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Samuel D Hanlon, Justin Andrew Courson, Thao T Do, Clifton Wayne Smith, Alan Robert Burns; Neuron-epithelial cell fusion in the murine cornea revealed with serial block-face sem imaging . Invest. Ophthalmol. Vis. Sci. 2017;58(8):2616. doi: https://doi.org/.
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It is generally accepted that corneal stromal nerves penetrate the epithelial basal lamina giving rise to intra-epithelial nerves. However, we observed a sub-group of stromal nerves fusing with basal epithelial cells where the axoplasm merged with the epithelial cytoplasm, which to our knowledge was a novel observation. This study characterizes stromal nerve-epithelial cell fusion and how it differs from stromal nerve-epithelial penetration.
Corneas from C57BL/6 male mice aged 8-12 weeks were fixed and embedded in resin blocks for ultrastructural sectioning. Serial block-face images (100 nm sections) were acquired using a Gatan 3 View system mounted in a Tescan MIRA3 SEM. Z-stacks were obtained from the anterior stroma in the paralimbus/paracentral and central cornea. Amira 6.0 software was used to manually segment representative nerves and for 3-D image reconstruction.
Within the central region of 5 corneas, we observed stromal nerves fused with basal epithelial cells without evidence of cell plasma membranes separating axoplasm from epithelial cytoplasm; i.e., neuron-epithelial cell fusion. These interactions did not result in nerve fibers extending into the epithelium. In several cases the epithelial cell protruded through the basal lamina, toward the stromal nerve fiber. We noted, predominantly outside the central cornea, stromal nerves penetrating the basal lamina with a characteristic double plasma membrane separating axoplasm from epithelial cytoplasm. In these cases, sub-basal nerve branches extended into the epithelium. Stromal nerves that fused with the epithelium were typically larger in diameter (2-4µm) than those (1-2 µm) penetrating the basal lamina and forming the sub-basal plexus.
Stromal nerve fusion with basal epithelial cells is a common occurrence in the central region of the mouse cornea. While cell fusion results in direct cytoplasmic contact between nerve and epithelium, it does not result in the formation of intra-epithelial nerve fibers. Stromal nerve fibers that penetrate the epithelial basal lamina, and do not fuse with epithelial cells, contribute to the formation of intra-epithelial nerve fibers. The larger diameter of the fusing nerves compared to the penetrating nerves suggests different types of nerve fibers are involved.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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