Efficacy testing: anti-dandruff

The elongated part of the hair that extends above the surface of the skin is called the hair shaft. It grows from an elongated tube-like structure in the skin called thehair follicle.

A hair follicle is lined by epithelial cells and is often associated with one or moresebaceous glands, which secrete sebum into the follicle, and an arrector pili muscle, whose contraction can cause the hair to stand upright. At the base of the follicle is the dermal papilla, a connective tissue structure containing blood vessels and signaling cells that help nourish and regulate the growing hair follicle.

The main structural component of hair is the protein keratin. The hair shaft consists of many thin keratin fibres arranged into thicker bundles. It is generally made up of three layers. The innermost region is the medulla, which may be absent or discontinuous along the length of the hair. Its presence can influence some optical properties of hair, although hair colour is mainly determined by melanin in the cortex. The middle and largest layer is the cortex, which provides much of the hair's strength, elasticity, and colour. It is composed of keratinized cells that were formed in the hair bulb and became hardened and dead as they moved upward during hair growth. The outermost layer is the cuticle, a thin protective layer that surrounds the hair shaft and helps protect the cortex. The cuticle surface is covered by a hydrophobic lipid layer that includes 18-methyleicosanoic acid (18-MEA).