It is a mucous membrane, similar to mucous membranes in other parts of the body.
The conjunctiva is the main supporting tissue of the ocular surface .
The conjunctiva surface of the eye is composed of non-keratinized squamous epithelium, mixed with goblet cells (mucus), Langerhans cells (dendritic-looking cells that express a class II antigen), and occasional dendritic melanocytes.
Function of the conjunctiva of the eye
The conjunctival epithelium has multiple functions. Covers the inside of the eyelids and provides a covering to the sclera.
The conjunctiva functions to maintain normal homeostasis and the integrity of the eye.
In addition to being a physical barrier that provides protection, it also has a lubricating function for the eye by producing mucus and tears.
The goblet cells of the conjunctiva secrete mucin, which is part of the tear film of the eye.
This allows the ocular surface to maintain its layer of healthy moisture and helps prevent dry eye syndrome .
When the front surface of the eye is kept moist and lubricated, it allows the eyelids to open and close without friction or eye irritation.
It prevents the entry of microbes into the eye and plays a role in immune surveillance, as it also has some immune cells that help in the defense of the ocular surface.
It is highly vascular and houses extensive lymphatic vessels that provide nutrients to the eye and eyelids.
Conjunctiva areas of the eye
The conjunctiva covers the anterior, non-corneal parts of the globe, as well as the veins and palpebrae.
The conjunctiva is a continuous membrane, and for practical and clinical purposes it is subdivided into palpebral, forniceal, and bulbar areas.
The palpebral conjunctiva is divided into marginal, tarsal, and orbital regions. The palpebral conjunctiva lines the eyelids.
The palpebral conjunctiva extends from the mucocutaneous junction at the lid margin to the upper and lower margins of the tarsal plate.
Its surface is smooth and contains several crypt-shaped folds of epithelium called “crypt of Henle.”
The forniceal conjunctiva is attached to the orbital septum.
The forniceal conjunctiva extends temporarily behind the lateral canthus and nasally to the semilunar ducts of the lacrimal gland that open in the temporal portion of the superior base, into the open superior and inferior accessory glands of Krause and Wolfring.
The bulbar conjunctiva is divided into scleral and limbal parts.
The bulbar conjunctiva is located in the eyeball on the anterior sclera.
Tenon’s capsule attaches it to the underlying sclera. This conjunctiva has an average thickness of 33 microns.
Finally, the conjunctival fornices form the junction between the palpebral and bulbar conjunctivae.
This protective covering is loose and flexible, unlike its bulbar counterpart, which allows movement of the globe and eyelids.
Finally, the conjunctival fornices are divided into the superior, inferior, lateral and medial regions.
The conjunctiva of the eye consists of an epithelial layer composed of stratified columnar epithelium.
Also present within this epithelial layer are blood vessels, fibrous tissue, lymphatic channels, melanocytes, T and B cell lymphocytes, Langerhans cells, and accessory lacrimal glands.
A deeper layer, the substantia propria or the conjunctival submucosa, consists of lymphoid and superficial fibrous tissue.
The substantia propria is a layer of tissue that only exists in the conjunctiva, but not in other ocular tissues.
Numerous lymphocytes, mast cells, plasma cells, and neutrophils are present within this layer of connective tissue.
Finally, the deeper fibrous layer contains the nerves and vessels that provide innervation and blood supply to the conjunctiva.
Krause’s glands are also found within this deep layer.
The conjunctival epithelium is 3 to 5 cell layers thick.
The basal cells of the epithelium are cuboidal and flatten as they approach the surface.
In the area closest to the fornix, the conjunctiva has the highest number of goblet cells.
These unicellular mucous glands are especially common in the inferior and medial conjunctiva and near the medial canthus.
Away from the fornix, the number of goblet cells decreases, in addition, the fornix has a greater number of lymphocyte patches, which are mostly suppressor T cells.
There are patches of immune cells such as T and B lymphocytes that form conjunctiva-associated lymphoid tissue.
Structure of the conjunctiva of the eye
The histomorphology of the conjunctiva differs in the three areas.
The conjunctiva is covered by two or more layers of stratified squamous and columnar epithelium at the limbus, and the conjunctiva of the palpebral margins exhibits a stratified squamous pattern.
Histologically, the bulbar and marginal conjunctiva are composed of squamous, stratified, non-keratinized epithelium, while the tarsal and forniceal components are arranged in varying degrees of stratified columnar and cuboidal epithelium.
At the mucocutaneous junction of the eyelid margin, an abrupt transition is found from the epithelium of the nonkeratinized stratified squamous mucosa of the palpebral conjunctiva to the keratinized epithelium of the skin.
Conjunctival basal cells have a thin basement membrane similar to the basal cells of the corneal epithelium.
The medial epithelial and superficial cells appear polygonal and tend to flatten as they approach the surface.
Mucus-secreting goblet cells are normally present in the middle and superficial layers of the epithelium, and represent up to 10% of the basal epithelial cells of the conjunctiva.
The conjunctiva contains accessory lacrimal glands, lymphoid tissue, mast cells, and goblet cells.
Accessory lacrimal glands and seibaceous meibomian glands are concentrated in the upper tarsal conjunctiva, but are also evident in the lower one.
They help produce the aqueous and lipid constituents of the tear film, respectively.
Goblet cells abound on the tarsus and fornix, providing the mucinous component of the tear film through MUC5AC, central gel-forming mucins for many ocular surface disorders.
Embryology of the conjunctiva of the eye
The conjunctiva develops from the ectoderm of the ocular surface.
This is the same origin for all other epithelia in the eye, including the cornea and limbus.
Blood and lymphatic supply to the conjunctiva of the eye
The conjunctiva receives its blood supply from the marginal tarsal arches, the peripheral tarsal arches, and the anterior ciliary arteries.
The marginal tarsal arches provide blood supply to the palpebral conjunctiva along with the fornices.
The proximal arch lies along the upper edge of the cap, feeding the forniceal conjunctiva and then becomes the posterior conjunctival arteries feeding the bulbar conjunctiva.
The anterior ciliary arteries supply the bulbar conjunctiva and are derived from the ophthalmic artery.
Also, there is connectivity between the blood supplies to the conjunctiva, so ultimately there is an overlap between the blood supply from the two largest vessels to the conjunctiva to some extent.
The basin area of the anterior ciliary arteries, which supply the conjunctiva at the limbus, and the arteries of the posterior conjunctiva, which travel from the posterior bulbar conjunctiva anteriorly, lie approximately 3–4 mm from the limbus.
The nerves of the conjunctiva of the eye
The conjunctiva receives nerves from sensory, sympathetic, and parasympathetic sources. The sensory nerves, which are of trigeminal origin, reach the conjunctiva through branches of the ophthalmic nerve.
Sensory innervation is received from the supraorbital nerve, the supratrochlear nerve, the infratrochlear nerve, the infraorbital nerve, the lacrimal nerve, and the long ciliary nerves.
The main function of these fibers is to equip the conjunctiva with the ability to detect a variety of sensations, for example, touch, pain, heat and cold.
Sensory nerve endings include both free (unspecialized) nerve endings and more complex corpuscular endings (classically referred to as Krause’s terminal bulbs).
The upper portion is supplied by the supraorbital nerve, the supratrochlear nerve, and the infratrochlear nerve.
The infraorbital nerve supplies the lower portion.
Finally, the lateral portion is supplied by the lacrimal nerve, while the long ciliary nerves supply the circumcorneal portion.
Conjunctival blood vessels receive double autonomic innervation.
Parasympathetic fibers emerging from the pterygopalatine ganglion and sympathetic fibers from the superior cervical ganglion are responsible for vasodilation and vasoconstriction, respectively.
Muscles in the conjunctiva of the eye
There are no muscles that originate or insert directly into the conjunctiva.
Instead, some muscles interact with parts of the eye near the conjunctiva.
For example, the superficial to deep layers of the eyelid include the skin, the orbicularis oculi muscle, the tarsus, and then the palpebral conjunctiva.
The extraocular muscles of the eye, such as the four rectus muscles (superior, inferior, medial, and lateral rectus), insert into the globe at various distances, posterior to the limbus.
These muscle attachments lie deep in the bulbar conjunctiva.
Physiological variants of the conjunctiva of the eye
The term Conjunctivechalasis refers to the redundant conjunctiva, which typically involves the inferotemporal conjunctiva. This can be asymptomatic in some people.
It is a normal physiological variant with incidence that increases with age. However, the redundant conjunctiva can be associated with symptoms of dry eye syndrome.
Some postulate that normal tear meniscus formation is impaired and that normal tear movement is blocked. However, no definitive pathological mechanism for this association has been discovered.
Surgical Considerations of the Conjunctiva of the Eye
There are many routine procedures that involve the conjunctiva.
In conjunctiva chalasis, the conjunctiva is grasped with forceps and thermocautery is used to cause the redundant tissue to contract.
Clinical significance of the conjunctiva of the eye
The surface of the eye is exposed to numerous external influences.
It is susceptible to many disorders, such as dryness, allergic reactions, chemical irritation, trauma, and infections.
Many systemic diseases cause irritation of the conjunctiva.
Sickle cell anemia, type 2 diabetes, high blood pressure, carotid artery occlusion, and leptospirosis are just a few diseases that result in changes in the blood supply or the structure of the conjunctiva.
The bulbar conjunctiva can usually be assessed in the slit lamp by having the patient look nasally to assess the temporal conjunctiva and temporally to assess the nasal conjunctiva.
The upper and lower parts of the bulbar conjunctiva can be assessed by gently holding the patient’s eyelid open while the patient looks up or down.
The palpebral conjunctiva and vessels may be more difficult to assess, but can be assessed with a small cotton swab to invert the lid.
An ophthalmologist can clinically evaluate the conjunctiva using common slit lamp stains.
A small amount of fluorescein dye is applied to the ocular surface, and blue filter light is used to detect areas of increased uptake of spots, indicative of damaged epithelium.
Lissamine green and rose bengal can also be used to tint the ocular surface with appropriate absorption filters.
There are several methods for rating abnormalities, including the Oxford rating scale and the National Eye Institute scales.
Both scales are developed for the classification of the conjunctiva and the cornea.
A number of conditions can affect the conjunctiva. Among the most common conjunctival problems are: