Merkel-like cells in the murine conjunctival stroma

Merkel cells, discovered by Friedrich Sigmund Merkel in 1875, are found in multiple regions of the skin and some mucosa and make contact with specialized nerve fibers, participating in the perception of touch. However, Merkel cells have thus far not been described on the ocular surface. The purpose of this study was to investigate the distribution and ultrastructure of Merkel cells on the ocular surface and study changes in their number and ultrastructure after corneal epithelial wounding. Entire mouse ocular surfaces were obtained and stained by antibodies and tracking dye on fixed whole-mount specimens. The distribution and ultrastructure of specific fluorescence-positive cells were analyzed using deconvolution microscopy and transmission electron microscopy (TEM), respectively. The corneal epithelial wound-healing model was employed to observe the ultrastructural changes of these CK8-positive cells. We found that CK8-positive cells and FM1-43-positive cells were mainly located in the stromal layer, but not in the epithelial basal layer, of the forniceal conjunctiva. Our TEM results indicate that these cells possess the unique characteristic structures of Merkel cells, including electron-dense membrane-surrounded granules and spine-like protrusions of variable lengths, and demonstrate the formation of Merkel cell-neurite complexes. After corneal epithelial wounding, these cells exhibited rapid cell shrinkage and nuclear lobulation. Thus, Merkel-like cells were found in the conjunctival stroma of the ocular surface and may play an important role in maintaining the normal physiological function of the ocular surface.