OVERVIEW of ...

The Ocular Surface ... in more Detail

the ocular surface consists of

The ORGANS and TISSUES

The Organs of the Ocular Surface: The organs of the ocular surface proper (Cornea and conjunctiva) can be readily seen in clinical inspection, even though the lids must be everted to the the palpebral conjunctiva at the posterior side of the eye lids. The organs of the mucosal ocular adnexa can not readily by seen except when technical measures are applied.

  • related to the anterior mucosal ORGANS of the eye ball

    • directly as the cornea and conjunctiva of the ocular surface proper

      • and

    • indirectly ... but still anatomically continuous as the mucosal adnexa of the lacrimal gland & the lacrimal drainage system

  • together with the TEAR FLUID

    • that is an indispensable component - even though it is not a ´tissue´ - because Tear are the vehicle of moisture

The schematic animation shows a cross-section through a lower eyelid. with the bulbar surface of the cornea to the left. In the posterior part of the lid body are the Meibomian Glands that produce lipids (yellow) for the tear film lipid layer (TFLL) and in the anterior part is the lid muscle (M. orbicularis) that helps to deliver the oily secretum from the ductal system of the gland to the outside onto the posterior lid margin. So ... every one of us has multitude of little oil springs ... and we make great profit from the oil because it is indispensable for ocular surface health and for perfect visual acuity.

There are FURTHER ORGANS that are (also) related to the ocular surface in a functional way because they are of eminent importance for ocular surface health and function  ... even though they are not necessarily of mucosal nature themselves - These are:

  • the Meibomian glands, that provide the oily lipids for the superficial lipid phase of the tear film - the Tear Film Lipid Layer (TFLL)

  • the Muscles inside the eye lids that are necessary for blinking, which in turn is necessary to spread the tear fluid into a thin tear film ... which is then necessary for ´sharp´ vision, termed visual acuity.

Both of these "related organs" of the ocular surface happen to be relevant NOT for moisture as such... BUT for the formation of a Tear FILM from the tears - from this it can be assumed that the tear FILM must also be of high importance ... which is certainly the case as we will see.

In an EVEN WIDER sense ... further organs can possibly also be regarded as ´Part´ of the Ocular Surface- these are all SUPPORT Structures, that provide metabolic support, such as

  • ´nutrition´, gas exchange etc.

  • regulatory support, i.e. exchange of information what to do functionally

  • in an intermediary position are probably tropic factors such as growth factors, cytokines, etc.

  • these ´factors and signals´ are typically transferred via the

    • blood vessels and

    • nerve fibers

 

The ocular surface organs of the anatomical unit form a functional unit

The CORNEA and the CONJUNCTIVA with its mucosal adnexa form the ocular surface together with the Tears. Upstream of the conjunctiva are the associated GLANDS that secrete the new TEARS. Tears are a multi-purpose solution with many ingredients that is used for bathing the cornea. Downstream is the LACRIMAL DRAINAGE SYSTEM that drains and discharges the used tears into the nose.

The well-being of the CORNEA that is in the focus of ocular surface functionality depends on the surrounding organs of the anatomical unit.   

In addition to the introduced anatomical unit the respective organs need regulatory systems that activate and regulate them in a coordinate fashion to act in common for the common goal of vision. This transforms the Anatomical Unit into a Functional Unit - the regulated tear flow over the surface and the formation of a thin film is the key function.

Even though is is certainly no secret that organs are necessarily related to and depend on regulatory systems, the somewhat artificial appearing division between an anatomical and a functional unit may offer some deeper insight.

The ocular surface produces two main functional complexes – Tear secretion and Tear Film formation

The 2 Main FUNCTIONAL COMPLEXES that the Ocular Surface must establish in order to allow the first steps of the Vision process are Tear SECRETION and Tear FILM FORMATION. The Secretion can be divided into 3 Sub-Complexes according to the individual secretion of the 3 main components of the TEAR FILM.

In order to establish vision with a sufficient visual acuity, the ocular surface needs to produce two main functional complexes

  • SECRETION of the different components of the tears by the ocular glands

    • ... the glands could be indicated as 3 Sub-Complexes for Secretion (see in the diagram to the right)

    • ... which represent the 3 phases/ layers of the tear film

    • ... which is equivalent to the 3 main types of potential tear film deficiencies

 

  • FORMATION of a thin homogeneous Tear Film from the secretions

    • the lack of this Tear FILM constitutes another type of Dry Eye due to a lack of lid motility. or due to morphological alterations of the lid shape and integrity

    • this was termed as ´Lid and Blinking Deficiency´ (abbreviated as LBD) by Members of the OSCB

The establishment of both of these functional complexes (Secretion & Film Formation) is not as simple as it may seem and they are so important and indispensable that even a sufficient deficiency in in only one of the three SUB-Complexes concerning the secretion of a single tear film component may lead to the onset of Dry Eye Disease - as we can learn from the lipid-deficient main type of Dry Eye Disease.

The target of the main functional complexes is a (stable pre-corneal) Tear Film … and the aim is vision

The two MAIN FUNCTIONAL COMPLEXES of the Ocular Surface are illustrated here schematically: (1) SECRETION by the Ocular Glands to provide Moisture and (2) FORMATION of a Tear FILM from the Moisture of the Tear Fluid by the BLINK mechanism of the Eye Lids (Actually, Tear Secretion with running tear drops, as seen here, resembles more the tear overflow in emotional tearing than the normal tear production - it is still used here in order to illustrate some basic things)

What is the function of the ocular surface ?

The main function of the ocular surface is however not primarily the production for tears ... as may be assumed by now.

Therefore it may be more suitable to ask for the AIM of all the Ocular Surface´s functional efforts ... and this is certainly to aid the vision process.

This is done by the CORNEA and occurs in two ways:

(1) to allow the ENTRANCE of light into the eye and                

(2) to perform REFRACTION of the light for clear vision.

 

Moisture is essential for the ocular surface

The Lacrimal Gland is not quite like the Niagara Falls ... but upon emotional tearing while crying or during stimulated secretionbecause of other reasons the lacrimal gland can produce surprising amounts of aqueous fluid.

It becomes immediately obvious that a key factor at the ocular surface is tears and tear flow.

This points to the necessity of moisture in a dry environment on earth - which makes the ocular surface complex and vulnerable.


Many other functions and functional systems at the ocular surface (such as nerves, vessels, hormones, immune system etc.) are there to provide and maintain sufficient moisture without excessive pathological side effects of any kind such for example infection. 

 

"I wish it was moist or the prussians came" ... unproven quote after Arthur Wellesley the first Duke of Wellington during the battle of Waterloo

 

Why is the ocular surface moist then ?

Now – what is the link between vision, moisture and tears ?

Only a MOIST Cornea can stay TRANSPARENT

The cornea must be moist in order to achieve its function – which is vision – and tears are the vehicle of this MOISTURE for vision

  • (1) Moisture with all the ingredients of tears maintains the clarity of the cornea and thus allows the entrance of light into the eye – Only a moist cornea can stay TRANSPARENT !

  • Moisture is a prerequisite for vision at the ocular surface ... but it alone is not sufficient. Other factors such as the continuous reformation of the epithelial surface of the cornea from its marginal stem cell pool are also necessary ... but all this comes later, or, in the ´language of tears´ ... ´downstream´ of moisture

  • (2) Moisture of the tears is also a basis for the formation of a pre-corneal Tear Film – Its anterior layer is the air-to-tear interface where most of the REFRACTION of the incoming light is performed

Therefore, moisture is a special characteristic of the ocular surface in contrast to the outside skin.

… by the way, the skin certainly also (usually) does a wonderful job – but this is an almost totally different one compared to that of the ocular surface. In contrast to the ocular surface, the skin even becomes ill when it is constantly moist.

Additional effects of moisture – lubrication

Eye Movements and Blinking produce FRICTION

The constant maintenance of moisture is a pre-requisite for the health and visual function of the whole eye. Moisture has some main functions for the ocular surface, in particular for the cornea as explained earlier, but there are additional effects as e.g.:

(3)    moisture maintains LUBRICATION - that is necessary e.g. 

- when the eye ball moves along the surrounding lid tissue in order to focus on what the brain desires to see

- during an eye lid blink that is necessary to continuously refresh the thin tear film that is unstable by nature

Potential risks of moisture are managed by accessory systems

The moist mucosal ocular surface is endangered by numerous microbes

INFECTION by microbes is a main risk factor for the ocular surface apart from trauma or trauma with infection.

On the other hand – ocular infection is surprisingly rare if we take into consideration that the ocular surface is basically an (almost) closed moist chamber full of nutrients with a very pleasant temperature for microbial colonization.  
This little example illustrates how perfect the ocular surface functional unit usually works … as long as the moisture is preserved that is even used here for defense systems.

However, when moisture is not sufficient this leads to some basic problems and medical issues. Therefore, dry eye disease is a prototypic disease at the ocular surface.

 

How is the ocular surface moisture established and maintained ?

Internal Regulatory Networks

Different Systems regulate the Ocular Surface

Networks of internal regulatory factors and their interaction guarantee the moisture.

First of all, the surface tissues including the ocular glands have to grow intact and healthy. Their growth and later activity is adjusted by many soluble regulatory factors in the tears or in the blood stream.

Nerves are important for functional regulation but also for sensation of irritation and for feelings of pain.

The endocrine hormonal system, particularly sex hormones, and the action of the immune system, with its cells and soluble factors, also have large impact.

The activity of all regulatory factors is mainly influenced by age and sex, i.e. different levels of sex hormones, are therefore important factors for ocular surface health.

 

Which external influence factors affect ocular moisture and the tear film

Negative External Influence Factors are termed RISK FACTORS because they increase the likeliness of Dry Eye Disease

There are many factors that influence the the ocular surface organs (anatomical unit) and the tears that they produce and spread into a tear film by the help of the regulatory systems as the functional unit.

Influence Factors that have a negative effect on the Ocular Surface and Tears are termed RISK Factors because they can increase the likeliness of the onset or worsening of Dry Eye Disease. (for more details please see under ´Dry Eye Disease´).

There is an influence of the external environment

Temperature, moisture, wind speed, or contact lens wear are known to be associated with dry eye disease.  Visual tasks such as concentrated work at video display terminals with low blinking frequency can typically lead to the onset or worsening of dry eye conditions.

Particularly when more than one negative influence factor is present, this makes the maintenance of sufficient moisture often a difficult task for the ocular surface. This applies e.g. to concentrated display work in air-conditioned dry environment - Dry Eye conditions in this context are termed "office eye".

 

Internal risk factors

Daily experience indicates that nutrition, sufficient drinking volume as well as certain topical and systemic medications play a role for ocular surface integrity and tear production.  

Further known negative influence factors can come from the decline of body functions with advancing age alone or in combination with the important effect of hormones, particularly from a lack of androgens,

Further negative factors can be inherited genetic disease and other chronic diseases ... as well as the negative effects of chronic systemic medication or of eye drops.

 

What happens when the ocular surface becomes dry? – signs & symptoms

Some typical SYMPTOMS in DRY EYE DISEASE concern those that can easily be verified and quantified, such as alteration of visual acuity, ... and those that involve very subjective feelings such as dryness, irritation and pain.

When the ocular surface becomes dry for any reason it can no longer stay healthy and intact. The vision becomes typically unstable or blurred and the eyes feel tired, ´heavy´, gritty or simply irritated and painful.

The eyelids are typically involved and accumulate lipid deposits and foam from the tear film or show increased cornification, plugging of the minute orifices of Meibomian glands at the posterior lid border and increased vascularity and redness.

All of this is typically observed in ´Dry Eye Disease´ (DED).

  • DED is the most frequent pathological condition in ophthalmological practice.

  • DED affects hundreds of millions of people world-wide and

  • DED condition needs timely diagnosis and treatment

    • in order to prevent progression into an chronic inflammatory, painful and potentially sight-threatening condition.

 

How to understand the ocular surface and dry eye disease ? – don´t worry ... just join the tour !

This is the aim of the Ocular Surface Center Berlin (OSCB) and of this homepage  !

It is our aim to increase the knowledge and understanding 0f the ocular surface structure (anatomy and cell biology) and of its functional regulation (physiology) in order to understand how diseases of the ocular surface, such as dry eye disease, arise (pathophysiology) and how they get worse when they continue untreated.

All of this contributes to deeper insight, how ocular surface disease can be treated more effectively.

The Ocular Surface Center Berlin (OSCB) contributes to promote the knowledge on the mechanisms of health, of disease, and of therapy of ocular surface disease by own scientific efforts, by teaching on various levels and by consulting with stakeholders in ocular surface health and disease.

 

The road to understanding of the ocular surface

... is paved by the following steps (shown in the animation above)

... A ´Trailer´for the Complete New Concept is shown below

This animation is part of the complete

NEW HOLISTIC DYNAMIC UP-TO-DATE CONCEPT

ON THE PATHOPHYSIOLOGY OF DRY EYE DISEASE

that was elaborated by members of the Ocular Surface Center Berlin (OSCB)  

- it can be studied of this OSCB siteby following the LINK