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E. Knop, M. Ludescher, N. Knop; Keratinization of the Human Meibomian Gland and Its Contribution to Meibomian Gland Dysfunction (mgd). Invest. Ophthalmol. Vis. Sci. 2010;51(13):2366.
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© ARVO (1962-2015); The Authors (2016-present)
The Meibomian gland (MG) is an important and as yet underestimated component of the functional anatomy of the ocular surface. Increasing evidence points to a high impact of MG hyper-keratinisation as a major cause of obstructive MG dysfunction (MGD) and evaporative dry eye disease. Information of the normal keratinisation status and cytokeratin composition of the human MG is limited.
Conjunctival whole-mount specimens including the lid margin from ten body donors were embedded in paraffin. Serial sections were stained by H&E and Masson-Goldners stain and by immunohistochemistry (IHC) with antibodies to cytokeratins and related proteins.
In conventional histology, the MG showed distinct similarities with the pilo-sebaceous unit of the cilia. The keratinised skin epithelium extended into the terminal part of the central duct and formed an excretory duct similar to the opening of the hair follicle. Masson-Goldners stain of tangential sections showed that the whole ductal epithelium including the connecting ductules contained keratohyalin granules in the superficial epithelial cell layer. IHC staining showed that the epithelium of the excretory duct, similar to the epidermis, was strongly positive for the keratinisation marker CK10 which was lost more proximal in the central duct. However, filaggrin, a marker for incipient stages of keratinisation and located in keratohyalin granules continued in the superficial layer of the duct epithelium all along the normal Meibomian central ductal system.
The MG shares similarities with the cilia in embryology, in structure and in the cytokeratin expression. It can hence be regarded as a "hair without a hair shaft". All parts of the normal MG ducts have signs of incipient keratinisation and hence preserve a commitment to keratinisation. Its upregulation in MGD explains hyper-keratinisation as a typical event in obstructive MGD. A flow diagram is presented, that summarizes the interacting events that form vicious circles in obstructive MGD.
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