Novel insights into hair structure and the effects of chemical stressors on hair and skin using label-free advanced light microscopy

Abstract

There is a need for a better method to image hair as the current methods involve embedding the hair in resin, which may produce artefacts, or using dyes which are limited in their depth of penetration into the hair. The research performed in this thesis endeavours to characterise the cellular structure of human hair with label-free imaging using autofluorescence and fluorescence lifetime imaging. Wavelengths were shown to selectively excite the hair cuticle, cortex and medulla, and subcellular compartments. Development of an optical transverse imaging method enabled discoveries including different fluorescence lifetimes across the cuticle cell layers and suggests the cuticle layers possess differing chemical environments. A new method was developed to distinguish between eumelanin and pheomelanin using 405nm and 633nm wavelengths. The newly developed methods were additionally used in the characterisation of an unidentified hair and skin disorder, which found poorly differentiated cuticle cells and showed differences in the fluorescence lifetimes of the hair compared to control hairs.

The hair care industry needs more efficacious chemical depilatories and information into their action. This was elucidated using the developed methods and a new dynamic imaging method. Potassium thioglycolate was shown to cause drastic expansion of the hair which was amplified by the addition of guanidine carbonate, creating fissures through the cuticle and into the cortex. Other experimental depilatory formulations were tested and were found to have varying effects upon the structure of the hair.

New chemical depilatories require development because existing depilatories can cause irritation in the skin. Potassium thioglycolate and guanidine carbonate were tested on HaCaT cells, isolated cornified envelopes, and HEKn cells in a 3D epidermal model. An investigation into the differentiation, proliferation and acute stress response of the cells showed that the treatments had no significant effect on these markers. However, the chemicals negatively affected HaCaT cell viability and damaged the cornified envelopes. Despite this, the viability and structural integrity of the living cells of the epidermal model were maintained through the protection provided by the stratum corneum.