The meibomian glands are a special kind of sebaceous gland

Sebaceous gland

The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals...

 at the rim of the eyelid

Eyelid

An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

s inside the tarsal plate, responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 tear film. They prevent tear spillage onto the cheek, trapping tears between the oiled edge and eyeball, and make the closed lids airtight. There are approximately 50 glands on the upper eyelids and 25 glands on the lower eyelids.
The glands are named after Heinrich Meibom (1638–1700), a German physician.

Lipids

Lipids 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. Lipids are universally recognized as major components of human and animal meibum. Recently, an update on the composition of human meibum and on the structures of various positively identified meibomian lipids was published.
Currently, the most sensitive and informative approach to lipidomic analysis of meibum is mass-spectrometry in combination with liquid chromatography.

Proteins

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

Dysfunction

Dysfunctional meibomian glands often cause dry eyes, one of the more common eye conditions. They may also cause blepharitis

Blepharitis

Blepharitis is an ocular condition characterized by chronic inflammation of the eyelid, the severity and time course of which can vary. Onset can be acute, resolving without treatment within 2–4 weeks , but more generally is a long standing inflammation varying in severity...

, as the dry eyeball rubs off small pieces of skin from the eyelid, which may get infected. Inflammation of the meibomian glands (also known as meibomitis, meibomian gland dysfunction, or posterior blepharitis) causes the glands to be obstructed by thick waxy secretions. Besides leading to dry eyes, the obstructions can be degraded by bacterial lipase

Lipase

A lipase is an enzyme that catalyzes the formation or cleavage of fats . Lipases are a subclass of the esterases.Lipases perform essential roles in the digestion, transport and processing of dietary lipids in most, if not all, living organisms...

s, resulting in the formation of free fatty acid

Fatty acid

In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s, which irritate the eyes and sometimes cause punctate keratopathy

Thygeson's superficial punctate keratopathy

Thygeson's superficial punctate keratopathy is a disease of the eyes. The causes of TSPK are not currently known, but details of the disease were first published in the Journal of the American Medical Association in 1950 by Phillips Thygeson - after whom it is named.- Symptoms :A patient with TSPK...

.