The conjunctiva is the specialised tissue which coats the anterior eyeball and internal surface of the eyelids.
Structure
The conjunctiva is a mucous membrane important for the health of the eyeball. It assists in the lubrication and movement of the eyeball. There are several parts to the conjunctiva:
- The marginal conjunctiva is a narrow region extending from the mucocutaneous junction to the tarsal plate of each eyelid.
- The tarsal conjunctiva overlies the internal surface of the tarsal plate. This region is notable for the minimal submucosal layer.
- The orbital conjunctiva covers the inner eyelid and extends to each conjunctival fornix. It has abundant loose submucosal tissue which allows free movement. Beneath this region of conjunctiva there is a significant quantity of lymphoid tissue (mucosa associated lymphoid tissue or MALT).
- The bulbar conjunctiva overlies the sclera, extending from the fornix to the corneo-scleral junction. It is thin and transparent.
- The limbic conjunctiva is a ring about 1 mm in width, and contains a network of capillaries which assist in the nourishment of the cornea.
The conjunctiva is reflected from the orbit onto the eyeball at the superior and inferior fornices, which lie just in front of the equator of the eyeball.
Microscopic Apperance
The conjunctiva has a varied epithelium based on its parts.
- The marginal conjunctiva is a non-keratinising squamous epithelium about 8 - 12 cells thick
- The tarsal conjunctiva is cuboidal or columnar, about 2 - 3 cells thick
- The orbital and bulbar conjunctiva is usually 3 cells thick and contains numerous goblet cells
- The limbal conjunctiva is stratified squamous and assists in the replenishment of the corneal epithelium
The underlying stroma varies greatly in thickness. It is nearly absent in the tarsal conjunctiva and quite thick at the fornices.
Routes of Cancer Spread
Tumours of the conjunctiva are exceedingly rare. Local invasion may lead to involvement of the eyelid, orbit or eyeball depending on the location. Lymphatics of the lateral two thirds of the upper conjunctiva and lateral third of the lower conjunctiva typically spread to parotid lymph nodes. Lymph from the medial conjunctiva may pass to submandibular nodes.
Neurovascular Supply
Arterial Supply
The conjunctiva is nourished mostly by branches of the ophthalmic artery. The palpebral part receives supply from the arcades of the eyelids. The bulbar part is supplied by branches of the anterior ciliary arteries.
Venous drainage
Venous blood from the palpebral portion passes externally to the facial veins or internally to the ophthalmic veins; blood from the bulbar parts tends to pass mostly to the ophthalmic veins.
Lymphatic drainage
Lymphatics pass either to pre-auricular/parotid nodes (lateral parts) or to submandibular nodes (medial parts).
Innervation
The inferior conjunctiva is preferentially innervated by branches of the infraorbital nerve; the superior by the supraorbital nerve. Medially the trochlear nerves play a small role whereas laterally the lacrimal nerve provides a small portion of sensory innervation.
