Optic nerve head morphology of the Eastern gray squirrel, Sciurus carolinensis

The objective of this study was to characterize the morphology of the Eastern gray squirrel optic nerve head, using histochemical techniques and scanning electron microscopy, in order to make comparisons with other mammalian species. Eyes of three Eastern gray squirrels were enucleated, fixed in 10% neutral buffered formalin, and embedded in paraffin. Sections through the optic nerve head were made in the three different planes, and staining was accomplished with hematoxylin and eosin and Gomori's trichrome stain. Stained sections were examined using light microscopy. The surface of the optic nerve head and the peripapillary retina were evaluated using field emission scanning electron microscopy. The anterior optic nerve head of the Eastern gray squirrel was found to be horizontally elongated and tapers to a more conventional round shape after it exits the eye. Retinal nerve fiber axons converge on the optic nerve head from all directions. Anterior to the lamina cribrosa, the mean (+/- SD) optic nerve axon bundle diameter was 27 +/- 11 mum. The lamina cribrosa measured 363 mum dorsal to ventral and 3.60 mm nasal to temporal. The thickness of the lamina was 125 mum anterior to posterior with a mean (+/- SD) laminar pore size of 45 +/- 12 mum. The optic nerve head measured 3.65 mm nasal to temporal and 330 mum dorsal to ventral. Trichrome staining indicated the presence of collagen in the lamina cribrosa. This is the first detailed description of the squirrel optic nerve head. It has an unusual shape among mammals with a dramatic horizontal elongation. The purpose of this shape is not apparent. The presence of collagen in the scleral lamina cribrosa indicates a similar extracellular matrix composition as in other species.