This paper reviews our current understanding of age-related meibomian gland dysfunction (MGD) and the role of the nuclear receptor, peroxisome proliferator-activated receptor gamma (PPARγ), in the regulation of meibomian gland function, meibocyte differentiation and lipid synthesis. The studies suggest that PPARγ is a master regulator of meibocyte differentiation and function, whose expression and nuclear signaling coupled with meibocyte renewal is altered during aging, potentially leading to atrophy of the meibomian gland as seen in clinical MGD. Study of meibomian gland stem cells also suggest that there is a limited number of precursor meibocytes that provide progeny to the acini, that may be susceptible to exhaustion as occurs during aging and other environmental factors. Further study of pathways regulating PPARγ expression and function as well as meibocyte stem cell maintenance may provide clues to establishing cellular and molecular mechanisms underlying MGD and the development of novel therapeutic strategies to treating this disease.

Purpose

While meibography has proven useful in identifying structural changes in the meibomian gland (MG), little is known regarding the MG spectral transmission and absorption characteristics. The purpose of this study was to measure the transmission/absorption spectra of the MG compared to other eyelid tissues.

Methods

Human and rabbit eyelids were fixed in paraformaldehyde, serial sectioned (50 μm) using a cryotome and imaged by brightfield and reflectance microscopy. Eyelid regions (MG, muscle, tarsus and dermis) were then illuminated by a 100 μm spot using a infrared enhanced white light source. Transmission spectra over a 550-950 nm range were then measured using a spectrometer and differences compared using two-way analysis of variance.

Results

Brightfield microscopy of both human and rabbit eyelid tissue showed a marked decrease in light transmission for MG acini compared to other eyelid tissues. In rabbit, the dermis showed 5× and the muscle showed 2× more light transmission compared to MG (P < .001 and P < .001, respectively). For human, the muscle showed 14× and the tarsus showed 84× more light transmission compared to MG (P < .01 and P < .001, respectively). No specific spectral region of light absorption could be detected in either rabbit or human MG. Loss of light transmission in MG was localized to acini containing small lipid droplets, averaging 2.7 ± 0.8 μm in diameter.

Conclusions

The data suggest that light transmission is dramatically reduced in the acini due to light scattering by small lipid droplets, suggesting that Meibography detects active lipid synthesis in differentiating meibocytes.