OVERVIEW of ...
The Ocular Surface ... in more Detail
The OCULAR SURFACE consists of
The ORGANS and TISSUES
- related to the anterior mucosal ORGANS of the eye ball
- directly as the cornea and conjunctiva of the ocular surface proper
- indirectly ... but still anatomically continuous as the mucosal adnexa of the lacrimal gland & the lacrimal drainage system
- directly as the cornea and conjunctiva of the ocular surface proper
- together with the TEAR FLUID
- that is an indispensable component - even though it is not a ´tissue´ - because Tear are the vehicle of moisture
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 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
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
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
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 ?
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
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
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
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
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
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
that was elaborated by members of the Ocular Surface Center Berlin (OSCB)
- it can be studied of this OSCB siteby following the LINK