Construction of the root and strand

Hair Follicle Structure

Hair is mainly composed of the protein keratin. Keratin is made of rope-like intermediate filaments. The structure of the filaments provides strength to the hair shaft. Like other proteins, keratin is non-polar and is not miscible in polar substances, like water. For this reason, hair is waterproof.

Hair growth begins under the skin at the root. Each individual strand of hair is housed in a hair follicle. Only the “living” portion of the hair is found in the follicle. The hair that is visible is considered dead and is called terminal hair. The base of the root is called the bulb, which contains the matrix and the papilla. These two structures promote hair growth. Other structures of the hair follicle include the sebaceous gland and the erector pili muscles, which, respectively, produce a natural oil, called sebum, so that the shaft does not dry out, and are responsible for causing goose bumps.

Strand of human hair

Each strand of hair is made up of the medulla, cortex, and cuticle. The medulla is the innermost region of the hair strand and reflects light to give hair its various colors and tones. Finer hairs and naturally blond hairs have a fragmented or nonexistent medulla layer. Like the medulla, the cortex, or middle layer of the hair, contributes to hair color. The cortex contains melanin, which predisposes hair color based on the type of melanin that is present. The presence of eumelanin suggests brown or black hair, while pheomelanin suggests red hair. Blond hair is the result of having little pigmentation in the hair strand. The cortex also determines the shape of the hair strand. If the cortex is round, the hair is straight and if it is oval-shaped, the hair is wavy or curly. The cuticle coats and protects the hair strand.

Hair pigment

All natural hair colours are the result of two types of hair pigment. Both these pigments are a type of melanin produced inside the hair follicle:Phaeomelanin is responsible for the yellowish-blond to red colors and Eumelanin is responsible for the brown to black shades. Gray hair occurs when these melanin molecules are no longer produced, so there is no pigment left to colour the hair naturally.

Human hair growth

Human hair grows everywhere on the body except for the palms of the hands, soles of the feet, the lips, and the eyelids, apart from eyelashes. Like skin, hair is an epithelium. Unlike skin, it is stratified, squamous, keratinized epithelium because it is made of multi-layered, flat cells and contains the protein keratin, whose rope-like filaments provide structure and strength to the hair shaft.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.