Polarity-driven distribution of the non-aqueous tear components during microdesiccation on flat glass surfaces

Abstract

Spontaneous desiccation of microvolumes of tear from a healthy subject on flat glass surfaces produces highly reproducible supramolecular structures called tear microdesiccate. In the process, non-aqueous tear components organize four main radially distributed morphological domains (zones I, II, III, and transition band or TB) through non-well understood mechanisms. Tear lipids have been located in the outermost domains (ZI, TB), thus suggesting that tear components in microdesiccate are partitioned according to electric polarities. Using a combination of dark-field and contrast phase transmitted-light microscopy, we have now studied the impact on preformed tear microdesiccate of a series of six small-sized monohydric alcohols (methanol, ethanol, n-propanol, n-butanol, n-pentanol, 2-propanol; C1 through I-C3) ranked according to polarity index (P.I. 0.762–0.546). Water (P.I. 1.000) served as control. The study aimed to assess the influence of the electric polarity of the alcohols on the organization of the microdesiccate domains.The study revealed self-assembly as a mechanism underlying formation of the entire microdesiccate as shown by the fully disruptive effect of water on preformed tear microdesiccate and their reorganization following a new desiccation cycle. Devastating effects of the most polar alcohol in the assay (methanol) were observed particularly on the centermost domain (zone III), and secondarily on zone II. Contrarily, the less polar alcohols (C2 to C5) showed both fixative effects on crystalloids of ZII and ZIII and differential unveiling of filamentous structures in ZI and TB probably due to selective extraction of lipid-like components from these latter two domains. Those effects on microdesiccate from different subjects were highly reproducible, thus supporting the view that these tear specimens bear a common design.Microdesiccates from normal tears produced on flat glass surfaces are self-assembled structures based on a design including a radial gradient of charge polarities spanning from the lipid-rich outermost hydrophobic domain (ZI) to the centermost hydrophilic fern-like crystalloid-rich domain (ZIII).