Introduction
The tear film covers the ocular surface and is vital for keeping the conjunctiva and the avascular cornea healthy, shielding the eye from the elements, and lubricating the surface. It also helps to maintain a smooth surface for light refraction. The tear film is secreted at an average of 1 to 2 μL/min (microliter per minute) and has a volume of around 3 to 10 μL and a thickness of 3 μm (micrometer). Tear pH is around 7.45, ranging from 7.14 to 7.82 based on seasonal and daily variations.
What Is the Composition of a Tear Film?
The inner mucin, middle aqueous, and outermost lipid layers comprise the three separate and heterogeneous layers of the tear film. However, there is mixing and overlap between strata in a more practical sense. Most of the tear film is composed of an aqueous phase with a thin surface lipid layer that is 50 to 100 nm (nanometer) thick and varying mucin concentrations depending on the layer. The aqueous layer is necessary for the ocular surface to remain lubricated and protected. It comprises proteins, inorganic salts, metabolites, glucose, electrolytes, and oxygen, all of which are necessary for cleaning the ocular surface and removing pollutants and detritus. The goblet cells in the conjunctival epithelium, the acinar cells of the lacrimal gland, and the epithelial cells in the cornea and conjunctiva release mucins that create the inner layer. It serves to keep the aqueous layer stable.
The lipid layer is present at the interface between the environment and the tear and is crucial in slowing the pace of tear evaporation. Fatty acids, phospholipids, wax esters, and cholesterol are all in this outermost lipid layer. Tears include more than 600 distinct lipids from 17 different lipid groups. Mucins help to ensure that the ocular surface is uniformly lubricated by anchoring the aqueous layer to the hydrophobic corneal epithelium through the glycocalyx. They also improve the stability and decrease the surface tension of the tear film. More specifically, both nonpolar and polar lipids are present in the meibum. The main ingredients of the polar component, which acts as a surfactant, include phospholipids, sphingomyelin, and phosphatidylcholine. The main ingredients of this layer are wax esters and cholesterol.
What Are the Functions of Tear Film?
The antibacterial tear film acts as a barrier to protect the ocular surface from the outside world. The tear film's aqueous layer contains important antimicrobial components such as complement, transferrin, glycoprotein, anti-proteinase, lactoferrin, ceruloplasmin, immunoglobulins (IgG, IgA, and IgE), as well as lysozyme. The tear film acts as antibacterial protection, lubricates the ocular surface and eyelids, creates a smooth surface for refraction, and feeds and oxygenates the avascular or corneal epithelium. Because they contain bacterial decoy receptors that prevent them from attaching to ocular tissue, mucins, and glycoproteins released by goblet cells also contribute to ocular defense by entrapping foreign objects or bacteria.
Moreover, they focus IgA on the mucosal surface, a potential breeding ground for bacteria. Bacteriolytic lysozyme hydrolyzes the peptidoglycan cell walls of bacteria. When compared to other physiological fluids, tears have the highest concentration of it. Lactoferrin is an iron chelator that sequesters iron from bacteria that need it for development and metabolism. Immunoglobulins are essential for the body's defense against infections by bacteria, viruses, and parasites. IgA levels are elevated in infectious or inflammatory conditions, such as acute bacterial conjunctivitis, blepharoconjunctivitis (inflammation of eyelid and conjunctiva), keratomalacia (drying and clouding of the cornea), and ocular graft response. Another antibacterial compound present in tears is alpha-lysin, which ruptures cells.
What Is the Mechanism of Tear Film?
Both parasympathetic and sympathetic innervation are received by the lacrimal and meibomian glands, which generate the lipid and aqueous layers of the tear film, respectively. They work together to create a neural circuit that controls the release of tears. Increased aqueous and mucin production and tear formation are the results of parasympathetic activation in reaction to pain, irritation, and cold. Conjunctival epithelial cells secrete more when they get sympathetic inputs.
The regulation of tear generation is mostly dependent on sensory innervation. Through mechanical, polymodal, and cold receptors, trigeminal nerve (V1) afferent neurons found in the cornea and eyelid receive sensory inputs in reaction to temperature changes and painful stimuli. They trigger somatic and autonomic reactions in response, causing an increase in blinking and tear production.
What Is the Clinical Significance of Tear Film?
After allogeneic hematopoietic stem cell transplantation, aberrant tear film formation is seen in ocular graft-versus-host disease, characterized by extreme dry eye and ocular surface disease. A persistent metabolic disorder or illness conditions in both ocular surface and systemic disease might be indicated by abnormalities in tear film biomarkers. Tear film is a useful tool for tracking the evolution of an illness or for further elucidating disease processes since it is easily accessible for noninvasive collection and analysis, and it reflects the ocular environment.
When corneal neovascularization (CNV) occurs, there is also an increase in proinflammatory mediators in the tear film. New vascular structures growing on the cornea are known as CNV, which can lead to corneal opacification and visual loss. Other ocular surfaces and systemic disorders have been similarly defined, even if just a few disease states and their corresponding tear films have been presented here. To clarify the pathophysiology, etiology, and potential therapeutic target of a given disease, additional analysis of the tear film in different disorders may provide unique biomarkers.
Conclusion
The intricate structure and composition of the tear film preserve eye comfort and sharp vision while shielding the cornea and accelerating wound healing following injury. Vision impairment, corneal epithelial and nerve injury, and eye discomfort are all brought on by altered tear content and stability. Point-of-care tear biomarker testing is made possible by technological advancements such as the simplicity of collecting tear fluid, the discovery of pertinent biomarkers in health and disease, and the development of more sensitive immunoassays that smartphones can read.
