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Human tears. Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all land mammals. [1] Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. [2] The different types of tears—basal, reflex, and emotional—vary significantly in composition. [2]
The layer of mucin allows tears to glide evenly across the eye's surface. The crypts of Henle are named after German anatomist Friedrich Gustav Jakob Henle (1809-1885). Another anatomical structure called the glands of Manz perform a similar function.
Mucin genes encode mucin monomers that are synthesized as rod-shaped apomucin cores that are post-translationally modified by exceptionally abundant glycosylation. The dense "sugar coating" of mucins gives them considerable water-holding capacity and also makes them resistant to proteolysis , which may be important in maintaining mucosal barriers.
The tear film is composed of three layers: the lipid, aqueous, and mucin. [8] These play a role in creating a smooth surface to facilitate refraction, lubricating the movement of the eyelid, passively transporting gases such as oxygen and carbon dioxide, and protecting the cornea.
The epithelial layer contains blood vessels, fibrous tissue, and lymphatic channels. [6] Accessory lacrimal glands in the conjunctiva constantly produce the aqueous portion of tears. [6] Additional cells present in the conjunctival epithelium include melanocytes, T and B cell lymphocytes. [6]
The lacrimal gland is a compound tubuloacinar gland, it is made up of many lobules separated by connective tissue, each lobule contains many acini.The acini composed of large serous cells which, produce a watery serous secretion, serous cells are filled with lightly stained secretory granules and surrounded by well-developed myoepithelial cells and a sparse, vascular stroma.
The mucous layer contains mucins, gelatinous glycoproteins, which allow for an even distribution of the tear film over the corneal surface and decreased friction during blinking. [3] Artificial tears are used to supplement a patient's naturally-produced tears and increase eye lubrication by mimicking characteristics of the tear film. [4]
Disorders of the eyelid can impair the complex blinking motion required to spread tears. [16] Abnormalities of the mucin tear layer caused by vitamin A deficiency, trachoma, diphtheric keratoconjunctivitis, mucocutaneous disorders, and certain topical medications are also causes of keratoconjunctivitis sicca. [14]