CONJUNCTIVA : ANATOMY AND PHYSIOLOGY OVERVIEW

In this article we discuss in detail about the applied anatomy and physiology of the conjunctiva. If you are an audio visual learner you can visit my video on the anatomy of conjunctiva here. Also you may refer to the text below as a read along notes for the same.

What is Conjunctiva?

 The term conjunctiva has originated from the word “cojoin” which means to join together. Conjunctiva joins the Eyeball to the eyelid and hence the term .

The conjunctiva is a thin, translucent mucous membrane covering the eyeball and the inner aspect of the eyelid. 

image depicting parts of conjunctiva namely bulbar and palpebral conjunctiva

Parts of Conjunctiva

The conjunctiva mainly has three main parts based on the structure it covers in eye.

  • PALPEBRAL CONJUNCTIVA: lines the posterior surface of the eyelids. It is further divided into marginal, tarsal and orbital conjunctiva.
    • Marginal Conjunctiva: Part of the palpebral conjunctiva covering the eyelid margin.
    • Tarsal Conjunctiva: The area of the palpebral conjunctiva covering the tarsal plate.
    • Orbital conjunctiva : The part pf palpebral conjunctiva covering the orbital septum.
  • FORNICEAL CONJUNCTIVA : part of conjunctiva covering the fornices. It acts as a connection between the palpebral and bulbar conjunctiva. 
  • BULBAR CONJUNCTIVA :lines the anterior surface of the globe. It is further subdivided into Scleral and limbal conjunctiva.
    • Scleral conjunctiva : Covers the sclera.
    • Limbal Conjunctiva : Covers the limbal zone
Flow shoeing that various parts of the conjunctiva

CLINICAL NUGGET: SULCUS SUBTARSALIS AND ARLT’S LINE 

  • Groove/ sulcus present 2 mm away from the lid margin
  • It’s a common site for lodgement of the foreign bodies
  • Perforating branches of the marginal arcade present in the submuscular space pierce the tarsal plate to reach and supply the conjunctiva at the sulcus subtarsalis 
  • In trachoma :Scarring of the conjunctiva leads to development of a white line . This is known as Arlts Line
Diagram of sulcus subtarsalis and arlts line

Important Clinical Aspects of Palpebral Conjunctiva

MARGINAL  CONJUNCTIVA 

  • The marginal conjunctiva is bounded by the anterior lid margin anteriorly and the posterior lid margin.
  • The part in between is known as the intermarginal strip.
  • The sulcus subtarsalis lies 2mm away the posterior lid margin 
  • Marginal conjunctiva covers the intermarginal strip and the distance between the posterior lid margin and sulcus subtarsalis.

  • IMPORTANT : The marginal conjunctiva is considered to be a transitional zone covered with stratified epithelium with the characteristics of both skin and conjunctiva.

Image depicting the limits of marginal conjunctiva

TARSAL CONJUNCTIVA

  • Highly vascular and adherent to the underlying tarsal plate
  • More adherent in upper lid
  • Tarsal glands are visible through thin tarsal conjunctiva

ORBITAL CONJUNCTIVA

  • Loosely present between the tarsal plate and fornix covering the orbital septum 
  • The orbital conjunctiva is trown into horizontal folds during the eye movements. Therefore it helps in easy eye movements.
  • It coveres not only the orbital septum but also the  mullers muscle in upper lid and retractors in lower lid.

FORNICEAL CONJUNCTIVA

  • Part of conjunctiva lining the fold formed between the palpebral conjunctiva and the bulbar conjunctiva
  • It is thicker and is loosely present to allow movement of globe.
  • The pocket or sac like structure formed by the meeting of the palpebral and bulbar conjunctiva is known as the Conjunctival sac 

BULBAR CONJUNCTIVA

  • It Covers the globe and underlying white sclera
  • Loosely adherent except at a 3mm zone near limbus where firmly attached and at the insertion of the rectus muscles
marginal conjunctiva, tarsal conjunctiva, orbital conjunctiva, scleral conjunctiva, limbal conjunctiva

IMPORTANT FACT ABOUT LIMBAL CONJUNCTIVA 

  • At the limbus the conjunctiva , tenons and sclera fuse together . 
  • This makes the conjunctiva less mobile at the limbus.
  • This of advantage to the surgeon , as holding and stabilizing the globe at limbus offers a firm grip during the surgery.

Conjunctival Sac

  • As discussed above, the conjunctiva forms a sac that opens into the palpebral fissures. It contains the tears.
  • It contains 7 microlitre of tear fluid, but it has the capacity to accommodate up to 30 microlitre .

DIMENSIONS OF THE CONJUNCTIVAL SAC FROM THE LID MARGIN 

  • SUPERIOR FORNIX : It is the largest fornix . It is about 13 mm 
  • INFERIOR FORNIX : it is about 9 mm 
  • LATERAL FORNIX : It is about 5 mm 
  • MEDIAL FORNIX : It is the shallowest . It contains two structures namely the plica semilunaris and the caruncle.
  • (Also read anatomy of eyelid )

DIMENSIONS OF THE CONJUNCTIVAL SAC FROM THE LIMBUS  

  • SUPERIOR FORNIX : 8-10mm 
  • INFERIOR FORNIX : 8-10mm 
  • LATERAL FORNIX : 14mm
  • MEDIAL FORNIX : 7mm
diagram depicting the dimensions of the conjunctival sac

Histology Of Conjunctiva

The conjunctiva has three layers. from outside to inwards they are as follows :-

  1. Epithelium 
  2. Adenoid layer /lymphoid layer 
  3. Fibrous layer

The adenoid layer and fibroid layer together are known as substantia propria.

Layers of conjunctiva

Epithelium of the Conjunctiva

  • Stratified non keratinized epithelium
  • There are two layers of epithelium over the palpebral conjunctiva.
  • The epithelium becomes gradually thicker from the fornices to the limbus.
  • Marginal conjunctiva is known as the transitional epithelium as discussed above 

Number of layers of epithelium in different parts of conjunctiva:-

  •  Marginal conjunctiva – 5 layers of stratified squamous epithelium.
  • Tarsal conjunctiva –2 layers of stratified cuboidal  epithelium
  • Forniceal conjunctiva- 3 layers of stratified squamous epithelium
  • Bulbar conjunctiva- 10 layers of stratified columnar epithelium

CLINICAL NUGGET : SQUAMOUS METAPLASIA :

Many disorders of the conjunctiva cause the moist non keratinized conjunctiva to become dry and keratinized .This pathologic transition is called squamous metaplasia.

Clinically the conjunctiva looks thickened and dry due to loss pf goblet cells 

Goblet cells in the epithelium of the conjunctiva

  • Goblet cells are present through out the epithelium.
  • They arise from the basal layer, enlarge as they reach surface of the epithelium and then discharge mucin and disintegrate
  • Produces 2.2 ml of mucin everyday.
  • Ensures stability of the tear film by decreasing surface tension
Goblet cells of conjunctiva in epithelium

Adenoid layer /Lymphoid layer of Conjunctiva

  • The adenoid layer contains lymphoid tissue from which follicles are formed.
  • Follicles are aggregations of the lymphocytes.
  • Within the follicles are germinal centers with lymphoblasts in the center.
  • Also adenoid layer consists of the mast cells (6000/mm3), plasma cells and neutrophils.
  • Thickest in the fornices and terminates at the sulcus subtarsalis
  • The marginal conjunctiva is devoid of the adenoid/lymphoid layer.

The adenoid layer is responsible for forming the Conjunctiva associated lymphoid tissue(CALT). The CALT like the Mucosa associated lymphoid tissue has immune function.

NOTE :Lymphoid or adenoid layer is absent in the newborn, arises in 3-4 months of age in fornices

Fibrous Layer Of Conjunctiva

  • The fibrous layer is composed of connective tissue, which attaches the conjunctiva to the underlying structures.
  • The fibrous layer is firmly attached to the tarsal plate and the limbus.
  • This form attachment contributes to the characteristic appearance of papillae.

Accessory Lacrimal Glands

They are present within the substance of the conjunctiva. These are namely : 

Krause’s glands: 

  • Krause’s glands are located within the stroma of conjunctival fornix
  • They are about 20 to 40 in upper fornix, and 6-8 glands in lower fornix.
  • Their ducts unite to form one single duct to open in fornix

Wolfing’s glands(or Ciaccio’s glands)

  • These are located near the upper border of tarsus
  • They are larger than Krause
  • About 2-5 located along the superior tarsal border

Popov’s glands

  • Popov’s glands are located within the substance of the caruncle

For more about anatomy of lacrimal gland, click here.

NOTE :

  • All together Contribute to basal tear secretion,
  • They have no role in reflex tear secretion.
accessory lacrimal glands - glands of krause and glands of wolfring

Blood Supply of the Conjunctiva

Lets enumerate the arteries first before diving into the details :-

  • The marginal arcade of the eyelid for the marginal conjunctiva
  • The peripheral arcade of the eyelid for the forniceal conjunctiva
  • The posterior conjunctival artery which is a branch of the peripheral arcade to within 4 mm of the limbus
  • The anterior conjunctival artery which is a branch of the anterior ciliary for the limbus
  • The capillary arcades extending 1 mm into the cornea.

The marginal and peripheral arcae supply the Palpebral conjunctiva whereas the conjunctival arteries supply the bulbar conjunctiva.

Arcades Of the Conjunctiva

  • The marginal and peripheral arcade is formed by the two palpebral arteries.
  • The medial part is formed by the medial palpebral artery that is a branch of the dorsal nasal artery.
  • The lateral part of the arcade is formed by the lacrimal artery .
  • The dorsal nasal artery and the lacrimal artery are the branches of the ophthalmic artery which inturn is a branch of the internal carotid artery (For video on the blood supply of eye- click here).
  • Note : Both arcades are present in upper eyelid; but only on arcade is present in lower eyelid.

Important Facts about arcades

  • The arcades are present in the submuscular plane .( To understand further click on the anatomy of eyelid post).
  • To supply the conjunctiva the arcade, need to perforate the tarsus and reach the conjunctiva.
  • The marginal arcade branches pierce the tarsus at the sulcus subtarsalis.
  • After emerging from the sulcus subtarsalis it divided into an ascending branch and a descending branch.
  • The peripheral arcade pierces however the mullers muscle and reached the conjunctiva giving again ascending and descending branch.
  • The descending branch of the peripheral arcade anastomosis with the ascending branch of the marginal arcade
  • The ascending branch of the peripheral arcade travels the fornix and reached the bulbar conjunctiva, forming the posterior conjunctival artery.
diagram depicting peripheral and marginal arcades of eye

Conjunctival arteries : Anterior ciliary and Posterior Conjunctival

  • The ascending branch of the peripheral arcade travels the fornix and reached the bulbar conjunctiva, forming the posterior conjunctival artery.
  • The anterior ciliary arteries travel along the extraocular muscles.
  • These are branches of the Muscular artery.
  • There are 2 anterior ciliary arteries per muscle except the lateral rectus which has only one .
  • For more details about the blood supply of the muscles , click here
diagram depicting posterior conjunctival artery and anterior ciliary artery

Venous Supply of the conjunctiva

  • Conjunctival veins are more in number than the conjunctival arteries
  • Most of the veins drain into the superior and inferior ophthalmic veins from the eyelid plexus
  • Limbal area drains into the anterior ciliary vein

Lymphatic Drainage of the Conjunctiva

  • The lateral part of the conjunctiva drains into the preauricular group of the lymph nodes
  • The medial part drains into the submandibular group of lymph nodes 

Nerve Supply of the Conjunctiva

 The conjunctival nerve supply consists of branches of the ophthalmic division and branches of the maxillary division of the fifth cranial nerve.

Why is Conjunctiva Important? Functions of Conjunctiva !

  • Tear production (mucin by the goblet cells and aqueous by the accessory lacrimal glands)
  • Supply of oxygen directly to the cornea when the eyes are open
  • Tear film washes off debris
  • Maintain a smooth ocular surface
  • Protection of the eye by defense mechanisms which include nonspecific measures such as presence of an intact epithelial barrier, lacrimation and  provision of rich blood supply and specific immunological mechanisms such as outpouring of mast cells, leucocytes, presence of an active mucosal-associated lymphoid tissue( known as CALT in conjunctiva) and antibodies in the form of secretory IgA.

IMPORTANT NON PATHOGENIC COMMENSALS OF CONJUNCTIVA.

  • Staphylococcus albus
  • S. epidermidis
  • Diphtheroids
  • Propionibacterium acnes
  • Neisseria catarrhalis
  • Corynebacterium xerosis

Conclusion

So that was the detailed anatomy of the conjunctiva. I hope that was useful! Do not  forget to subscribe our youtube channel. 

If you are an audio visual learner you can check out our video on the conjunctiva anatomy here.

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