Differential scanning calorimetric evaluation of human meibomian gland secretions and model lipid mixtures: Transition temperatures and cooperativity of melting

• Meibomian gland secretions (meibum) are formed from a large number of various lipids.
• These lipids, if melted individually, have very high transition temperatures (Tm).
• Also, they melt highly cooperatively with cooperativity unit size, n, being 50–100.
• However, the meibum's Tm is about 30 °C, while n is only about 10.
• Easy melting of meibomian lipids is important for their physiological functions.

Meibomian gland secretions (or meibum) are produced by holocrine meibomian glands and are secreted in melted form onto the ocular surface of humans and animals to form a protective tear film lipid layer (TFLL). Its protective effect strongly depends on the composition and, hence, thermotropic behavior of meibum. The goal of our study was to quantitatively evaluate the melting characteristics of human meibum and model lipid mixtures using differential scanning microcalorimetry. Standard calorimetric parameters, e.g. changes in calorimetric enthalpy, transition temperatures Tm, cooperativity of melting, etc. were assessed. We found that thermotropic behavior of meibum resembled that of relatively simple mixtures of unsaturated wax esters, but showed a lower change in calorimetric enthalpy, which can be indicative of a looser packing of lipids in meibum compared with pure standards and their simple mixtures. The cooperativity of melting of meibomian lipids was comparable to that of an equimolar mixture of four oleic-acid based wax esters. We demonstrated that the phase transitions in meibum start at about 10–15 °C and end at 35–36 °C, with Tm being about 30 °C. The highly asymmetrical shape of the thermotropic peak of meibum is important for the physiology and biophysics of TFLL.