The conjunctiva is a thin
mucous membrane
A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It i ...
that lines the inside of the
eyelid
An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eye ...
s and covers the
sclera (the white of the
eye).
It is composed of non-keratinized,
stratified squamous epithelium with
goblet cells,
stratified columnar epithelium
Stratified columnar epithelium is a rare type of epithelial tissue composed of column-shaped cells arranged in multiple layers. It is found in the conjunctiva, pharynx, anus, and male urethra. It also occurs in embryo.
Location
Stratified col ...
and
stratified cuboidal epithelium (depending on the zone). The conjunctiva is highly vascularised, with many
microvessels easily accessible for imaging studies.
Structure
The conjunctiva is typically divided into three parts:
Blood supply
Blood to the bulbar conjunctiva is primarily derived from the ophthalmic artery. The blood supply to the palpebral conjunctiva (the eyelid) is derived from the
external carotid artery. However, the circulations of the bulbar conjunctiva and palpebral conjunctiva are linked, so both bulbar conjunctival and palpebral conjunctival vessels are supplied by both the ophthalmic artery and the external carotid artery, to varying extents.
Nerve supply
Sensory innervation of the conjunctiva is divided into four parts:
Microanatomy
The conjunctiva consists of unkeratinized, both stratified squamous and stratified columnar epithelium, with interspersed
goblet cells.
The epithelial layer contains blood vessels, fibrous tissue, and lymphatic channels.
Accessory lacrimal glands
Krause's glands, Wolfring's glands (or Ciaccio's glands) and Popov's gland are the accessory lacrimal glands of the lacrimal system of human eye. These glands are structurally and histologically similar to the main lacrimal gland. Glands of Kra ...
in the conjunctiva constantly produce the aqueous portion of tears.
Additional cells present in the conjunctival epithelium include
melanocytes,
T and B cell lymphocytes.
Function
The conjunctiva helps lubricate the
eye by producing
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
and
tears
Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different types of ...
, although a smaller volume of
tears
Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different types of ...
than the
lacrimal gland
The lacrimal glands are paired exocrine glands, one for each eye, found in most terrestrial vertebrates and some marine mammals, that secrete the aqueous layer of the tear film. In humans, they are situated in the upper lateral region of each or ...
.
It also contributes to
immune surveillance and helps to prevent the entrance of
microbes
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
into the eye.
Clinical significance
Disorders of the conjunctiva and
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...
are common sources of eye complaints, in particular because the surface of the eye is exposed to various external influences and is especially susceptible to
trauma,
infections
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable di ...
, chemical irritation,
allergic reactions
Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic derm ...
, and
dryness.
* The conjunctival microvascular hemodynamics are affected by
diabetic retinopathy
Diabetic retinopathy (also known as diabetic eye disease), is a medical condition in which damage occurs to the retina due to diabetes mellitus. It is a leading cause of blindness in developed countries.
Diabetic retinopathy affects up to 80 perc ...
(DR), hence can be useful for DR diagnosis and monitoring, and discriminating stages of DR.
*
Type II diabetes
Type 2 diabetes, formerly known as adult-onset diabetes, is a form of diabetes mellitus that is characterized by high blood sugar, insulin resistance, and relative lack of insulin. Common symptoms include increased thirst, frequent urinatio ...
is associated with conjunctival
hypoxia, increased average blood vessel diameter, and capillary loss.
*
Sickle-cell anemia
Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red bl ...
is associated with blood vessel sludging, altered blood flow and blood vessel diameter, and capillary
micro-haemorrhages.
*
Hypertension is associated with an increase in the
tortuosity of bulbar conjunctival blood vessels and capillary and arteriole loss.
*
Carotid artery occlusion is associated with slower conjunctival blood flow and apparent capillary loss.
* With age, the conjunctiva can stretch and loosen from the underlying sclera, leading to the formation of conjunctival folds, a condition known as
conjunctivochalasis
Conjunctivochalasis, also known aMechanical Dry Eye (MDE) is a common eye surface condition characterized by the presence of excess folds of the conjunctiva located between the globe of the eye and the eyelid margin.
Symptoms
Symptoms range fro ...
.
* The conjunctiva can be affected by
tumors which can be benign, pre-malignant or malignant.
*
Leptospirosis, an infection with Leptospira, can cause
conjunctival suffusion, which is characterized by chemosis, and redness without exudates.
Bulbar conjunctival microvasculature
Vessel morphology
The bulbar conjunctival
microvasculature
The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. ...
contains
arteriole
An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.
Arterioles have muscular walls (usually only one to two layers of smooth muscle cells) and are the primar ...
s,
meta-arterioles,
venule
A venule is a very small blood vessel in the microcirculation that allows blood to return from the capillary beds to drain into the larger blood vessels, the veins. Venules range from 7μm to 1mm in diameter. Veins contain approximately 70% of t ...
s,
capillaries
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
, and communicating vessels. Vessel morphology varies greatly between subjects and even between regions of the individual eyes. In some subjects, arterioles and venules can be seen to run parallel with each other. Paired arterioles are generally smaller than corresponding venules. The average bulbar conjunctival vessel has been reported to be 15.1 microns, which reflects the high number of small capillaries, which are typically <10 microns in diameter.
Blood oxygen dynamics
The bulbar conjunctival microvasculature is in close proximity to ambient air, thus
oxygen diffusion from ambient air strongly influences their
blood oxygen saturation. Because of oxygen diffusion,
hypoxic
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the t ...
bulbar conjunctival vessels will rapidly reoxygenate (in under 10 seconds) when exposed to ambient air (i.e. when the eyelid is open). Closing the eyelid stops this oxygen diffusion by placing a barrier between the bulbar conjunctival microvessels and ambient air.
Blood vessel imaging methods
The bulbar conjunctival microvessels are typically imaged with a high-magnification
slit lamp
A slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope. The lamp facilitates an examination of the anterior segme ...
with green filters.
With such high-magnification imaging systems, it is possible to see groups of individual red blood cells flowing in vivo.
Fundus cameras may also be used for low-magnification wide field-of-view imaging of the bulbar conjunctival microvasculature. Modified fundus cameras have been used to measure conjunctival blood flow and to measure
blood oxygen saturation.
Fluorescein angiography
Fluorescein angiography (FA), fluorescent angiography (FAG), or fundus fluorescein angiography (FFA) is a technique for examining the circulation of the retina and choroid (parts of the fundus) using a fluorescent dye and a specialized camera. S ...
has been used to study the blood flow of the bulbar conjunctiva and to differentiate the bulbar conjunctival and
episcleral microcirculation.
Vasodilation
The bulbar conjunctival microvasculature is known to dilate in response to several stimuli and external conditions, including allergens (e.g. pollen), temperature,
time-of-day,
contact-lens wear,
and acute mild hypoxia.
Bulbar conjunctival vasodilation has also been shown to correlate changes in emotional state.
Type 2 diabetes is associated with an increase in average bulbar conjunctival vessel diameter and capillary loss.
Sickle-cell anemia
Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red bl ...
is associated with altered average vessel diameter.
See also
*
Conjunctivitis
Conjunctivitis, also known as pink eye, is inflammation of the outermost layer of the white part of the eye and the inner surface of the eyelid. It makes the eye appear pink or reddish. Pain, burning, scratchiness, or itchiness may occur. The ...
(pink-eye)
*
Conjunctivochalasis
Conjunctivochalasis, also known aMechanical Dry Eye (MDE) is a common eye surface condition characterized by the presence of excess folds of the conjunctiva located between the globe of the eye and the eyelid margin.
Symptoms
Symptoms range fro ...
*
Dry eye
*
Pinguecula
A pinguecula is a common type of conjunctival stromal degeneration in the eye. It appears as an elevated yellow-white plaque in the bulbar conjunctiva near the limbus. Calcification may also seen occasionally.
Etiology
The exact etiology is unk ...
*
Pterygium
Pterygium (plural ''pterygia'' or ''pterygiums'') refers to any wing-like triangular membrane occurring in the neck, eyes, knees, elbows, ankles or digits.
The term comes from the Greek word ''pterygion'' meaning "wing".
Types
* Popliteal pter ...
*
Rougine
*
Subconjunctival hemorrhage
Subconjunctival bleeding, also known as subconjunctival hemorrhage or subconjunctival haemorrhage, is bleeding from a small blood vessel over the whites of the eye. It results in a red spot in the white of the eye. There is generally little to no ...
*
Diabetes
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...
*
Sickle-cell disease
*
Slit lamp
A slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope. The lamp facilitates an examination of the anterior segme ...
Additional images
Image:Gray893.png, Sagittal section through the upper eyelid.
File:Slide2www.JPG, Extrinsic eye muscle. Nerves of orbita. Deep dissection.
References
External links
*
{{Authority control
Human eye anatomy