As a layer of mucous membrane that partially rests on the eyeball and lies against the eyelids from the inside, the conjunctiva serves in particular to protect the eye and the immune system. Diseases are often manifested by a reddish to brick-red discoloration of the conjunctiva.
What is the conjunctiva?
Conjunctiva (conjunctiva, tunica conjunctiva) is the term used to describe the transparent, mucosa-like continuation of the skin in the area of the eye that covers the eyelid on the posterior surface to continue on the ventral (anterior) surface of the sclera (cornea) and then connect to the cornea (cornea) at the limbus corneae, the transition zone between the sclera and cornea. The conjunctiva also provides a connection between the bulbus oculi (eyeball) and the eyelids, with which it is firmly fused. The numerous vessels of the conjunctiva, which are displaceable in a healthy state and barely visible to the naked eye, become more prominent with their brick-red color when irritated.
Anatomy and structure
The conjunctiva is commonly divided into three distinct sections. The portion of the conjunctiva that covers the posterior surface of the eyelid and lines its inner surface is called the conjunctiva palpebrarum (also conjunctiva tarsi). This then continues as the conjunctiva fornicis with the formation of an upper and lower fold (fornix conjunctivae superior and inferior, respectively) and merges into the conjunctiva bulbi covering the anterior surface of the sclera. At the limbus, the conjunctiva firmly attaches to the cornea. While it is firmly fused to the eyelids, the conjunctiva is merely loosely attached to the bulb and covers it on the ventral portion up to the limbus corneae. The visible part of the sclera is completely covered by the conjunctiva. Histologically, the conjunctiva consists of a multilayered epithelial tissue and a layer of connective tissue underneath (lamina propria). Within the non-keratinizing epithelial layer there are also the so-called goblet cells, which are involved in the synthesis of the tear film as mucus-forming cells. Sensory innervation of the conjunctiva is provided primarily by branches of the trigeminal nerve.
Function and Tasks
Conjunctivitis is the most common disease of the conjunctiva in the eye and should be examined and treated by an ophthalmologist. The conjunctiva first connects the eyeball with the eyelids (Latin “coniungere” = “to connect”) as a transparent mucous membrane layer. In addition, it acts as an outer protective covering of the eye and provides an additional protective mechanism through the mucus-forming goblet cells located in it, which participate in the synthesis of the tear film. Among other things, the tear film protects the eye from foreign bodies and, through its antimicrobial components, the anterior bulb from infection. In addition, it serves as a lubricating layer for the upper lid and nourishes the avascular cornea via diffusion. The tarsal conjunctiva (conjunctiva palpebrarum) has a large number of follicle-like collections of plasma cells and lymphocytes, which are designed to prevent invasion by foreign pathogens. In the presence of inflammation, these enlarge and form follicles that bulge out (so-called follicular swelling). In addition, so-called Langerhans cells are found in the tarsal conjunctiva in particular. These cells, which belong to the dendritic system (immune defense), play an essential role in antigen presentation through their interaction with T lymphocytes. Conjunctival Langerhans cells, along with corneal dendritic cells, are thought to play an important function within the immune system as regulators of immune response and modulators between immune tolerance and defense.
Diseases and disorders
The conjunctiva can be affected by a variety of disorders. One of the most common disorders is represented by inflammatory changes of the conjunctiva (conjunctivitis), which may be due to various causes such as chemical-physical stimuli (including foreign bodies, injuries, radiation, burns, chemical burns), bacterial (including conjunctivitis pseudomembranosa, swimming pool conjunctivitis, conjunctivitis trachomatosa), and viral infections (including Conjunctivitis follicularis), pathological processes in the adjacent structures (e.g. meibomian carcinoma), wetting disorders due to reduced tear secretion (e.g. keratoconjunctivitis sicca) and allergies (e.g. conjunctivitis vernalis).Symptomatically, acute conjunctival inflammation is characterized by redness, swelling, strong secretion, photosensitivity and blepharospasm, while chronic conjunctivitis is characterized by absent edema, reduced secretion and proliferation of the papillary body. Since the conjunctiva contains a high number of plasma cells, leukocytes and lymphocytes, allergies, irritations, inflammations (especially from the paranasal sinuses) and blood vessel congestion (e.g. due to tumors or endocrine orbitopathy) lead to glassy, edematous swellings (chemosis). After traumatic events, during strong stress (e.g. labor, strong cough) and/or pathological changes of the blood and vascular system (e.g. arteriosclerosis, hypotension) hyposphagmata (hemorrhages into the subconjunctival space) are frequently observed. These subconjunctival hemorrhages are characterized by their sharp borders, while the conjunctiva has an intense red coloration. Conjunctival hemorrhages are generally harmless and are resorbed within 1 to 2 weeks.
