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C ORNEA and protection of the cornea against viruses, bacteria, external elements, and more," Dr. Tassignon said. Cytokines are small cell-signaling protein molecules that are secreted by numerous cells of the immune system and are a category of signaling molecules used extensively in intercel- lular communication. They can be classified as proteins, peptides, or glycoproteins. "The term 'cytokine' encompasses a large and diverse family of regulators produced throughout the body by cells of diverse embry- ological origin. Unlike hormones, cytokines are not stored in glands as preformed molecules, but are rapidly synthesized and secreted by dif- ferent cells mostly after stimulation. The same cytokine can be both inhibitory or excitatory, and their roles in tears are very complex," Dr. Tassignon said. "When you have deficient limbus cells, you will have epithelialization of the cornea, mean- ing you have conjunctivalization and epitheli- alization on top of the cornea, causing corneal thickening," she noted. Tear film Factors affecting tear composition are either systemic or local. Systemic factors include: inborn errors of metabolism such as in Tay- Sachs disease, infections, general pathological conditions that affect the lacrimal system, and drug-related conditions affecting tear produc- tion. Local factors are infections, injury, con- junctival leakage, and tarsorrhaphy. "The tear film is not only an important refractive element, but it also protects the cor- nea. You have a lot of cytokines in the tear film playing a very important role in the preservation continued from page 72 The four corneal layers relevant to the patho- logic response: (1) epithelium, (2) subepithelial zone (basement membrane, Bowman's layer, and anterior stroma), (3) stroma, and (4) endo- thelium and Descemet's membrane Epithelial bullae resulting from endothelial cell damage and edema Corneal defects can be limited to the epithelium and tend to recover within 24–48 hours. Corneal cyst, a focal collection of fluid, most commonly in the corneal epithelium but rarely in the other layers Stromal corneal defects typically occur after infections and tend to scar with or without vascularization. Deficient limbus cells causing epithelialization of the cornea, i.e., conjunctivalization and epithelialization on top of the cornea Source (all): Marie-Jose Tassignon, MD 74 | EYEWORLD | SEPTEMBER 2019