Meibomian glands (also called tarsal glands, palpebral glands, and tarsoconjunctival glands) are

sebaceous gland A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number ...

s along the rims 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 eyel ...

inside the

tarsal plate The tarsi (tarsal plates) are two comparatively thick, elongated plates of dense connective tissue, about in length for the upper eyelid and 5 mm for the lower eyelid; one is found in each eyelid, and contributes to its form and support. They ...

. They produce meibum, an oily substance that prevents evaporation of the eye's

tear film Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all Mammal, land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different ty ...

. Meibum prevents

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 ...

from spilling onto the cheek, traps them between the oiled edge and the eyeball, and makes the closed lids airtight. There are about 25 such glands on the upper eyelid, and 20 on the lower eyelid. Dysfunctional meibomian glands is believed to be the most often cause of

dry eye Dry or dryness most often refers to: * Lack of rainfall, which may refer to **Arid regions **Drought * Dry or dry area, relating to legal prohibition of selling, serving, or imbibing alcoholic beverages * Dry humor, deadpan * Dryness (medical) * ...

s. They are also the cause of posterior blepharitis.

History

The glands were mentioned by

Galen Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be one of ...

in 200 AD and were described in more detail by Heinrich Meibom (1638–1700), a German physician, in his work ''De Vasis Palpebrarum Novis Epistola'' in 1666. This work included a drawing with the basic characteristics of the glands.

Anatomy

Although the upper lid have greater number and volume of meibomian glands than the lower lid, there is no consensus whether it contributes more to the tearfilm stability. The glands do not have direct contact with eyelash follicles. The process of blinking releases meibum into the lid margin.

Function

Meibum

Lipids

Lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...

s are the major components of meibum (also known as "meibomian gland secretions"). The term "meibum" was originally introduced by Nicolaides ''et al.'' in 1981. The biochemical composition of meibum is extremely complex and very different from that of

sebum A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest nu ...

. Lipids are universally recognized as major components of human and animal meibum. An update was published in 2009 on the composition of human meibum and on the structures of various positively identified meibomian lipids Currently, the most sensitive and informative approach to lipidomic analysis of meibum is

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

, either with direct infusion or in combination with

liquid chromatography In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...

. The lipids are the main component of the lipid layer of the tear film, preventing rapid evaporation and it is believed they lower the surface tension help helps to stabilize the tear film.

Proteins

In humans, more than 90 different proteins have been identified in meibomian gland secretions.

Clinical significance

Dysfunctional meibomian glands often cause

s, one of the more common eye conditions. They may also contribute to

blepharitis Blepharitis is one of the most common ocular conditions characterized by inflammation, scaling, reddening, and crusting of the eyelid. This condition may also cause swelling, burning, itching, or a grainy sensation when introducing foreign objects ...

. Inflammation of the meibomian glands (also known as ''meibomitis'', ''meibomian gland dysfunction'', or ''posterior blepharitis'') causes the glands to be obstructed by thick, cloudy-to-yellow, more opaque and viscous-like, oily and waxy secretions, a change from the glands' normal clear secretions.Peter Bex, Reza Dana, Linda Mcloon, Jerry Niederkorn (2011)
''Ocular Periphery and Disorders''
/ref> Besides leading to dry eyes, the obstructions can be degraded by bacterial

lipase Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...

s, resulting in the formation of free

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

s, which irritate the eyes and sometimes cause punctate keratopathy. Meibomian gland dysfunction is more often seen in women and is regarded as the main cause of

dry eye disease Dry eye syndrome (DES), also known as keratoconjunctivitis sicca (KCS), is the condition of having dry eyes. Other associated symptoms include irritation, redness, discharge, and easily fatigued eyes. Blurred vision may also occur. Symptoms range ...

. Factors that contribute to meibomian gland dysfunction can include things such as a person's age and/or hormones, or severe infestation of

Demodex brevis ''Demodex brevis'' is one of the two species of face mite that inhabit humans (the other being ''Demodex folliculorum''). They are very similar to ''Demodex folliculorum'', with just a few differences. They are usually found in the sebaceous gla ...

mite. Treatment can include

warm compresses A warm compress is a method of applying heat to the body. Heating sources can include warm water, microwaveable pads, wheat packs and electrical or chemical pads. Some unorthodox methods can include warmed potatoes, uncooked rice, and hard-boiled ...

to thin the secretions and eyelid scrubs with a commercial eyelid cleanser as multiple studies have shown baby shampoo to be therapeutically ineffective, or emptying ("expression") of the gland by a professional.

Lifitegrast Lifitegrast, sold under the brand name Xiidra (), is a medication for the treatment of signs and symptoms of dry eye, a syndrome called keratoconjunctivitis sicca. Lifitegrast reduces inflammation by inhibiting inflammatory cell binding. It is of ...

and

Restasis Ciclosporin, also spelled cyclosporine and cyclosporin, is a calcineurin inhibitor, used as an immunosuppressant medication. It is a natural product. It is taken orally or intravenously for rheumatoid arthritis, psoriasis, Crohn's disease ...

are topical medication commonly used to control the inflammation and improve the oil quality. In some cases, topical

steroid A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...

s and topical (drops or ointment)/oral

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

s (to reduce bacteria on the lid margin) are also prescribed to reduce inflammation.

Intense pulsed light Intense pulsed light (IPL) is a technology used by cosmetic and medical practitioners to perform various skin treatments for aesthetic and therapeutic purposes, including hair removal, photorejuvenation (e.g. the treatment of skin pigmentation ...

(IPL) treatments have also been shown to reduce inflammation and improve gland function. Meibomian gland probing is also used on patients who experience deep clogging of the glands. Meibomian gland dysfunction may be caused by some prescription medications, notably

isotretinoin Isotretinoin, also known as 13-''cis''-retinoic acid and sold under the brand name Accutane among others, is a medication primarily used to treat severe acne. It is also used to prevent certain skin cancers (squamous-cell carcinoma), and in th ...

. A blocked meibomian gland can cause a

chalazion A chalazion (; plural chalazia or chalazions) or meibomian cyst is a cyst in the eyelid usually due to a blocked meibomian gland, typically in the middle of the eyelid, red, and not painful. They tend to come on gradually over a few weeks. A ...

(or "meibomian cyst") to form in the eyelid.

References

External links

*
Rethinking Meibomian Gland Dysfunction: How to Spot It, Stage It and Treat It
{{Authority control Human eye anatomy