LINKS to some pages:

 

GLIMPSE

on ...  The Ocular Surface

 

SUMMARY      (Every sentence here is LINKED to a respective CHAPTER below)

The Ocular Surface is a complex and delicate arrangement of organs at the front of the eye ball that provides a moist environment in order to perform the first steps of the Vision Process.   The moist mucosal organs at the anterior side of the Eye Ball form a continuous Anatomical Unit. It consists of the Ocular Surface Proper (Cornea and Conjunctiva - the organs that are continuously bathed by the tears) and of the Ocular Surface Mucosal Adnexa (Lacrimal Gland and Lacrimal Drainage System - the organs that manage the tears, i.e. that produce them or dispose them, respectively). Regulatory Systems maintain the intact function of the organs and thus join the Anatomical Ocular Surface into the Functional Unit. The Ocular Surface is a pre-requisite for Vision. Moisture - by the tears - is Key at the Ocular Surface. Moisture must be produced by the ocular surface glands - this is Secretion. Moisture must be spread into a Tear Film to provide permanent moisture in the palpebral fissure - this is Tear Film Formation. Dysfunction of the Ocular Surface Functional Unit leads to Disease - prototypic is Dry Eye Disease.

 

This is a (very) quick overview of the main OCULAR SURFACE components and functions that form an intricate network - repeated viewing helps and proves your interest in the topic (every move you do will of course be tracked) ... in slower versions it may become too boring ;-)

The OCULAR SURFACE is a complex and delicate arrangement of organs at the front of the eye ball that provides a moist environment in order to perform the first steps of the  V I S I O N Process

 

Contribution of the Ocular Surface Center Berlin (OSCB) to Ocular Surface Research

Members of the Ocular Surface Center Berlin (OSCB) have devoted their scientific energy to the concept of an Ocular Surface Unit for about 3 decades. Nowadays this is a cornerstone for the conceptual understanding of health and disease at the ocular surface.

 

Early Concept about the FUNCTIONAL UNIT of the Ocular Surface

The ocular surface consists of the moist mucosal organs  at the anterior surface of the eye. They are a continuous anatomical unit from the lacrimal gland over the conjunctiva & cornea into the lacrimal drainage system

The ocular surface organs are joined by several systems e.g. the flow of tears along the mucosal surface and they are regulated by the same systems, e.g. nerves, hormones and the immune system - thus they also form a functional unit.

The main function of  the Ocular Surface is to provide the necessary equipment for VISION ...  in the form of permitting the entrance of light into the eye and for its refraction.

Any dysfunctions in the unit, due to underlying regulatory systems or environmental influence factors, potentially results in ocular surface disease mainly Dry Eye Disease

 

The Ocular Surface consists of the moist mucosal organs at the anterior side of the eye ball that form an ANATOMICAL UNIT

 

The ORGANS of the OCULAR SURFACE form a continuous the Anatomical Unit of

The Complete Ocular Surface ANATOMICAL UNIT. It consists of the OCULAR MUCOSA, i.e. the organs that are bathed by the tear fluid and of the MUCOSAL ADNEXA, i.e. the organs that manage the tear fluid, either by producing it, as the lacrimal gland, or by draining it after bathing, as the lacrimal drainage system.

the Ocular Surface PROPER

´... the (in-visible) organs that are bathed by the tears´
  • CORNEA
    • as a transparent window of the brain to the outside world

 

The cornea is the ´nerd´ of the ocular surface with high-tech function ... however in a very limited spectrum

 

 

 

 

 

 

 

 

  • CONJUNCTIVA
    • main support tissue for the Cornea that hat many functions and provides, moisture, nutrition and protection
    • has a much larger surface because it covers the
      • anterior surface of the globe and the
      • posterior surface of the eyelids
    • contains all components that the Cornea can not have in the interest of transparency, e.g. 
      • blood and lymph vessels
The conjunctiva is the ´maintenance genius´  for the ocular surface ... and thus for the whole eye and for vision

the Ocular Surface Mucosal ADNEXA

´... the (in-visible) organs that manage the tears´
  • LACRIMAL GLAND
    • ´Home of the aqueous tears´ - it produces the bulk aqueous tear flow that reaches the downstream Conjunctiva and Cornea via its several excretory duct
      • ... which is certainly a simpli-fication because the lacrimal gland produces many other important ingredients apart from just water

 

  • LACRIMAL DRAINAGE SYSTEM
    • ´The sewer pipe for the aqueous tears´  - it drains the tear fluid, after bathing the ocular surface, into the nose
      • ... which is again a simplification, as there are also several other functions - but it is good enough for now
All the mucosal organs of the OCULAR SURFACE PROPER and of its mucosal ADNEXA are anatomically continuous and form an anatomical unit.

 

REGULATORY SYSTEMS maintain the intact function of the organs and thus join the Anatomical Ocular Surface into the Functional Unit 

    The intact function of the Ocular Surface depends on REGULATORY SYSTEMS that support the growth and maturation of the organs and are also necessary for the actual functional regulation, e.g.  of the volume of secretion or the frequency of blinking and many other functions that make the ocular surface feel well ... and also the individual who owns it.

    The healthy function of the ocular surface organs is established and maintained by an intricate interacting network of regulatory systems such as

    • the nervous system,
    • the endocrine hormonal system and
    • the immune system.
    • Furthermore there is a plethora of other soluble factors that come with the blood circulation or are locally produced and ensure the growth, maturation, regeneration and ´special functions´, such as protective reactions and inflammation, of the tissues and organs.

    The regulatory systems transform the anatomically continuous organs of the anatomical unit into the ocular surface functional unit. For more details please see e.g. the Section on ´Regulatory Systems´.

     

    The Ocular Surface is a pre-requisite for VISION

    The Ocular Surface is the first part of the vision pathway and therefore of utmost importance for everything that happens later and more posterior in the eye.

    The ocular surface provides the clear optical media for entrance of light into the eye and for its refraction to have a clear image of the outside world.

    All later events inside the eye for

    • further transmission and refraction/ accomodation of the light for near and far distances ... or
    • for its reception by the photo-receptors of the retina ... or
    • for the neural transduction of signals in the nerve fibers to the visual cortex of the brain

    can be drastically impaired or become meaningless IF  the ocular surface is not intact.

    The ocular surface is thus of utmost importance for vision and any dysfunction or disease of the ocular surface immediately endangers visual acuity.

     

    MOISTURE - by the tears - is KEY at the Ocular Surface

    1. Moisture must be produced by the glands - this is SECRETION

    The FUNCTIONAL UNIT of the Ocular Surface.  The organs of the Mucosal ADNEXA, the lacrimal gland and the lacrimal drainage system. are not readily seen whereas the Cornea and Conjunctiva that constitute the Ocular Surface ´Proper´ can easily be seen and investigated in clinics.  The Organs of the Mucosal Adnexa both ´manage´ the tears in the sense that they either produce the new tears or drain the ´used´tears - whereas the Cornea and Conjunctiva are bathed in between by the tears.

    The organs of the mucosal adnexa, that are not readily seen in clinics, can be termed upstream and downstream of the ocular surface proper (cornea and conjunctiva).

     

    This points to the high importance of the tear fluid, that ´flows´ along this paths, to provide ocular surface integrity, health and function.

     

     

    "MOISTURE is key at the ocular surface"  because

    The Tear Flow is along the Organs that constitute the Ocular Surface Functional Unit, i.e. from the Lacrimal Gland, through its excretory ducts on the Cornea and Conjunctiva and further through the Lacrimal Drainage System into the Nose.

    • moisture keeps the optical medium of the cornea TRANSPARENT for the entrance of light into the eye and, probably equally important,

     

     

     

     

     

    2. Moisture must be spread into a tear film to provide permanent moisture in the palpebral fissure- this is Tear Film FORMATION

    The BLINK movement of the Eye Lids spreads the Tears into a very thin and homogenous Layer of Tear Fluid - the Tear FILM - that keeps the mucosal Ocular Surface constantly moist even in the opened palpebral fissure during the interblink period.

    Moisture must be spread into a FILM of Tears in front of the cornea and conjunctiva at the anterior side of the eye ball in the palpebral fissure. The FORMATION of the pre-corneal Tear FILM is achieved by the Blinking movement of the eye lids.

    This importance of the pre-corneal TEAR FILM has at least two main reasons:

    • For the Mucosal Organs the essential MOISTURE must be be guaranteed always & everywhere. This puts the Ocular Surface into a DILEMMA:
      • in the palpebral fissure the bulbar cornea and conjunctiva are exposed to the ambient air atmosphere and are thus at risk of desiccation
      • on the other hand, when the eye lids were constantly closed this would be safe for the mucosa ... but no light could get into the eye for vision
    • This Dilemma is elegantly solved by the Ocular Surface: the transparent cornea is constantly covered with a very thin and homogeneous tear film. By this ´trick of nature´ the mucosal surface is kept moist every-where and every-time and light can still get into the eye. 
    • The solution wouldn´t be half as elegant, if this ´trick´ wouldn´t provide another important function, because the very smooth surface of the pre-corneal tear film is at the same time the main refraction element of the eye and thus of the of utmost importance to achieve optimal visual acuity.
      • ... here we already have a hint to understand, why one frequent symptom of Dry Eye Disease, that typically goes along with alterations of the pre-corneal tear film,  is an unstable visual acuity.

    Dysfunction of the Ocular Surface Functional Unit leads to Disease - prototypic is Dry Eye Disease

    Dry Eye Disease is a prototypical disease of the Ocular Surface due to the dysfunctions that eventually impair the most basic functional  complex of preserving moisture.

    When components of the ocular surface functional unit are impaired and thus become dys-functional beyond a certain limit, this includes a risk of downstream pathological consequences.

    All together the intricate network of structure and function at the ocular surface is keeping a delicate functional balance to preserve the moist mucosal niche in a dry environment ... and may easily become ill.

    In view of the complex task that the ocular surface has, it is fair to say, that the system turns out to be surprisingly robust in our daily lives.   

    Nevertheless, many disturbances of the ocular surface functional unit, at least when they are chronic, can lead to pathology.

    This will eventually also affect the most basic functional complex of the ocular surface - which  is the provision of  moisture. Therefore Dry Eye Disease is a prototypic disease of the ocular surface.