The outmost layer of the epidermis, the stratum corneum, is the
predominant barrier layer. The stratum corneum is described as
the Brick and Mortar model [9], with the bricks being annucleated,
keratin-rich corneocytes and the mortar the extracellular, lipidenriched
matrix organized into lipid bilayers [10]. IHL is bound to
the cuticle cell or cortical cell surface to make an environmentally
resistant lipid envelope, as stratum corneum lipid. The IHL in hair is the only continuous structure that among the total components of
hair [11]. Lipid plays two important roles in the body: one is a
structural function and the other is an energy storage function [12].
The major structural function of lipid is to form cell membranes
[13]. Lipid in most of the keratinized layers are mainly composed of
ceramide, cholesterol, and free fatty acid unlike the normal
membranes [14]. As normal membranes include phospholipid,
sphingomyelin, and cholesterol and have high permeability to
water-soluble materials, they are not functioning as a barrier. On
the contrary, the keratinized layer plays an important role as a
barrier because of its linear lipid configuration.