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

Abstract

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 T(m), 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 to 15 °C and end at 35-36 °C, with T(m) being about 30 °C. The highly asymmetrical shape of the thermotropic peak of meibum is important for the physiology and biophysics of TFLL.

Sebum/Meibum Surface Film Interactions and Phase Transitional Differences

“…Squalene has a much lower melting point (−75°C) than the transition temperature of both sebum (Table 2) and meibum (28–35°C). 57,69,70 A ratio of SQ well above expected physiological levels was used to clearly delineate an effect should one occur. Squalene caused significant ( P < 0.05) changes in the phase transition parameters for a pool of human meibum (Figs.…”

TFOS DEWS II Tear Film Report

“…Monolayers of tear film-like wax esters can retard evaporation when used at temperatures 3 °C lower than their bulk melting temperature [258,260,261]. However, meibum has a much more complex composition, structure and thermotropic behavior compared to the model films, and the melting temperature of meibum (~30 °C) [262–264] is lower than the ocular surface temperature (35 °C). Recently in vitro studies with the Interfacial Dewetting and Drainage Optical Platform (i-DDrOP) on silicone hydrogel (SiHy) contact lenses have shown that the mucoaqueous subphase beneath thinner parts of meibomian films thins and dewets faster compared to that seen below thicker “islands” in the meibomian layer [265].…”

“…Data on the thermotropic phase transitions (via spectroscopy, differential scanning calorimetry and X-ray diffraction studies) [262,264,301–303] and interfacial and bulk rheology [29,301,302] of tear lipids in vitro and in vivo , suggest a thick viscoelastic duplex structure composed of a monomolecular layer of amphiphilic polar lipids at the mucoaqueous surface and a generally unstructured shear thinning lipophilic suspension at the lipid-air interface. The lipophilic suspension consists of lipid lamellar-crystallite particulates immersed in a continuous liquid phase with no long-range order.…”

“…Differential scanning calorimetry reveals that the phase transitions in meibum start at 10–15 °C and end at 35–36 °C, with a melting temperature (Tm) of ~30 °C [264,302]. Thus, it is reasonable to assume that in the meibomian gland orifices meibum is both disorganized and liquid to facilitate excretion.…”

“…Combining differential scanning calorimetry with small- and wide-angle X-ray diffraction points to meibum as a liquid suspension consisting of lipid lamellar-crystallite particulates immersed in a continuous liquid phase, with no long-range order at physiological temperature [262,264,301–303]. Hot stage cross-polarized light microscopy confirms such a view and reveals that in meibum from patients with MGD there is an increased presence of non-lipid, non-melting, chloroform-insoluble inclusions of protein, including cytokeratin [262].…”