Known and Unknown Features of Hair Cuticle Structure: A Brief Review

Abstract

The cuticle is the outermost layer of overlapping flattened cells of hair and has been subjected to many years of study to understand its structure and how it develops in the follicle. The essential function of the cuticle with its tough inelastic protein content is to protect the inner cortex that provides the elastic properties of hair. Progress in our knowledge of hair came from studies with the electron microscope, initially transmission electron microscopy (TEM) for internal structure and later the scanning electron microscope (SEM) for cuticle surface shape and for investigating changes caused by various environmental influences such as cosmetic treatments and industrial processing of wool. Other physical techniques have been successfully applied in conjunction with proteomics. The outstanding internal features of the cuticle cells are the internal layers consisting of keratin filament proteins and the keratin-associated proteins. The stability and physical toughness of the cuticle cell is partly accounted for by the high content of disulphide crosslinking. The material between the cells that holds them tightly together, the cell membrane complex, consists of a layer of lipid on both sides of a central protein layer. The lipid contains 18-methyleicosanoic acid that is part of the hydrophobic lipid surface of hair. For the past decade there have been aspects that remained unanswered because they are difficult to study. Some of these are discussed in this brief review with suggestions for experimental approaches to shed more light.