from 0 to 1 g, 1 to 3 g, and 3 to 5 g. Both were measured in the sclera of both eyes from animals exposed to four experimental conditions: (1) Normal development, at intervals from the day of natural eyelid opening (day 1 of visual experience [VE]) to greater than 5 years of age; (2) Monocular form deprivation (MD), for varying lengths of time; (3) Recovery from MD; (4) Monocular -5 D lens treatment. The creep rate was low in normal animals (1-2% elongation/h), did not change significantly between day 1 and day 75 of VE, and was not significantly different between the two eyes. Four days of MD produced a 200-300% increase in creep rate in the sclera from deprived eyes. Creep rate remained similarly elevated after 11 and 21 days of MD. After 2 days of recovery, which followed 11 days of MD, the creep rate of sclera from the recovering eyes was below normal levels. In animals that wore a monocular -5 D lens for up to 21 days, creep rate increased, and then decreased, in concert with the increase, and decrease, in axial elongation rate as the eyes compensated for the lens. The modulus of elasticity of the sclera was not significantly affected by any manipulation. The temporal correspondence between changes in axial elongation rate and changes in creep rate support the hypothesis that regulation of the time-dependent mechanical properties of fibrous mammalian sclera plays a role in controlling axial elongation rate during both normal emmetropization and the development of refractive errors.

The susceptible period for deprivation-induced myopia in tree shrew.

To examine the susceptible period for deprivation-induced myopia, six groups of tree shrew pups (Tupaia glis belangeri) were monocularly deprived for 12 days with an opaque occluder starting 7, 15, 21, 33, 48, or 63 days after natural eyelid opening. Compared to the untreated fellow control eye, significant myopia and vitreous chamber elongation were produced by the deprivation in all six groups. The effect was greater in the middle three groups in comparison with the youngest and the two oldest groups and the amount of induced myopia and axial elongation was not proportional to the normal rate of axial growth. The peak period of susceptibility was between approximately 15 and 45 days after eye opening during the juvenile, slow-elongation phase of ocular development when the eye is within 7% of its adult axial length. Significant myopia and axial elongation were also induced in adult animals by 70 days of monocular deprivation. To examine recovery from deprivation-induced myopia, the occluder was removed at the end of the 12 day deprivation period. After an additional 48 days of binocular visual experience, no significant myopia was present in the previously deprived eyes in any experimental group. During the recovery period, the elongation rate of the previously deprived eyes was reduced in comparison with the control eyes while normal corneal flattening and lens development continued, thus reducing the myopia. No difference in corneal curvature, relative to the untreated control eyes, was found after deprivation or after the recovery period. Data are presented which suggests that changes in the thickness of the choroid may occur in this mammal during deprivation and recovery that are in the same direction, but of smaller magnitude, than those reported in the chicken. The results of this study provide evidence that visually guided emmetropization occurs in this mammalian species during a period of ocular development analogous to the juvenile period in humans.

Characterization of two new preproGnRH mRNAs in the tree shrew: first direct evidence for mesencephalic GnRH gene expression in a placental mammal.

Reproductive maturation and regulation is centrally orchestrated by gonadotropin-releasing hormone (GnRH). GnRH produced in the vertebrate hypothalamus acts on the pituitary to regulate gonadotropins. In nonplacental mammalian species, it has recently been shown that a second GnRH gene is expressed in mesencephalic cells. Here, we report the cDNA sequences and expression patterns for two distinct genes encoding the hypothalamic and mesencephalic GnRH forms in the brain of a placental mammal, the tree shrew (Tupaia glis belangeri). The novel mammalian GnRH form, designated here as [His5Trp7Tyr8]GnRH (often called chicken GnRH II), is expressed in neurons of the mesencephalon and is the first nonhypothalamic form to be isolated from a mammal. Its peptide sequence is identical to the form previously reported in fish, amphibians, reptiles, and birds, revealing that it has remained unchanged for 500 million years. In contrast, the sequences of the hypothalamic GnRH decapeptides vary by as much as 50% across vertebrate species. The remarkable sequence conservation of mesencephalic GnRH suggests that it has been highly constrained throughout evolution, perhaps indicating an important, conserved nongonadotropic role. The discovery and localization of two mRNAs encoding distinct GnRH forms in an advanced mammal suggest that other mammals, including primates, may also have a second GnRH gene with expression localized in the midbrain.

Laminar organization of receptive field properties in the dorsal lateral geniculate nucleus of the tree shrew (Tupaiaglis belangeri).

A laminar analysis of the receptive field properties of relay cells in the binocular region of the tree shrew dorsal lateral geniculate nucleus (LGN) found three main subdivisions. Lamina 1 (receiving ipsilateral eye input) and lamina 2 (contralateral) comprise a pair of layers that contain only ON-center neurons. Laminae 4 (contralateral) and 5 (ipsilateral) comprise a pair of layers with mostly OFF-center cells (86%). Laminae 3 and 6 (both contralaterally innervated) also form a distinct pair, although lamina 3 contains a mixture of cells with ON-centers (43%) or OFF-centers (57%), and lamina 6 contains mostly cells with ON-OFF centers and suppressive surrounds (81%). Cells located in the interlaminar zones resembled neurons in laminae 3 and 6. In comparison with the cells in the OFF-center laminae 4 and 5, the ON-center cells in laminae 1 and 2 had smaller, more elliptical receptive field centers with stronger responses to flashed visual stimuli. In addition, cells in the ipsilateral eye laminae 1 and 5 showed a greater change in center diameter, with eccentricity from the area centralis, than cells in the contralateral eye laminae 2 and 4. Principal components analysis using six receptive field properties (latency to optic chiasm stimulation, receptive field center diameter, maintained discharge rate, response onset latency, peak spike density, and phasic-tonic index) suggested that the cells in laminae 3 and 6 and the interlaminar zones are W-like. Principal components analysis of the same receptive field properties in laminae 1, 2, 4, and 5 did not reveal differences clearly related to X-like (parvocellular) and Y-like (magnocellular) categories. Ninety-seven percent of the cells tested for linearity of spatial summation in laminae 1, 2, 4, and 5 were linear. We conclude that the dominant organizational features of the tree shrew LGN are the ON-center, OFF-center, and W pairs of layers that project to different regions within the striate cortex.

Animal models of emmetropization: matching axial length to the focal plane.

BACKGROUND: It has long been recognized that more people are emmetropic than would be expected from a random combination of the refractive and axial components of the eye. However, it has been difficult to determine whether this is the result of an active emmetropization mechanism. METHODS: This paper reviews some of the studies in animals that have been conducted during the past 20 years. Four basic paradigms have been used to determine whether the visual environment helps guide eyes to emmetropia: 1) observing the normal pattern of ocular development, 2) shifting the location of the focal plane with minus- (and plus-) power lenses, 3) removing focused images by visual form deprivation and, 4) restoring form vision after a period of visual deprivation. RESULTS: Data from many studies suggest that an active emmetropization mechanism guides the postnatal development of the eye, matching the axial length to the focal plane. In normal development, the axial length initially is generally short so that the photoreceptors are in front of the focal plane of the unaccommodated eye. The subsequent axial elongation eventually moves the photoreceptors to, but not past, the focal plane. When animals are raised with the focal plane shifted posteriorly with minus-power lenses, the eyes elongate to approximately match the displaced focal plane. When information about the location of the focal plane is removed by visual deprivation, the eyes elongate past the point of emmetropia and become myopic. When developing eyes that have become myopic from a brief period of form deprivation are re-exposed to patterned images, they can slow their axial elongation, gradually eliminating the myopia. Data from several species suggest that the axial length is regulated within the eye itself, involving direct, spatially local communication from the retina to the sclera. It also appears that the regulation of axial elongation involves active control of the scleral extracellular matrix. CONCLUSIONS: If humans have a similar mechanism, then successful emmetropization in children may involve two components. One is to inherit a fully functional emmetropization mechanism. Equally important is exposure to a "normal" visual environment. Deficiencies in either, or an interaction between a compromised mechanism and a non-optimal visual environment might also prevent emmetropization.

Reduced extracellular matrix in mammalian sclera with induced myopia.

The purpose of this study was to learn whether visual form deprivation, which produces myopia in the deprived eye, alters the scleral extracellular matrix in tree shrew, a mammal closely related to primates. Axial myopia was induced in 10 tree shrews by monocular deprivation imposed with a translucent diffuser. The other eye in each animal was an untreated control. After 21 days of deprivation the refractive state and axial component dimensions were measured and the eyes were assayed for levels of DNA, hydroxyproline, and sulfated glycosaminoglycans (GAGs) in samples of the sclera and the cornea. In comparison to the open control eye, the deprived eyes became myopic and elongated. In the sclera, DNA levels were not significantly changed from the control eye. Sulfated GAG levels were significantly lower in the deprived eyes, as compared to the control eyes, at the posterior pole (-15.6%), at the nasal equatorial region (-18.1%), and in the rest of the sclera (-11.6%). The hydroxyproline level was significantly lower only at the posterior pole (-11.8%). Levels of sulfated GAGs were significantly reduced relative to DNA and relative to hydroxyproline in the total sclera. No significant changes were found in the cornea. The lower level of sulfated GAGs throughout the sclera of the deprived eyes, as compared with the control eyes, suggests that the deprived sclera contained less proteoglycan, or that the proteoglycans were less glycosylated or less sulfated. In contrast, the regional reduction of hydroxyproline suggests that collagen accumulation was specifically reduced only at the posterior pole of deprived eyes. These results suggest that form deprivation slows or reverses the normal process of extracellular matrix accumulation in the sclera of this mammal. This may allow the sclera to be more distensible, permitting the vitreous chamber elongation and resultant myopia.

Prevention of collagen crosslinking increases form-deprivation myopia in tree shrew.

To examine whether collagen crosslinking is important for the regulation of refractive development, tree shrews were treated with agents that block collagen crosslinking [beta-aminoproprionitrile (beta-APN), or D-penicillamine (DPA)] and underwent monocular deprivation (MD) of form vision by eyelid closure to induce myopia. MD began on the first day of visual exposure and continued for 75 days. After 15-20 days of visual exposure, daily intraperitoneal injections of beta-APN (beta-APN MD animals, n = 5) or DPA (DPA MD animals, n = 5) were administered for 17-21 days. beta-APN open animals (n = 5) received the same injection schedule, but both eyes remained open. Saline MD animals (n = 5) received i.p. saline and MD. At 75 days of visual exposure, the MD eyes of beta-APN treated tree shrews were highly myopic (-23.6 +/- 3.3 D) in comparison to their open control eyes. This was markedly greater than the difference in saline MD animals (-11.0 +/- 0.8 D). DPA MD animals showed a relative myopia of -14.3 +/- 2.2 D. The structural correlate of the myopia, elongation of the vitreous chamber in the deprived eyes relative to the control eyes, was significantly greater in the beta-APN MD animals 0.53 +/- 0.03 mm, which was not significantly different from the saline MD group. Thinning of the posterior sclera, but not the cornea, was observed in the deprived eyes of beta-APN treated tree shrews, along with a tessellated appearance to the fundus. The eyes of the beta-APN open animals showed no significant differences from normal. Beta-APN MD and DPA MD treated chickens did not develop greater myopia or vitreous chamber elongation than standard MD chickens. The selective effect of the lathyritic agents on the deprived eyes in tree shrew suggests that collagen crosslinking interacts, in the mammalian sclera, with a retinally-derived signal to regulate the elongation of the eye in myopia.

Lid-suture myopia in tree shrews with retinal ganglion cell blockade.

To determine whether central communication of retinal signals is necessary for the development of an experimentally induced myopia, tree shrews were exposed to monocular deprivation (MD) while the action potentials of retinal cells in the deprived eye were blocked with intravitreally injected tetrodotoxin (TTX-MD animals). TTX injections (0.6 microgram in 3 microL) and MD began about 15 days after eye opening, at the start of the susceptible period for the development of lid-suture myopia. Six injections were given, one every second day to produce 12 days of MD and TTX-blockade. Control TTX animals (TTX-open) received TTX in one eye, but not MD, on the same injection schedule and were always found to be behaviorally unresponsive to visual stimuli through the injected eye indicating that TTX blocked central communication of action potentials. Other control animals received intravitreally injected saline in either an open eye (saline-open), or an MD eye (saline-MD). A sham-injected group (sham-inj-MD) received MD and all anesthetic and surgical manipulations except for penetration of the sclera. In all groups, one eye in each animal was an untreated control. Two effects were found. All MD groups, including the TTX-MD animals, developed a significant vitreous chamber elongation in the deprived eye, indicating that an experimental myopia developed despite ganglion cell blockade. Thus, retinal mechanisms in tree shrew can detect the presence of a degraded visual image and produce an experimental myopia that does not depend on the receipt of visual messages by central neural structures. In addition, eyes in which the sclera was punctured had smaller vitreous chamber depths than comparable uninjected eyes, indicating that puncturing the sclera reduced the normal elongation. These data suggest that forces within the eye normally contribute to its expansion and may be resisted by the choroid and/or the sclera.

Center/surround relationships of magnocellular, parvocellular, and koniocellular relay cells in primate lateral geniculate nucleus.

As in other primates, the lateral geniculate nucleus (LGN) of the prosimian primate, bush baby (Galago crassicaudatus), contains three morphologically and physiologically distinct cell classes [magnocellular (M), parvocellular (P), and koniocellular (K)] (Norton & Casagrande, 1982; Casagrande & Norton, 1991). The present study examined quantitatively the center/surround relationships of cells in all three classes. Estimates of receptive-field center size (Rc) and sensitivity (Kc) and of surround size (Rs) and sensitivity (Ks) were obtained from 47 LGN relay cells by fitting a difference of Gaussians function to contrast-sensitivity data. For M and P cells, center size (Rc) increases with eccentricity but is about two times larger for M than for P cells at a given eccentricity. Surround size (Rs) increases with eccentricity for P but not for M or K cells. The center sensitivity (Kc) is inversely related to center size (Rc) and surround sensitivity (Ks) is inversely related to surround size (Rs) for cells in all classes, a result consistent with the sensitivity regulation that is produced by light adaptation. High spatial-frequency cutoff (acuity) is inversely related to center size (Rc). However, the peak contrast sensitivity is relatively independent of Rc. The ratio of the integrated strength (volume) of the surround to the volume of the center remains relatively constant (median, 0.87) across all three cell classes. This ratio is an excellent predictor of a cell's rolloff in contrast sensitivity at low spatial frequencies: cells with a low surround/center ratio have less low-frequency rolloff. Although M, P, and K cells generally display similar center/surround relationships, differences in center size and the other parameters between the classes distinguish most M, P, and K cells. These findings demonstrate that both similarities and differences in the visual-response properties of primate LGN cells in these three parallel afferent pathways can be explained by basic center/surround relationships.

Reduction in choroidal blood flow occurs in chicks wearing goggles that induce eye growth toward myopia.

Goggles that degrade the retinal image produce axial enlargement of the ocular globe and large myopic refractive errors. Many authors have assumed that visual image degradation itself leads to myopia. Hodos and co-authors have shown, however, that goggled eyes in chicks are considerably warmer than normal. Such temperature changes may either underlie or be a consequence of alterations in choroidal blood flow (CBF). Since alterations in CBF could affect eye growth, we explored the effect of monocular goggling on CBF in chicks. Plastic goggles were glued over one eye in four-day old chicks and the goggles were left in place for 12 or 14 days. Fourteen days after the goggling, CBF was measured using laser Doppler velocimetry. Three groups of chicks were studied: 1) chicks with goggles for 14 days; 2) chicks with goggles for 12 days followed by no goggles for the two days; 3) age matched non-goggled chicks. A -scan ultrasonography confirmed that the visual deprivation produced vitreous chamber elongation in the goggled eye and that the degree of elongation for the goggled eye was the same for the two goggled groups. The results were: 1) blood flow in non-goggled chicks was similar in both eyes; 2) blood flow was significantly reduced in the goggled eye in chicks wearing goggles for 14 days- 37% of control; and 3) blood flow was still significantly reduced in the goggled eye in chicks whose goggles were removed two days before measurement- 51% of control. These results show that CBF is reduced by goggles that result in myopic eye growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal development of refractive state and ocular component dimensions in the tree shrew (Tupaia belangeri).

The normal development of refractive state, ocular components and simple visually-guided behaviors was examined in maternally-reared tree shrews. Six groups consisting of 5 animals each were anesthetized and examined after 0, 15, 30, 45, 60 and 75 days of normal binocular visual exposure. Measures in the 75-day group provided values for an improved schematic eye of the tree shrew. Cycloplegic refraction showed a marked hyperopia (+25 D) at eye opening which decreased rapidly during the first 15 days of visual exposure and stabilized near the value (+5 D) expected in an eye of this axial length (approx. 7.8 mm). Corneal radius increased slightly during development. Anterior segment depth, measured by A-scan ultrasonography, seemed to complete most of its development at an earlier age (15-30 days of visual exposure) than did other ocular parameters. Lens thickness increased steadily throughout development. Vitreous chamber depth increased rapidly until 15 days of visual exposure, and then decreased because the lens thickness increased more rapidly than axial length. Crude orienting to, and following of, large objects developed shortly after eye opening (median age at onset, 5 and 6 days, respectively). Triggered visual placing responses developed at about the same time that the refractive state completed the rapid drop from highly hyperopic values. The slowed rate of ocular development after 15 days of visual exposure may be related to increased retinal activity that is permitted by neural maturation and by the presence of a relatively well-focussed retinal image. The increased activity may influence the final dimensions of the eye to coordinate the axial length with the focal length of the eye.

The development of experimental myopia and ocular component dimensions in monocularly lid-sutured tree shrews (Tupaia belangeri)

Tree shrews were monocularly deprived (MD) from the day of eye opening for periods of 15, 30, 45, 60 and 75 days. The initial structural change after 15 days of MD was a flattening of the corneal curvature in the deprived eye causing a hyperopic increase in refraction, relative to the fellow control eye. A relative myopia was first observed after 30 days of deprivation and increased as the length of MD increased. Animals monocularly deprived for 75 days consistently showed high degrees of myopia (greater -10 D). An increase in vitreous chamber depth was found after 30 days of deprivation and continued to increase, relative to the control eye, throughout the developmental period under investigation. There was a strong correlation (r = 0.84) between increase in vitreous chamber depth and the amount of experimentally-induced myopia. Anterior chamber depth was shallower in the deprived eyes of all animals. The crystalline lens was also consistently thinner in the deprived eye. Based on optical modeling, the observed myopia was consistent with the changes in ocular component dimensions. The susceptible period for experimental myopia begins about 15 days after eye opening.