The conjunctiva is a mucous membrane which lines the posterior surface of the eyelids and anterior aspect of eyeball.
The name conjunctiva (conjoin: to join) has been given to this mucous membrane owing to the fact that it joins the eyeball to the lids.
It stretches from the lid margin to the limbus, and encloses a complex space called conjunctival sac which is open in front at the palpebral fissure.
Parts of conjunctiva
Conjunctiva can be divided into three parts:
1. Palpebral conjunctiva.
It lines the lids and can be subdivided into marginal, tarsal and orbital conjunctiva.
i. Marginal conjunctiva extends from the lid margin to about 2 mm on the back of lid up to a shallow groove, the sulcus subtarsalis. It is actually a transitional zone between skin and the conjunctiva proper.
ii. Tarsal conjunctiva is thin, transparent and highly vascular. It is firmly adherent to the whole tarsal plate in the upper lid. In the lower lid, it is adherent only to half width of the tarsus. The tarsal glands are seen through it as yellow streaks.
iii. Orbital part of palpebral conjunctiva lies loose between the tarsal plate and fornix.
2. Bulbar conjunctiva.
It is thin, transparent and lies loose over the underlying structures and thus can be moved easily.
It is separated from the anterior sclera by episcleral tissue and Tenon's capsule. A 3-mm ridge of bulbar conjunctiva around the cornea is called limbal conjunctiva.
In the area of limbus, the conjunctiva, Tenon's capsule and the episcleral tissue are fused into a dense tissue which is strongly adherent to the underlying corneoscleral junction.
At the limbus, the epithelium of conjunctiva becomes continuous with that of cornea.
3. Conjunctival fornix.
It is a continuous circular cul-de-sac which is broken only on the medial side by caruncle and the plica semilunaris.
Conjunctival fornix joins the bulbar conjunctiva with the palpebral conjunctiva. It can be subdivided into superior, inferior, medial and lateral fornices.
Structure of conjunctiva
Histologically, conjunctiva consists of three layers namely,
(1) epithelium,
(2) adenoid layer
(3) fibrous layer
1. Epithelium.
The layer of epithelial cells in conjunctiva varies from region to region and in its different parts as follows:
� Marginal conjunctiva has 5-layered stratified squamous type of epithelium.
� Tarsal conjunctiva has 2-layered epithelium: superficial layer of cylindrical cells and a deep layer of flat cells.
� Fornix and bulbar conjunctiva have 3-layered epithelium: a superficial layer of cylindrical cells, middle layer of polyhedral cells and a deep layer of cuboidal cells.
Limbal conjunctiva has again many layered (5 to 6) stratified squamous epithelium.
2. Adenoid layer.
It is also called lymphoid layer and consist s of fine connective tissue reticulum in. the meshes of which lie lymphocytes. This layer is most developed in the fornices. It is not present since birth but develops after 3-4 months of life. For this reason, conjunctival inflammation in an infant does not produce follicular reaction.
3. Fibrous layer.
It consists of a meshwork of collagenous and elastic fibres. It is thicker than the adenoid layer, except in the region of tarsal conjunctiva, where it is very thin.
This layer contains vessels and nerves of conjunctiva. It blends with the underlying Tenon's capsule in the region of bulbar conjunctiva.
Glands of conjunctiva
The conjunctiva contains two types of glands.
1. Mucin secretory glands.
These are goblet cells (the unicellular glands located within the epithelium), crypts of Henle (present in the tarsal conjunctiva) and glands of Manz (found in limbal conjunctiva).
These glands secrete mucus which is essential for wetting the cornea and conjunctiva.
2. Accessory lacrimal glands.
These are Glands of Krause (present in subconjunctival connective tissue of fornix, about 42 in upper fornix and 8 in lower fornix) and Glands of Wolfring (present along the upper border of superior tarsus and along the lower border of inferior tarsus).
Plica semilunaris
It is a pinkish crescentric fold of conjunctiva, present in the medial canthus. Its lateral free border is concave.
It is a vestigeal structure in human beings and represents the nictitating membrane (or third eyelid) of lower animals.
Caruncle
The caruncle is a small, ovoid, pinkish mass, situated in the inner canthus, just medial to the plica semilunaris.
In reality, it is a piece of modified skin and so is covered with stratified squamous epithelium and contains sweat glands, sebaceous glands and hair follicles.
Blood supply of conjunctiva
Arteries supplying the conjunctiva are derived from three sources
(1) peripheral arterial arcade of the eyelid;
(2) marginal arcade of the eyelid
(3) anterior ciliary arteries.
� Palpebral conjunctiva and fornices are supplied by branches from the peripheral and marginal arterial arcades of the eyelids.
� Bulbar conjunctiva is supplied by two sets of vessels: the posterior conjunctival arteries which are branches from the arterial arcades of the eyelids; and the anterior conjunctival arteries which are the branches of anterior ciliary arteries.
Terminal branches of the posterior conjunctival arteries anastomose with the anterior conjunctival arteries to form the pericorneal plexus. Veins from the conjunctiva drain into the venous plexus of eyelids and some around the cornea into the anterior ciliary veins.
Lymphatics of the conjunctiva are arranged in two layers: a superficial and a deep. Lymphatics from the lateral side drain into preauricular lymph nodes and those from the medial side into the submandibular lymph nodes.
Nerve supply of conjunctiva
A circumcorneal zone of conjunctiva is supplied by the branches from long ciliary nerves which supply the cornea. Rest of the conjunctiva is supplied by the branches from lacrimal, infratrochlear, supratrochlear, supraorbital and frontal nerves.
OPTOMETRY-SHARP VISION
Optometrist
0 Comments