The retina of the horse differs considerably in the degree and quantity of neurons and glial elements as well as in vascular patterns compared to the retina of other domestic animals. Morphometric analysis revealed that the thickness of the retina varies between 80 Āµm at the ora serrata and 250 Āµm medial to the optic disc. Approximately 90% of the equine retina is comparatively thin (< 130 Āµm). This is a physiologic response to the distance that oxygen can diffuse in avascular retina. Ganglion cells form a single layer in all parts of the retina. The majority of ganglion cells are very large Nissl-positive cells. Small Nissl-negative ganglion cells are less abundant. A high ganglion cell density is found only in the central area. Vascularization is virtually absent from the retina with the exception of a narrow strip around the disc of the optic nerve, as revealed by lectin histochemistry.
Light microscopy of the eyes of older horses repeatedly revealed cystoid degenerations in the retina adjacent to the pars plana of the ciliary body, as well as a destruction of the regular layering of the peripheral region of the retina.
Source: Ehrenhofer, M.C.A., Deeg, C.A., Reese, S., Liebich, H.G., Stangassinger, M., Kaspers, B.(2002): Normal structure and age-related changes of the equine retina. In: Veterinary Ophthalmology 5 (1), pp 39-47.
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