The conjunctiva is typically divided into three parts:
Part
Area
Palpebral or tarsal conjunctiva
Lines the eyelids
Bulbar or ocular conjunctiva
Covers the eyeball, over the anterior
sclera: This region of the conjunctiva is tightly bound to the underlying sclera by
Tenon's capsule and moves with the eyeball movements. The average thickness of the bulbar conjunctival membrane is 33 microns.[2]
Fornix conjunctiva
Forms the junction between the bulbar and palpebral conjunctivas: It is loose and flexible, allowing the free movement of the lids and eyeball.[3]
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.[4]
Nerve supply
Sensory innervation of the conjunctiva is divided into four parts:[5]
The conjunctiva consists of unkeratinized, both stratified squamous and stratified columnar epithelium, with interspersed
goblet cells.[6] The epithelial layer contains blood vessels, fibrous tissue, and lymphatic channels.[6]Accessory lacrimal glands in the conjunctiva constantly produce the aqueous portion of tears.[6] Additional cells present in the conjunctival epithelium include
melanocytes,
T and B cell lymphocytes.[6]
Disorders of the conjunctiva and
cornea 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, chemical irritation,
allergic reactions, and
dryness.
The conjunctival microvascular hemodynamics are affected by
diabetic retinopathy (DR), hence can be useful for DR diagnosis and monitoring,[8] and discriminating stages of DR.[9]
Hypertension is associated with an increase in the
tortuosity of bulbar conjunctival blood vessels and capillary and arteriole loss.[17][18]
Carotid artery occlusion is associated with slower conjunctival blood flow and apparent capillary loss.[4]
With age, the conjunctiva can stretch and loosen from the underlying sclera, leading to the formation of conjunctival folds, a condition known as
conjunctivochalasis.[19][20]
The conjunctiva can be affected by
tumors which can be benign, pre-malignant or malignant.[21]
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 contains
arterioles,
meta-arterioles,
venules,
capillaries, 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.[22] 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.[23]
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 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.[24]
Blood vessel imaging methods
The bulbar conjunctival microvessels are typically imaged with a high-magnification
slit lamp with green filters.[25][26][27] With such high-magnification imaging systems, it is possible to see groups of individual red blood cells flowing in vivo.[25] 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 [28] and to measure
blood oxygen saturation.[24]Fluorescein angiography has been used to study the blood flow of the bulbar conjunctiva and to differentiate the bulbar conjunctival and
episcleral microcirculation.[29][30][31]
Vasodilation
The bulbar conjunctival microvasculature is known to dilate in response to several stimuli and external conditions, including allergens (e.g. pollen),[32] temperature,[33] time-of-day,[33]contact-lens wear,[13] and acute mild hypoxia.[24] Bulbar conjunctival vasodilation has also been shown to correlate changes in emotional state.[34]
Type 2 diabetes is associated with an increase in average bulbar conjunctival vessel diameter and capillary loss.[11][12]Sickle-cell anemia is associated with altered average vessel diameter.[14]
^
abDitzel J (1967-01-12). "The in vivo reactions of the small blood vessels to diabetes mellitus". Acta Medica Scandinavica. Supplementum. 476 (S476): 123–34.
doi:
10.1111/j.0954-6820.1967.tb12691.x.
PMID5236035.
^
abCheung AT, Ramanujam S, Greer DA, Kumagai LF, Aoki TT (2001-10-01). "Microvascular abnormalities in the bulbar conjunctiva of patients with type 2 diabetes mellitus". Endocrine Practice. 7 (5): 358–63.
doi:
10.4158/EP.7.5.358.
PMID11585371.
^Harper RN, Moore MA, Marr MC, Watts LE, Hutchins PM (November 1978). "Arteriolar rarefaction in the conjunctiva of human essential hypertensives". Microvascular Research. 16 (3): 369–72.
doi:
10.1016/0026-2862(78)90070-5.
PMID748720.
^Lee RE (August 1955). "Anatomical and physiological aspects of the capillary bed in the bulbar conjunctiva of man in health and disease". Angiology. 6 (4): 369–82.
doi:
10.1177/000331975500600408.
PMID13275744.
S2CID11589129.
^Horak F, Berger U, Menapace R, Schuster N (September 1996). "Quantification of conjunctival vascular reaction by digital imaging". The Journal of Allergy and Clinical Immunology. 98 (3): 495–500.
doi:
10.1016/S0091-6749(96)70081-7.
PMID8828525.
^
abDuench S, Simpson T, Jones LW, Flanagan JG, Fonn D (June 2007). "Assessment of variation in bulbar conjunctival redness, temperature, and blood flow". Optometry and Vision Science. 84 (6): 511–6.
doi:
10.1097/OPX.0b013e318073c304.
PMID17568321.
S2CID943038.
^Provine RR, Nave-Blodgett J, Cabrera MO (2013-11-01). "The Emotional Eye: Red Sclera as a Uniquely Human Cue of Emotion". Ethology. 119 (11): 993–998.
doi:
10.1111/eth.12144.
ISSN1439-0310.
External links
"Conjunctiva". Medicinenet.com. 1999. Archived from
the original on 7 June 2014. Retrieved 25 July 2004.