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Alan D Springer, Anita E. Hendrickson; All Foveas Are Not Created Equally. Invest. Ophthalmol. Vis. Sci. 2016;57(12):560.
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© ARVO (1962-2015); The Authors (2016-present)
Many species have a fovea, but foveal development has been studied extensively only in primates. The proposed mechanisms for fovea-genesis in primates are either unsupported (cell migration) or controversial (deformation by intraocular pressure). This study examined foveal development in a reptile (Pogona vitticeps; Bearded Dragon; BD) from egg-laying through hatching to determine similarities and differences between reptiles and primates.
BDs were bred in captivity. Embryonic day 1 (E1) was the day after egg laying. The eyes of embryos of different ages and of juveniles were removed, fixed in methyl Carnoy, paraffin sectioned and stained with Richardson's stain. The dorsal orbit of some BDs was removed to view the eye in situ.
Newly hatched (E67) BDs have a mature fovea and catch food soon after hatching. BD eyes, like those of birds, have an avascular inner retina that is nourished by optic disc blood vessels. The fovea appears as a dark spot located dorso-temporal to the disc. The dark spot is due to the thinness of the foveal retina (81 μm) relative to the thick surrounding parafoveal retina (340 μm), allowing the pigment epithelium to be seen in fixed eyes. Viewed dorsally, the posterior eye and retina undergo several marked changes in shape. Eyes are round at E20 and then develop a posterior staphyloma-like bulge at E28-38 that measures 1.36 mm high x 0.87 mm wide. This persists until E39 after which the posterior eye and retina become round by E43. An area centralis (AC), that has thickened inner and outer retinal laminae, appears at the apex of the retinal bulge at E38. A shallow pit appears in the thick AC at E45 and deepens progressively. The pit appears adult-like at hatching (E67) when it extends through all laminae including the photoreceptor layer. Unlike primates, no evidence was found for a central increase in foveal cone density before or after hatching.
Primate and reptile foveas develop in dramatically different ways, suggesting that different mechanisms have evolved to create this area of high visual acuity in primates vs. reptiles and birds. Unlike BDs, primates have a thick vascularized inner retina, an avascular zone centered on the fovea and a progressively increasing central cone density. No evidence for active cellular migrations was found in BDs.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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