SUMMARY: Tears are the Vehicle for the essential MOISTURE at the Ocular Surface. Tears are produced by the Ocular Glands – mainly the Lacrimal Gland. The Ocular Gland function is adjusted by the Regulatory Systems of the body – such as the nerve system, the hormones and the immune system.
These systems are somewhat different depending of the genetic sex and the level of hormones, particular sex hormones, of the individual. Furthermore, the activity of all systems certainly change with age which explains age-dependent changes in organ function, that occur e.g. in puberty as well as in advanced age. Negative external influence factors are Risk Factors that can disturb the tear fluid on the ocular surface.
Tears provide the ocular surface moisture and keep the Cornea clear ... but only the Tear Film provides perfect Vision. The homogeneity of tears is important for perfect vision ... so what we need is a thin Tear Film. This leads us to the question of how we come from tears to the tear film? The answer is: BLINKING ! The two most essential functions for ocular surface integrity and visual acuity are Tear secretion and Tear Film Formation – they are therefore the basic functional complexes of the Ocular Surface that keep it healthy and functioning.
Tears are the Vehicle for the essential Moisture at the Ocular Surface
The tear fluid provides moisture at the Ocular Surface – this is the most basic function of the ocular surface because it is a prerequisite for ocular surface health and integrity.
All other, more obviously "useful" functions of the ocular surface for the body, which is basically the provision of vision can only be achieved ´downstream´ of moisture - i.e. only when moisture is provided and maintained in the dry environment that we live in, can vision be realized.
Moisture is not only a pre-requisite for the integrity of the ocular surface but also for vision because it renders the cornea transparent ... which is already something – but not quite good enough as we will see.
Tears are produced by the Ocular Glands - mainly the Lacrimal Gland
The DIFFERENT OCULAR GLANDS are shown here in a schematic depiction of the conjunctival sac and an upper eye lid. The eye lid is slightly everted from the eye ball in order to better see the single mucus-producing goblet cells (indicated here as pink dots) that are distributed throughout the conjunctiva. The color code further indicates the aqueous glands in light blue color and the oil producing Meibomian glands in yellow.
The vehicle for moisture is the tear fluid and this, in turn, is produced by the Ocular Glands.
The ocular glands, however, have many own demands in terms of growth, regulation and nutrition etc. in order to stay healthy themselves and to do their job of secretion of the tear components.
The different components of the Tear Fluid are produced by different ocular glands that are associated, more of less closely, to the conjunctival sac whereto they deliver their secretory products.
The AQUEOUS tear fluid constitutes the main component of the tears and is produced by the (main) lacrimal gland and by the much smaller accessory lacrimal glands of Krause and Wolfring. The fluid is delivered via excretory ducts into the conjunctival sac. A minor amount comes from transport through the conjunctival epithelium.
MUCINS are mainly secreted by the goblet cells, single cells located directly inside the conjunctival epithelium and bind to integral membrane mucins (glycocalyx) on all conjunctival cells.
LIPIDS are produced by the Meibomian glands in the tarsal plates of the upper and lower eye lids. These are the only glands that are not directly associated with the ocular surface mucosa and do not deliver their secretum into the conjunctival sac but, instead, directly onto the Tear Film while this is formed by the blink movement of mainly the upper lid.
The Flow of TEARS at the Ocular Surface is shown by an animated diagram. The bulk amount of tears is produced by the lacrimal gland and delivered by several excretory ducts into the conjunctival sac. The tear fluid is bathing the ocular surface proper (conjunctiva and cornea) and is then disposed by the lacrimal drainage system into the nose.
The Ocular Gland function is adjusted by the Regulatory Systems of the Body
The ocular gland function depends on the pre-requisites of an intact tissue maturation and anadequate functional regulation.
Therefore, the realization of the very basic function of just moisture is only possible when a long process of keeping the underlying and interrelated basic regulatory systems of the body in balance has been achieved before - which works quite well ... normally.
These regulatory systems of most importance are the nervous system, the endocrine hormonal system and the immune system.
External risk factors disturb the tear fluid on the ocular surface
Environmental RISK FACTORS - mainly external desiccating environment, contact lenses and certain visual tasks - have a negative influence on the delivered tear fluid on the ocular surface and may lead to ocular dryness.
Even when the tears are produced and readily cover the ocular surface in order to keep it moist, they are still prone to disturbance by other mainly external environmental factors.
Such factors are chronic noxious external influences, termed external Risk Factors. These are mainly desiccating environmental influences that disturb the tear fluid when it is already delivered onto the Ocular Surface.
This can start quite unsuspicious e.g. by the chronic presence of challenging environmental conditions such as high temperatures, low ambient moisture or wind.
When environmental risk factors occur chronic and are of sufficient strength they may lead to failure of the most basic function of ocular surface - moisture ... and thus typically lead to what is called a ´Dry´ Eye ... or ocular dryness.
The important effect of external desiccating environmental risk factors on the development and progression of Dry Eye Disease has led to their application as challenging conditions in animal models of Dry Eye Disease.
If environmental Risk Factors are not timely diagnosed and treated, which can in the simplest case be done by forceful blinking and by removing the challenging stimulus, the condition may proceed to what is termed Dry Eye Disease.
With this in mind it is no surprise, that many important factors for and in Dry Eye Disease and their interaction can be derived from the thorough understanding of how moisture is maintained at the ocular surface and how it is endangered.
Tears provide the ocular surface Moisture and keep the Cornea clear ... but only the Tear Film provides Vision
The homogeneity of tears is important for vision ... so what we need is a thin Tear Film
TEARS provide the MOISTURE of the Mucosal Ocular Surface, that is a pre-requisite for its TISSUE INTEGRITY and thus for CORNEAL TRANSPARENCY.
Moisture is a necessary component to keep the cornea clear but it is not sufficient to produce perfect vision.
… as we can see from visual disturbance in Dry Eye Disease and in emotional tearing (crying) when the volume is not adequate.
More important than the volume of tears is actually their homogeneous distribution in from of our optical medium of the translucent cornea – What we additionally need is: A very homogeneous and very thin Tear FILM for perfect refraction.
This leads us to the question how we come from tears to the Tear Film
Corneal Transparency is a pre-requisite for vision but not quite sufficient for perfect visual acuity. Only a thin homogeneous pre-corneal FILM of Tears provides perfect VISION.
More important than the volume of tears is actually the homogeneity of tears in front of our optical medium – the translucent cornea.
What we additionally need is: A very homogeneous and very thin Tear Film for perfect refraction.
This leads us to the question how we come from tears to the tear film.
This tear film needs a certain stratification. A perfect smooth air to-tear interface is constituted by the very thin outer lipid layer that is the first and main refractive surface of the eye and accounts for about three quarters of the eye´s refractive power.
How do we get from Tears to a Tear Film ? - Blinking
Individual TEAR DROPS (here from increased tear flow in emotional tearing) provide moisture ... but an eye BLINK is necessary to transform the tears into a homogeneous thin FILM with a layered structure in order to provide regular refraction for perfect vision
More important than the volume of tears is actually the homogeneity of tears in front of our optical medium – the translucent cornea. What we additionally need is: A very homogeneous and very thin Tear FILM for perfect refraction.
Most importantly, blinking is necessary for the formation of a very homogeneous and thin Tear Film in front of the clear cornea.
The two most essential functions for Ocular Surface integrity and visual acuity are Tear Secretion and Tear Film Formation
The production of tear FLUID by the ocular glands as well as its transformation into a thin and homogeneous pre-corneal tear FILM by the BLINK movement of the eye lids are the two most essential functions for ocular surface integrity and visual acuity.
Ocular surface dysfunctions and disease states are therefore often related
to a deficiency in the production of tear components by any kind of problem with the ocular GLANDS or
to a deficiency in the formation of a sufficient tear film by any kind of alteration of the EYE LIDS - this is termed "Eye-LID-and-BLINKING-DYSFUNCTION-(LBD)".
Such problems may depend
on the underlying regulatory systems, such as the neural system, the endocrine hormonal system or the immune system. The occurrence of respective dysfunctions also depends on
external environmental influence factors that act on the gland secretory products such as temperature, humidity, wind or visual tasks that influence the pattern and frequency of blinking.