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N. Knop, E. Knop; Presence and Structure of the Lidwiper Zone in the Marginal Conjunctiva of the Rat and Rabbit. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4374.
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The lid wiper is a specialized zone of the marginal conjunctiva. It is located at the inner lid border directly apposed to the corneal surface and assumed to spread the preocular tears into a thin film. The lidwiper has only been described in the human conjunctiva although it may be more widespread among species. Therefore we have investigated common laboratory animals like the rat and rabbit for the presence of such a zone.
Conjunctival whole-mount specimens and total bulbi were investigated by serial section histology in ten rats (DA and Lewis) and in ten rabbits (NZW and Chinchilla).
Similar to man, several zones of different morphology occur at the inner border of the upper and lower lid. The keratinization of the lid margin epithelium (four layered about 20µm thick in the rat and four layered about 30µm in the rabbit) stops at the level of the Meibomian gland openings in both species. A squamous epithelium with increased density of the surface cells continues for about 100µm in the rat and 100-200µm in the rabbit and represents the muco-cutaneous junction. This is followed by a zone with a drastically thickened epithelium (8-12 cell layers) that forms a cushion or lip-like structure. It extends for about 250-350µm length and has a maximal thickness of about 40-60µm in the rat. In the rabbit it extends for 500-800µm and has a maximal thickness of 60-100µm. The lidwiper epithelium is cuboidal in the rabbit and contains goblet cells towards the tarsal side. In the rat it is squamous without goblet cells. Adjacent to the lidwiper, the epithelium becomes thinner than the usual tarsal conjunctival epithelium and forms a subtarsal fold. This is more obvious in the rat and less obvious in the rabbit.
At the marginal conjunctiva of the upper and lower lid of the rat and the rabbit is a structure that resembles the lid wiper in the human. It is about three time thicker than the epidermis and about four times thicker than the tarsal conjunctiva. It forms a cushion- or lip-like structure at the inner lid border, directly apposed to the cornea. Therefore it appears as a suitable device to spread the preocular tear film and is hence an important component for maintenance of ocular surface integrity. Accordingly, epitheliopathy of the lid-wiper is shown to be a very sensitive early indicator of the dry eye syndrome in the human. Better knowledge about the anatomy of this structure may also prove useful in dry eye research in laboratory animals.
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