Cornea Corneal-epithelial barrier

For in vitro toxicity studies and assessment of the barrier function, drug transport, cell physiology, and metabolism as well as the development of delivery systems, cell culture models provide powerful systems for scientific research. As the corneal epithelium is the main barrier for ocular penetration, various corneal epithelial cell cultures were established besides the corneal constructs that mimic the whole cornea and serve as reductionist models for the ocular barrier. In general, two types of cell culture models are available primary cell cultures and immortalized, continuous cell lines. 

In another approach, Parnigotto and coworkers reconstructed corneal structures in vitro by using corneal stroma containing keratocytes to which corneal epithelial cells from bovine primary cultures were overlaid [73], However, this particular corneal model did not contain an endothelial layer. This model was histochemically characterized and the toxicity of different surfactants was tested using MTT methods. This stroma-epithelium model has been reported to show a cornea-like morphology, where a multilayered epithelial barrier composed of basal cells (of a cuboidal shape) and superficial cells (of a flattened shape) is noted. Furthermore, the formation of a basement membrane equivalent and expression of the 64-kDa keratin were reported, indicating the presence of differentiated epithelial cells. The toxicity data for various surfactants obtained with this model correlate well with those seen by the Draize test [73], However, this corneal equivalent was not further validated or used as a model for permeation studies. 

Despite their apparent efficacy, the usefulness of sodium chloride solutions in the treatment of edematous corneas with a traumatized epithelium appears to be limited. The intact corneal epithelium exhibits limited permeability to inorganic ions. In the absence of an intact epithelium the cornea imbibes salt solutions, which reduces the osmotic effect. In the management of corneal edema associated with traumatized epithelium, hypertonic saline solutions may be of limited value due to their increased ability to penetrate the epithelial barrier. 

Porcine corneal models have been used to develop in vitro/ex vivo models able to detect recovery of ocular injury. In preliminary studies porcine corneas cultured for at least 120 h showed regeneration of the damaged stratified epithelium by treatment with 3 % SLS and Ethanol [84], The model was further optimized and developed towards an ocular irritancy assay based on porcine corneas with reversibility as an endpoint, called the Porcine Corneal Ocular Reversibility Assay (PorCORA) [85]. The assay uses an air-interface porcine corneal culture system, and is maintained in culture for 21 days, similar to the in vivo observation period, to determine reversibility of corneal injury as measured by sodium fluorescein and to detect potential compromised epithelial barrier function. 

The corneal epithelium (epithelium cornea anterior layer) is made up of epithelial tissue and covers the front of the cornea (Figure 51.4). It acts as a barrier to protect the cornea, resisting the free flow of fluids from the tears, and prevents bacteria and also therapeutic drugs from entering the epithelium and corneal stroma. The epithelium of the cornea consists of five to six layers of cells packed closely and connected by tight junctions. The cornea is composed of five layers epithelium, Bowman s membrane, stroma, Descemet s membrane, and endothelium, each of alternating polarity. This sandwich-like structure makes the cornea a crucial barrier to most lipophilic and hydrophilic drugs. To penetrate these layers, optimal lipophilicity for the permeant corresponds to log D values of 2-3. °... 

The epithelial cells are closely packed together like a pavement, forming not only an effective barrier to most micro-organisms, but also for active substance absorptirMi. The low permeability of the cornea is due to the presence of tight junctions between the epithelial cells. The superficial corneal epithelial cells are exfoliated from the ocular surface, their average life is 4—8 days. 

The penetration into the tissue follows the initial breakdown of the epithelial barrier. This results in an immediate and strong edema of the conjunctiva, known as chemosis, due to a water influx from the surrounding tissue, vascular leakage, tears, and applied fluids. The cornea itself loads up with ions to a measured osmolarity of 1,830 mOsmol/kg after a 1 mol NaOH burn for 30 s [24]. The penetration of strong alkali has been systematically tested on sodium hydroxide by means of evaluation of the anterior chamber pH. This pH change typically occurs within 2 min after exposure of the corneal surface. The change of the cornea... 

The limbus is the anatomic junction between the transparent cornea and the conjunctiva, a tissue in which the vessels circulate. At this level, there would be the limbal stem cells, cells generating the differentiated epithelial cells of the cornea. The essential property of the cornea is transparency. The seriousness of the ocular burn consists in the loss of the corneal transparency. Actually the limbus is a real barrier to the conjunctiva. In the following months, a serious ocular burn will result in the development of a conjunctival cover leading to a loss of vision. 

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