Optic Nerve Head Morphology of the Eastern Gray Squirrel, Sciurus carolinensis

Abstract

Abstract: : Purpose To characterize the morphology of the Eastern gray squirrel optic nerve head, using histochemical techniques and scanning electron microscopy, in order to make comparisons to other mammalian species. Methods Eyes of three Eastern gray squirrels were enucleated, fixed in 10% neutral buffered formalin, and embedded in paraffin. Sections were made in the three different planes and staining was accomplished with hematoxylin/eosin and Gomori's trichrome stain. Stained sections were examined by light microscopy and other sections by field emission scanning electron microscopy. Results The anterior optic nerve head of the Eastern gray squirrel is horizontally-elongated and tapers off to a round structure soon after it exits the eye. Nerve fiber bundles converge on the nerve head from all directions with bundle diameter ranging from 14-18µm before leaving the eye. The lamina cribrosa measured 390µm dorsal to ventral with a thickness of 90µm anterior to posterior and a pore size of 34-64µm. The optic nerve head measured 3.65mm nasal to temporal. Trichrome staining indicated the presence of collagen in the lamina cribrosa. Conclusions This is the first detailed description of the squirrel optic nerve head. It has a unique shape among mammals as it exhibits a dramatic horizontal elongation, although the purpose of this shape is not apparent. The presence of collagen indicates a similar extracellular matrix composition as in other contrast species studied.

Keywords: optic disc • lamina cribrosa • anatomy