Three different epithelial zones occur in the region of the posterior lid margin
Three different epithelial zones occur in the region of the posterior lid margin. Only two of these belong to the posterior lid border - the Lid Wiper and the Muco-Cutaneous Junction, MCJ, as transition zone from moist mucosa to oiy-dry skin. The preceding cornified epidermis does not belong to the posterior lid border but belongs histologically to the free lid margin.
The functionally important Meibomian Oil glands have their orifices still in the cornified epidermis and in pathology, and probably also with advancing age, the location of the posterior lid margin can migrate in anterior direction which affects the function of the Meibomian glands. Therefore the epidermis is considered here together with the MCJ and Lid Wiper.
- The cornified skin EPIDERMIS belongs still to the free lid margin and terminates in a sharp line typically just posterior to the orifices of the Meibomian glands. The sharp border line is clearly detectable in histology and in in-vivo confocal microscopy. The epidermis is a stratified squamous cornified epithelium but it is thinner and has only few remaining cornified squames on the free lid margin as compared to the outside of the eye lids.
Only two of these posterior zones belong to the posterior lid border - the MCJ and the Lid Wiper
- The MUCOCUTANEOUS JUNCTION (MCJ) is the anterior zone of the posterior lid border. It starts after the abrupt termination of the epidermis and extends towards the tarsal side up to the crest of the posterior margin. The MCI is the transition zone between the epidermis and the fully developed conjunctiva. It has a stratified squamous epithelium with flat para-keratinized cells as surface layers. The surface of the MCI is physiologically stainable with several vital stains and represents the so-called ´line of Marx´ - first described by Eugen Marx in the 1920s.
- The LID WIPER starts at the crest of the inner lid border and extends Into tarsal direction. The lid wiper is the marginal conjunctiva and forms a thickened epithelial lip directly at the location where the lid margin is in touch with the globe. Towards the tarsal side the lid wiper becomes thinner and forms a slope that typically transforms into the epithelium of the sub-tarsal fold. The lid wiper contains goblet cells and the cell shape in the superficial cell layer is cubical to columnar. The lid wiper is ideally suitable to serve as a device, similar to the windscreen wiper blade of a car, to spread the tear fluid from the marginal tear meniscus into the thin preocular tear film during the up-phase of every blink.
All three Posterior Lid Margin zones are invovlved in the maintenance of the tears and tear film - with different function
The MCJ of the posterior lid border is intimately associated in function with the preceding epidermis. The two posterior zones of the lid wiper and MJC are aqueous moist and the more anterior MCJ is still convered by the tear meniscus. Since the epidermis, in contrast, is cornified and has an oily-dry surface it may be assumed that the anterior flow of the aqueous tear film is limited by the start of the epidermis. In this respect all three posterior epithelial zones are involved, with different functions, in the maintenance of the tears. The lid wiper is in touch with the globe and distributes and renews the pre-ocular tears into a tear film for visual acuity. The MCJ, even thought is performs the transition to the epidermal cell differentiation, is still wettable and is the bottom of the tear meniscus. Finally, the basically un-wettable epidermis limits the outward flow of tears that would lead to a collapse of the meniscus and thus of the tear film.
=> A more detailed description of the ZONES of the Posterior Lid Margin can be found below on separate pages.