Functional Morphology of the Lipid Layer of the Tear Film

Meibomian glands secrete the oily layer of the tear film, which prevents excessive evaporation of tear fluid. Dysfunction of meibomian glands is not only one of the causes of evaporative dry eye but also one of the main causes of entire dry eye. To understand the pathophysiology of meibomian gland dysfunction, it is important to evaluate both the morphology and function of the meibomian gland. We previously reported that meibography enabled visualization of the morphology of the meibomian gland. Meanwhile, tear interferometry was introduced as an evaluation method for the function of the meibomian gland. We combined observations of the oily layer and the aqueous layer of the tear film and found that a tear film compensatory system may work toward maintenance of tear film homeostasis. In this review, we describe both morphological evaluation systems for the meibomian gland, including noninvasive meibography, and functional evaluation systems, including tear interferometry. We further describe the morphological changes of the meibomian glands in various ocular surface diseases. Finally, we demonstrate the concept of a tear film compensatory system and propose a method for tear film component-oriented diagnosis.