Tear film
The tear film is the layer of fluid formed by the tear fluid that covers the front part of the eyeball. The tear fluid is largely produced in the lacrimal glands and runs down the corner of the eye and the draining tear ducts into the nasal cavity . The entire tear film consists of two different types of tear films, the pre-ocular and the precorneal tear film. The preocular tear film covers the cornea and conjunctiva located within the fissure of the eyelids . The precorneal tear film, on the other hand, only covers the exposed cornea. The precorneal tear fluid forms a three-layer tear film on the surface of the eye.
function
The tear film serves to protect against foreign bodies and forms a sliding layer for the upper eyelid. Its components provide the diet of the front layers of the cornea ( cornea ) that contains no blood vessels and thus on diffusion is malnourished. Thanks to its antimicrobial components, the tear film protects the front of the eyeball from infections . After all, it ensures a smooth and shiny surface of the cornea and is therefore essential to the optical function of the cornea.
layers
Outer layer of the tear film (lipid layer)
The lipid layer is the top layer of the tear film and about 100 nm thick. It prevents rapid evaporation of the aqueous layer and forms the optically effective smooth boundary layer to the air. The lipid layer consists of cholesterol , cholesterol esters , triglycerides and phospholipids .
The lipid layer is formed from the secretion of the meibomian glands , which are sebum glands and release their secretion in the area of the lid edge. Only because of a balanced mixing ratio of the individual components of Meibom's secretion and an associated lowering of the freezing point are the components liquid. The non-polar lipids and hydrophobic components of the Meibomian secretion adjoin the non-polar sections of the phospholipids. The phospholipids thus form a stable intermediate layer between the aqueous layer of the tear film and the outer lipid layer. Around 14% of the phospholipids carry electrical charges. These repel each other and in this way promote the spreading spread of the lipid film on the eye. Disturbances in the composition of the secretion of the meibomian glands favor bacterial infections of the lid edge.
Middle layer of the tear film (aqueous layer)
The aqueous layer, which is located between the lipid and mucin layers, consists of 98% water and, with a thickness of 6 to 10 µm, forms the largest part of the tear film. This layer is mainly formed by the lacrimal gland (about 60%) and the nictitating gland . If one of the two glands fails, the other can largely compensate for its production. Since the transition to the mucin layer is fluid, an increase in the concentration of mucopolysaccharides to the mucin layer can be measured.
In addition to water, the aqueous layer contains glucose , oxygen and other nutrients for the cornea. The basic lysozyme is the most important protein in the tear film because it has an antibacterial effect. It forms a buffer system for the pH value together with the acidic albumin . In addition to these two proteins, lactoferrin also plays an important role in the immune system. It has a high iron binding capacity and deprives bacteria of the iron that is important for their reproduction. In addition, the aqueous layer contains immunoglobulins (mainly IgA ), vitamin A and growth factors such as epidermal growth factor , transforming growth factor beta , tissue growth factor and fibronectin .
According to more recent findings, the aqueous layer no longer serves to moisten the surface of the eye. The mucin layer takes on this task. The watery layer should therefore ensure the high water content of the mucins.
Inner layer adjacent to the cornea (mucin layer)
The mucin layer (mucous layer) borders the epithelium of the cornea and merges into the aqueous layer of the tear film. Their thickness is given in the literature as about 30-40 nm. There are three mucins detected in the external eye: Mucin1 ( MUC1 ), Mucin4 (MUC4) and Mucin5AC (MUC5AC). They are also divided into two groups, the membrane-resistant mucins and the secretory mucins. Accordingly, MUC1 is a membrane-resistant mucin which is firmly built into the cell membranes of the epithelial cells of the cornea. Due to its size and negative charges, MUC1 prevents the adhesion of other cells and microorganisms to the epithelial cells. It towers over the microvilli many times over and thus prevents bacteria from reaching the outer epithelial cell membranes. MUC4, a secretory mucin, is produced in the epithelial cells of the conjunctiva. So far, relatively little research has been done. MUC5AC, also a secretory mucin, is produced by the goblet cells of the conjunctiva. Because of its chemical composition, it has a gel-like character. It connects to the outside of the MUC1 bound in the epithelial cells. Since the negative charges of MUC1 and those of the secretory mucins repel each other, the MUC1 facilitates the spread of the MUC5AC over the surface of the eye. The water content of the gel-like MUC5AC continues to increase towards the outside and finally passes into the aqueous layer.
Blink
When blinking the eye , microscopic vesicles appear that temporarily reduce visual performance. They dissolve after approx. 0.5 to 1.5 seconds (depending on the consistency of the film) and the thickening caused by the blinking of an eye is evened out. After approx. 5 seconds the film breaks open and again creates irregularities, as a result of which the visual performance deteriorates again.
literature
Corinna Eule: The tear film - survival factor for the eye . In: kleintier specifically 10 (2007), issue 1, pp. 14-18.