Abstract
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.
Highlights
Merkel cells, first described by Fredrich S
The ocular surface whole-mounts show that CK8-positive cells were localized in the fornix and the palpebral conjunctiva, and CK8-positive immunofluorescence was observed in the membrane, nucleus and cytoplasm of specific cells
We found that the CK8-positive cells exhibited circular nuclei and visible nucleoli, and the cytoplasmic fluorescence revealed reticular and granular shapes (Figure 1)
Summary
Merkel cells have been shown in vertebrates and cyclostomes[2] and can be widely found in hairy skin, the eccrine glandular ridges of the glabrous skin (palms and soles), ectodermderived mucosal tissues (palatine mucosa), and whisker pads or taste buds[3,4]. Merkel cells are generally classified into two distinct subpopulations based on whether or not they are associated with nerves. Those that are associated with nerves are referred to as the Merkel cell-nerve complexes and have mechanoreceptive and chemo-sensitive functions. Those that do not associate with nerves possess endocrine functions[4,5,6]. Recent studies have shown that Merkel cells are important in light-touch responses and can work as neurotransmitters, it is suggested that Merkel cells are essential for the perception of space experienced by individuals[7]
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