Special Issue IMI Report on Experimental Models of Emmetropization and Myopia David Troilo, 1 Earl L. Smith III, 2 Debora L. Nickla, 3 Regan Ashby, 4 Andrei V. Tkatchenko, 5 Lisa A. Ostrin, 2 Timothy J. Gawne, 6 Machelle T. Pardue, 7 Jody A. Summers, 8 Chea-su Kee, 9 Falk Schroedl, 10 Siegfried Wahl, 11 and Lyndon Jones 12 1 SUNY College of Optometry, State University of New York, New York, New York, United States 2 College of Optometry, University of Houston, Houston, Texas, United States 3 Biomedical Sciences and Disease, New England College of Optometry, Boston, Massachusetts, United States 4 Health Research Institute, University of Canberra, Canberra, Australia 5 Department of Ophthalmology, Department of Pathology and Cell Biology, Columbia University, New York, New York, United States 6 School of Optometry, University of Alabama Birmingham, Birmingham, Alabama, United States 7 Biomedical Engineering, Georgia Tech College of Engineering, Atlanta, Georgia, United States31 8 College of Medicine, University of Oklahoma, Oklahoma City, Oklahoma, United States 9 School of Optometry, The Hong Kong Polytechnic University, Hong Kong, SAR, China 10 Departments of Ophthalmology and Anatomy, Paracelsus Medical University, Salzburg, Austria 11 Institute for Ophthalmic Research, University of Tuebingen, Zeiss Vision Science Laboratory, Tuebingen, Germany 12 CORE, School of Optometry and Vision Science, University of Waterloo, Ontario, Canada Correspondence: David Troilo, SUNY College of Optometry, State University of New York, 33 West 42nd Street, New York, NY 10036, USA; dtroilosunyopt.edu. Submitted: October 14, 2018 Accepted: October 20, 2018 Citation: Troilo D, Smith EL III, Nickla DL, et al. IMI Report on Experi- mental Models of Emmetropization and Myopia. Invest Ophthalmol Vis Sci. 2019;60:M31M88. doi/10.1167/iovs.18-25967 The results of many studies in a variety of species have significantly advanced our understanding of the role of visual experience and the mechanisms of postnatal eye growth, and the development of myopia. This paper surveys and reviews the major contributions that experimental studies using animal models have made to our thinking about emmetropization and development of myopia. These studies established important concepts informing our knowledge of the visual regulation of eye growth and refractive development and have transformed treatment strategies for myopia. Several major findings have come from studies of experimental animal models. These include the eyes ability to detect the sign of retinal defocus and undergo compensatory growth, the local retinal control of eye growth, regulatory changes in choroidal thickness, and the identification of components in the biochemistry of eye growth leading to the characterization of signal cascades regulating eye growth and refractive state. Several of these findings provided the proofs of concepts that form the scientific basis of new and effective clinical treatments for controlling myopia progression in humans. Experimental animal models continue to provide new insights into the cellular and molecular mechanisms of eye growth control, including the identification of potential new targets for drug development and future treatments needed to stem the increasing prevalence of myopia and the vision-threatening conditions associated with this disease. Keywords: myopia, emmetropization, animal models, visual regulation, eye growth 1. INTRODUCTION E mmetropization refers to the developmental process that matches the eyes optical power to its axial length so that the unaccommodated eye is focused at distance. Investigations using animal models have informed our understanding of the role of vision in postnatal eye growth, the mechanisms and operating characteristics of emmetropization, and the develop- ment of refractive errors (myopia, where the eye is typically too long for its optical power; and hyperopia, where the eye is too short for its optical power). Animal models have established the existence of visual regulation of eye growth and refractive development as well as local retinal control of eye growth. They have also revealed biochemical signaling cascades that transduce visual stimuli related to the sign of defocus into cellular and biochemical changes in the retina, which, in turn, signal changes in the retinal pigment epithelium (RPE), choroid, and eventually sclera, leading to altered eye growth and changes in refractive state. These studies provide a framework for the development of optical and pharmacologic treatments that can be used to effectively reduce the prevalence and progression of myopia, which has become a major public health concern. 1 In this paper, the findings of investigations using experi- mental animal models to study emmetropization and myopia development are reviewed. The contributions that studies with experimental animal models have made to understanding the mechanisms of emmetropization, the development of myopia, and new treatments to reduce myopia progression are summarized. Current models of eye growth control, areas of investigation and major findings, and frameworks for the Copyright 2019 The Authors iovs.arvojournals j ISSN: 1552-5783 M31 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Downloaded from iovs.arvojournals on 03/02/2019 development of new and effective treatments for myopia are described. 2. ANIMAL MODELS COMMONLY USED IN STUDIES OF EMMETROPIZATION AND MYOPIA Experimental models of myopia and the visual regulation of eye growth have been demonstrated in a wide variety of species from primates to invertebrates, including macaque and marmoset monkeys, tree shrews, guinea pigs, mice, chickens, fish, and squid. All of these species (with the exception of squid) have been shown to develop myopia in response to visual form deprivation (see Section 3.2), compensate for optically imposed myopic or hyperopic defocus by regulating axial length (see Section 3.4), and recover from the induced refractive error when form deprivation or optical defocus is removed (see Section 3.3). Even though the squid model is the least well-characterized, squid eye growth responds to improve focus under imposed visual conditions. 2 Considering that all these varied species possess visually guided eye growth despite differences in ecology, ocular anatomy, visual function, and visual acuity, these results suggest that visual regulation of eye growth is a fundamental property of the camera-type eye, that it may have evolved more than once, and the mechanisms in vertebrates are evolutionarily conserved. From an experimen- tal perspective, each species provides unique advantages to study the mechanisms of visually guided eye growth and key signaling pathways that regulate refractive eye development across species; however, anatomical and physiological differ- ences must be taken into account when interpreting and translating results to humans. General retinal cellular organization and neural signaling circuitry are highly conserved among vertebrate species 3,4 ; however, there are significant variations between species. Diurnal primates, like humans, have a single fovea for high acuity, whereas other species may be multifoveal, or have an area centralis or visual streak, which are retinal areas with higher photoreceptor and ganglion cell density. The visual photopigment types underlying color vision also vary between species, as does retinal vascular anatomy. Table 1 summarizes structural similarities and differences between the retinas of the most commonly used experimental species. There are also significant species differences in the mechanisms and amount of accommodation, which regulates the dioptric power of the eye and may be indirectly involved in myopia development through its effects on retinal defocus. In many species, including human, accommodation is achieved by changing the power of the crystalline lens by contraction of the ciliary muscle, whereas in other species it is achieved by moving the lens. 5 Changes in corneal power have also been observed in some species. 68 For another recent review of different species used for experimental studies of emmetropization and myopia, see Schaeffel and Feldkaemper. 9 2.1 Comparative Ocular Anatomy and Visual Physiology of Animal Models 2.1.1 Nonhuman Primates. Macaque monkeys were used in the original studies showing form-deprivation myopia (FDM) and visual influences on eye growth. 10,11 Since then, both Old World (rhesus macaque Macaca mulatta) and New World (common marmoset Callithrix jacchus) monkeys have been used for myopia research. Both species have foveal retinas, eyes that are optically scaled down versions of human eyes, and visual physiology which is essentially identical to that of humans. 1215 The rhesus monkey retina is most similar to the human. It is rod-dominated (rod to cone ratio 20:1) with a cone-dominated fovea and possesses three cone types, with short-, middle- and long-wavelength sensitivities, in addition to rods. 16 The fovea provides visual acuity of approximately 44 cyc/deg. 13,14 The marmoset retina is cone-dominated with a well-developed fovea. 12,15 The marmoset retina contains rods as well as cones, which exhibit a polymorphism of visual pigments, in which three photopigments are in the middle- to long-wavelength range, with peak sensitivities at 543, 556, and 563 nm. 17 With this polymorphism, some animals are dichromatic (males and some females) while others are trichromatic (females). Visual acuity in marmosets is approx- imately 30 cyc/deg. 12,18 Both rhesus and marmoset monkeys have vascular inner retinas with a foveal avascular zone. In rhesus monkeys, the optic nerve head contains a collagenous TABLE 1. Retinal Differences in Species Used for Myopia Models Species Inner Retinal Blood Supply High Cell Density Region Photoreceptor Types and Peak Sensitivities Central Retinal Thickness Optic Nerve Head and Lamina Cribrosa Chick Avascular (Pecten) Area centralis (24,000 ganglion cells/mm 2 ) 83 Rods, S1 (415 nm, S2 (455 nm), M (508 nm), L (571 nm) 781 295350 lmat area centralis 84,782 Sparse glial and connective tissue 19,783 Zebrafish Vascular Area centralis (37,000 ganglion cells/mm 2 ) 112 Rods (503 nm), UV (361 nm), S (411 nm), M (482 nm), L (565 nm) cones 784 191 lm 785 Glial 120 Mouse Vascular Visual streak (6000 ganglion cells/mm 2 ) 786 Rods, UV (370 nm) and M (505 nm) cones 63 202 lm 787 Glial 788 Guinea pig Avascular Visual streak (2272 cells/ mm 2 ) 39 Rods, S (429 nm) and M (529 nm) cones 475 150 lm 789 Collagenous 41 Tree shrew Vascular Area centralis 27 Rods, S (428 nm) and L (555 nm) cones 32 213 lm 790 Collagenous 33 Marmoset Vascular Fovea 12 Rods, M/L (543, 556, 563 nm) cones 791 230 lm 12 Collagenous 19 Rhesus Vascular Fovea (33,000 ganglion cells/mm 2 ) 792 Rods, S 440 nm, M (536 nm), L (565 nm) cones 16,793 207 lm 794 Collagenous 795 Human Vascular Fovea (38,000 ganglion cells/mm 2 ) 796 Rods, S (419 nm), M (531 nm), L (558 nm) cones 797 182 lm at fovea 798 Collagenous 799 S, short wavelength; M, medium wavelength; L, long wavelength. IMI Experimental Models of Emmetropization and Myopia IOVS j Special Issue j Vol. 60 j No. 3 j M32 Downloaded from iovs.arvojournals on 03/02/2019 lamina cribrosa, closely resembling that in humans. In marmosets, the optic nerve also has a collagenous lamina cribrosa with characteristic sieve-like structure. 19 The accommodative system in rhesus monkeys and marmosets is closely related to that in humans and other primates. 20,21 The ciliary muscle and its pharmacology are similar to those of humans allowing cycloplegia (paralysis of accommodation) to be produced with muscarinic antagonists as in humans. Juvenile macaques and marmosets have an accommodative response of at least 20 diopters (D). 22,23 In previous studies, accommodation was successfully stimulated in awake-behaving marmosets and measured with photo- refraction, showing stimulus response slopes similar to humans. 22 Additionally, rhesus monkeys have been shown to develop presbyopia at a similar rate as humans, once corrected for life span. 20,24 Low availability due to low reproduction rate in macaques is a challenge, and the eyes and visual systems in macaques develop more slowly than in other species commonly used for myopia research. Marmosets give birth to twins or triplets approximately twice a year and are sexually mature at approximately 18 to 24 months. 25 2.1.2 Tree Shrew. Tree shrews belong to the order of Scandentia, which are closely related to primates. They are among the first species shown to develop FDM 26 and have since been used by several laboratories for myopia research. Tree shrews have a cone-dominated retina with rods compris- ing approximately 14% of the photoreceptor population. 27 Tree shrews do not have foveas, but the retina has an area centralis, 2729 which provides a visual acuity of approximately 2.4 cyc/deg. 30,31 Tree shrews are dichromatic, with short- and long-wavelength sensitive cones. 32 The tree shrew inner retina is vascular. The optic nerve contains a collagenous lamina cribrosa with radially oriented laminar beams. 33 Tree shrew eyes have relatively large crystalline lenses and relatively small vitreous chambers compared with primates. They do not appear to exhibit substantial accommodation 31,34 ; however, when stimulated with carbachol, tree shrews can accommodate up to 8 D. 35 Tree shrews typically give birth to two small litters a year. 2.1.3 Guinea Pig. Guinea pigs are diurnal rodents, which have been increasingly used as a model for myopia research. Guinea pigs develop FDM and can compensate appropriately for both imposed myopic and hyperopic defocus. 36,37 Guinea pigs are dichromatic. In addition to rods, the retinas of guinea pigs include middle- and short-wavelength-sensitive cones, which occupy superior and inferior areas of the retina, respectively, while the transition zone contains both cone types and cells with both pigments. 38 Guinea pigs do not have a fovea; however, the retinas have a visual streak, 39 which provides a visual acuity of approximately 2.7 cyc/deg. The guinea pig retina is avascular, having the retinal blood supply provided solely by the choroidal circulation. Because retinal nutrients must diffuse from the choroid, the retina is typically thinner than in animals possessing inner retinal vasculature. 40 The optic nerve contains a collagenous lamina cribrosa with connective tissue beams. 41 Guinea pig eyes have relatively large crystalline lenses and relatively small vitreous chambers compared with primates. 42 Guinea pigs do not appear to have an active accommodative response 43 ; however, approximately 5 D of accommodation can be elicited pharmacologically in juvenile animals. 44 Guinea pigs are able to breed year-round and grow rapidly, which allows large-scale studies.