Components of the Ora Serrata
The ora serrata lies approximately 5mm anterior to the equator of the eye, is approximately 2 mm wide, and is the transition site between the single, nonpigmented layer of ciliary epithelium and the multilayered retina. It is composed of sharp projections of retinal tissue that are separated by round bays of the pars plana and appears as a serrated line when visualized with an ophthalmoscope. There is notable asymmetry between the nasal and temporal portions of the ora serrata, as the nasal side is located 1 mm closer to the limbus, has broader bays, and has a narrower pigment band and much easier to discern compared to the temporal ora.
Nonpigmented epithelial cells
The nonpigmented epithelial cells of the pars plana are uniform up to the ora serrata and are tightly attached to each other with tight junctions, adherens junctions, and desmosomes. There is a sharp transition at the ora serrata, where the cells abruptly become undifferentiated cells that are similar in appearance to Mullerian cells but also have characteristics of neural cells. Posterior to these cells are typical Mullerian cells.
Pigmented Epithelial Cells
The pigmented epithelial cells on both sides of the ora serrata are similar in appearance and are in close contact with the pars plana epithelium and retina; however, the pigmented epithelium is more firmly attached to the pars plana. On the pars plana side, nonpigmented cells are attached to pigmented epithelium by many tight junctions and desmosomes. At the ora serrata, the number of tight junctions and desmosomes decrease but a narrow band of them still attaches the pars optica retinae to the pigmented epithelium. The retina immediately posterior to the ora serrata is also attached to the pigmented epithelium by desmosomes, but not tight junctions. Further posteriorly, the desmosomes disappear, and the retina is held in place solely by processes of the pigmented epithelium and outer segment.
A well-developed photoreceptor layer does not start immediately at the ora serrata, but instead begins as a graded series of photoreceptors in progressive stages of development. Closest to the ora serrata are undifferentiated photoreceptors that do not have outer, inner, or central connections. Moving posteriorly, the photoreceptors begin to differentiate into either cones or rods, and some may begin to possess a primitive outer segment. Further posteriorly, the photoreceptors begin to resemble those in the fundus of the retina, as the receptor elements become longer, and their outer segments become more prominent. The size of this transition zone between the ora serrata and normal photoreceptors is usually about 280 µm.
The anatomy of the ora serrata has been suggested to play a role in the pathogenesis of peripheral retinal detachment. It is well known that peripheral retinal holes are more dangerous than macular holes; a comparison of the fine structures at the ora serrata and the macula may elucidate why. Near the macula, there is an abundance of rods with sheath-like extensions that are surrounded by pigment epithelial cells. This creates a large surface area where retinal cells and pigment epithelium are held firmly together. However, closer to the ora, the undifferentiated photoreceptors are not able to have this same firm contact with the pigment epithelial cells. This decreased capillary attraction, coupled with the decreased number of attachment bodies at the ora serrata, explain why peripheral holes are more likely to lead to retinal detachment. The asymmetry of the nasal and temporal ora serrata may also explain why most retinal dialyses occur in the lower temporal quadrant of the eye. However, a definitive correlation between these anatomic differences and pathologic changes remains yet to be established.
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