Composition of the Tears in the Conjunctival Sac and Origins of Secretions
Several different collection techniques have been used, but usually collection is from the lower meniscus, or sometimes from the conjunctival surface of the slightly everted lid, or among the folds in the lower fornix. Some workers have used absorbent sponges placed in the lower fornix, which is effective but has the disadvantage of picking up mucus as well as fluid tears. It is still not possible to collect from the actual film, e.g. by blotting the ocular surface, without some damage to epithelial cells and contamination by cellular contents. One should be clear whether the aim is to collect stimulated or unstimulated tears. Stimulation of flow may be by bright lights, a cold stream of air on the cornea, tickling inside the nose or tweaking nasal hair, or by exposure to specific lacrimatory substances such as onion vapour, ammonia or chloracetophenone. If unstimulated tears are needed (for example, for osmolarity measurement), with collection at the slit-lamp, one must avoid passing the light beam across the pupil. We can classify the various components of the secretion as intrinsic or accessory in origin.
Intrinsic Secretions
Intrinsic secretions are produced in the main lacrimal gland (and presumably also from accessory lacrimal tissue since there is no apparent histological difference between the two types of tissue).
Aqueous Component
The aqueous part of the tears forms the bulk of the lacrimal secretion; it is actively secreted, and linked to the secretion of proteins . Although there is some input via aquaporin-controlled water chan¬nels in the corneal or conjunctival epithelium, its main source is the lacrimal tis¬sue, where it is produced by the acinar epithelium and collected by the ductules. There is some modification and reabsorption in the ductules before delivery via the main lacrimal ductules to the outer upper fornix. It is possible by everting the temporal portion of the upper temporal lid and by finger pressure prolaps¬ing the lacrimal gland slightly into the fornix to see one or two of the orifices, and if fluorescein is added then clear rivers can be seen in the fluorescing tears indicating the position of their orifices. During sleep or prolonged eye closure, the output of both proteins and water from the lacrimal gland changes .
The rate of secretion of lacrimal fluid varies considerably between the quiet eye and active stimulation (see 'Volume Flow of Tears into and from the Eye'). The ageing lacrimal gland suffers progressive fibrosis and loss of func¬tional acinar tissue so its output gradually falls, creating tear film conditions similar to the earlier stages of the aqueous-tear-deficient form of dry eye.
Salts
Electrolytes are actively secreted by acinar and ductal epithelium of the lacrimal gland, and can be seen from the relative proportions of various ions not to be a serum filtrate (table 2) . The pH of tears usually lies within the range 7.2-7.6 but may be higher on prolonged eye-opening through loss of CO2; the value in neonates is about 6.8. Tears exert a buffering action due to their content of bicarbonate ion, proteins and other components, although the turnover rate has also been shown to be part of the response to pH challenge .
The osmolarity of the tears is determined almost entirely by their electrolyte content, since the molarity of even the major proteins is low in comparison. For normal unstimulated tears the generally accepted value is 302 ± 6mosm • kg-1 .
Major Proteins
Human tears contain four major proteins (each 15-20% or more of total protein) - lysozyme, lactoferrin, lipocalin and secretory IgA . The pro¬tein of unknown function previously referred to as 'tear-specific prealbumin' is now known as tear lipocalin, a member of the lipocalin superfamily of small proteins with lipid-binding properties . There is some evidence for interactions between lipocalin and both lysozyme and lactoferrin . Lysozyme, lactoferrin and lipocalin are secreted by the acinar tissue of the lacrimal gland. The secretory form of IgA, in contrast, is produced by interstitial plasma cells embedded in the gland but external to the acini; the IgA dimer, consisting of two monomeric IgA molecules held together by a J or joining piece, are trans¬ported through the acini and the secretory component characteristic of com¬pleted sIgA is added . Control of secretion of lacrimal gland proteins
Several different collection techniques have been used, but usually collection is from the lower meniscus, or sometimes from the conjunctival surface of the slightly everted lid, or among the folds in the lower fornix. Some workers have used absorbent sponges placed in the lower fornix, which is effective but has the disadvantage of picking up mucus as well as fluid tears. It is still not possible to collect from the actual film, e.g. by blotting the ocular surface, without some damage to epithelial cells and contamination by cellular contents. One should be clear whether the aim is to collect stimulated or unstimulated tears. Stimulation of flow may be by bright lights, a cold stream of air on the cornea, tickling inside the nose or tweaking nasal hair, or by exposure to specific lacrimatory substances such as onion vapour, ammonia or chloracetophenone. If unstimulated tears are needed (for example, for osmolarity measurement), with collection at the slit-lamp, one must avoid passing the light beam across the pupil. We can classify the various components of the secretion as intrinsic or accessory in origin.
Intrinsic Secretions
Intrinsic secretions are produced in the main lacrimal gland (and presumably also from accessory lacrimal tissue since there is no apparent histological difference between the two types of tissue).
Aqueous Component
The aqueous part of the tears forms the bulk of the lacrimal secretion; it is actively secreted, and linked to the secretion of proteins . Although there is some input via aquaporin-controlled water chan¬nels in the corneal or conjunctival epithelium, its main source is the lacrimal tis¬sue, where it is produced by the acinar epithelium and collected by the ductules. There is some modification and reabsorption in the ductules before delivery via the main lacrimal ductules to the outer upper fornix. It is possible by everting the temporal portion of the upper temporal lid and by finger pressure prolaps¬ing the lacrimal gland slightly into the fornix to see one or two of the orifices, and if fluorescein is added then clear rivers can be seen in the fluorescing tears indicating the position of their orifices. During sleep or prolonged eye closure, the output of both proteins and water from the lacrimal gland changes .
The rate of secretion of lacrimal fluid varies considerably between the quiet eye and active stimulation (see 'Volume Flow of Tears into and from the Eye'). The ageing lacrimal gland suffers progressive fibrosis and loss of func¬tional acinar tissue so its output gradually falls, creating tear film conditions similar to the earlier stages of the aqueous-tear-deficient form of dry eye.
Salts
Electrolytes are actively secreted by acinar and ductal epithelium of the lacrimal gland, and can be seen from the relative proportions of various ions not to be a serum filtrate (table 2) . The pH of tears usually lies within the range 7.2-7.6 but may be higher on prolonged eye-opening through loss of CO2; the value in neonates is about 6.8. Tears exert a buffering action due to their content of bicarbonate ion, proteins and other components, although the turnover rate has also been shown to be part of the response to pH challenge .
The osmolarity of the tears is determined almost entirely by their electrolyte content, since the molarity of even the major proteins is low in comparison. For normal unstimulated tears the generally accepted value is 302 ± 6mosm • kg-1 .
Major Proteins
Human tears contain four major proteins (each 15-20% or more of total protein) - lysozyme, lactoferrin, lipocalin and secretory IgA . The pro¬tein of unknown function previously referred to as 'tear-specific prealbumin' is now known as tear lipocalin, a member of the lipocalin superfamily of small proteins with lipid-binding properties . There is some evidence for interactions between lipocalin and both lysozyme and lactoferrin . Lysozyme, lactoferrin and lipocalin are secreted by the acinar tissue of the lacrimal gland. The secretory form of IgA, in contrast, is produced by interstitial plasma cells embedded in the gland but external to the acini; the IgA dimer, consisting of two monomeric IgA molecules held together by a J or joining piece, are trans¬ported through the acini and the secretory component characteristic of com¬pleted sIgA is added . Control of secretion of lacrimal gland proteins
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