[Meibomian glands. Part II: physiology, characteristics, distribution and function of meibomian oil].

The oily secretion (meibum) of the Meibomian glands forms the superficial layer of the pre-ocular tear film and reduces evaporation of the aqueous phase. Meibum is a complex mixture of various lipids and minor protein components as well as other components of the secretory meibocytes, which form a clear liquid at body temperature. The exact composition and functions of meibum are still partly unknown, in particular the interaction of the water insoluble non-polar lipids with the polar, partly water soluble, lipids and potentially with proteins, which altogether interact to maintain the connection with the underlying aqueous tear phase. Meibum is transported within the gland by the force of secretory pressure from continuous secretion and by muscular action of the orbicularis muscle and Riolans muscles during blinking. After delivery of meibum onto the posterior lid margin the oil moves from the posterior lid margin reservoir onto the tear meniscus and is pulled as a thin layer onto the pre-ocular tear film every time the lid opens. During lid closure it is compressed and a small part is continuously renewed. Meibum also has a barrier function against the spillage of tears over the inner border of the lid and against the entry of skin lipids (sebum) from the free lid margin.