Accessory Secretions
Several components are added to the aqueous tears within the conjunctival sac, and it is the combination of all these which produces the physiologically functional tear film and influences its formation and stability.
Lipids
A complex mixture of lipids is delivered from the meibomian glands opening on the lid margin at the mucocutaneous junction. These glands are large, tubuloacinar structures lying within the tarsal plate and related to the sebaceous glands of skin; although the surrounding tissue is richly innervated, no specific fast-acting nervous stimulation is known, and they appear to be free-running, secreting lipid continuously. As with sebaceous glands of skin, modification of systemic hormonal status may affect output, but the response is on a scale of months.
Compression of the tarsal plate in blinking causes a small amount of oil to be squeezed out of each gland, but repeated heavy or forcible blinking can deplete the supply within the duct of the gland so that delivery is reduced until synthesis catches up with excretion. Conversely, during sleep there is no squeezing of the glands, so the elastic ducts fill up until some critical pressure is reached and excess leaks out onto the closed lid margins, where it either flows or is rubbed away, or forms flakes on the lashes .
In the lid-opening phase after a blink, a fresh air/water interface is rapidly generated, and oil (or at least the more surface-active components) spreads onto the tear film, probably forming a largely monomolecular film. It is thought that this initial spreading is followed by a second phase in which a fluid but less surface-active fraction spreads over the first to produce a multilayered oil film structure. Its thickness can be estimated from its interference colours; normal thickness is in the range 40-90 |xm. The surface tension gradient created within the film by this spreading may cause Marangoni flow, pulling aqueous tears from the upper and lower menisci and thickening the overall tear film.
The meibomian oil contains several phospholipids, principally phos- phatidylcholine and phosphatidylethanolamine, which with a small amount of free fatty acids and cholesterol make up the surface-active fraction. The non- polar fraction consists largely of wax esters (fatty acid + long-chain fatty alcohol) and cholesterol esters; branching in many of the acyl chains ensures that the melting range of the mixture is close to lid-margin temperatures. Together, they form a layer shown to retard the evaporation of water from the surface of the tear film. Recently a model has been proposed for the structure of the oil film .
Lipids of non-meibomian origin have also been found in the tears, although reports are still incomplete. A mixture of non-polar lipids, mainly tri- acylglycerides, a small amount of phospholipid, and a substantial proportion of unidentified glycolipids has been described. Since no free lipids are found in tear fluid, it is presumed that these are bound to lipocalin, which is the only major protein with strong lipid-binding characteristics.
Several components are added to the aqueous tears within the conjunctival sac, and it is the combination of all these which produces the physiologically functional tear film and influences its formation and stability.
Lipids
A complex mixture of lipids is delivered from the meibomian glands opening on the lid margin at the mucocutaneous junction. These glands are large, tubuloacinar structures lying within the tarsal plate and related to the sebaceous glands of skin; although the surrounding tissue is richly innervated, no specific fast-acting nervous stimulation is known, and they appear to be free-running, secreting lipid continuously. As with sebaceous glands of skin, modification of systemic hormonal status may affect output, but the response is on a scale of months.
Compression of the tarsal plate in blinking causes a small amount of oil to be squeezed out of each gland, but repeated heavy or forcible blinking can deplete the supply within the duct of the gland so that delivery is reduced until synthesis catches up with excretion. Conversely, during sleep there is no squeezing of the glands, so the elastic ducts fill up until some critical pressure is reached and excess leaks out onto the closed lid margins, where it either flows or is rubbed away, or forms flakes on the lashes .
In the lid-opening phase after a blink, a fresh air/water interface is rapidly generated, and oil (or at least the more surface-active components) spreads onto the tear film, probably forming a largely monomolecular film. It is thought that this initial spreading is followed by a second phase in which a fluid but less surface-active fraction spreads over the first to produce a multilayered oil film structure. Its thickness can be estimated from its interference colours; normal thickness is in the range 40-90 |xm. The surface tension gradient created within the film by this spreading may cause Marangoni flow, pulling aqueous tears from the upper and lower menisci and thickening the overall tear film.
The meibomian oil contains several phospholipids, principally phos- phatidylcholine and phosphatidylethanolamine, which with a small amount of free fatty acids and cholesterol make up the surface-active fraction. The non- polar fraction consists largely of wax esters (fatty acid + long-chain fatty alcohol) and cholesterol esters; branching in many of the acyl chains ensures that the melting range of the mixture is close to lid-margin temperatures. Together, they form a layer shown to retard the evaporation of water from the surface of the tear film. Recently a model has been proposed for the structure of the oil film .
Lipids of non-meibomian origin have also been found in the tears, although reports are still incomplete. A mixture of non-polar lipids, mainly tri- acylglycerides, a small amount of phospholipid, and a substantial proportion of unidentified glycolipids has been described. Since no free lipids are found in tear fluid, it is presumed that these are bound to lipocalin, which is the only major protein with strong lipid-binding characteristics.
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