When it comes to human appearance, few things are as recognizable and as distinctive as one’s hair. From its length and color to its texture and shape, a person’s hair can be an important reflection of their character and personality. Hair can serve several purposes for animals as well, from thermal regulation and camouflage to signaling and warnings.
Hair is a modified protein called keratin. It is a tough fiber that extends from the hair follicle beneath the skin to the epidermis above, where it sticks out. Hair grows in a cycle, with the stem cells in the root dividing rapidly to produce new cells that eventually build up the hair shaft. When the hair reaches the epidermis, it begins to separate from the follicle, entering the transitional phase of growth known as the catagen phase. Once the hair is completely separated from the follicle, it enters the final resting phase called the telogen phase. This phase lasts up to three months before the hair is pushed out of the scalp and falls out. Then, as soon as the empty follicle is exposed to light and the surrounding skin, the cell cycle in that hair follicle starts again, producing a new hair.
A hair’s color is a result of pigmentation in the follicle cells, which are responsible for producing specialized cells that contain pigment. The pigment is deposited into the keratin and arranged in a specific way to create the hair’s color. The keratin in the hair also protects the underlying epidermis from damage and stress.
The structure of hair is very complex, but it can be divided into three distinct parts: the cuticle, cortex, and medulla. The outer layer of the hair is the hydrophobic lipid epicuticle, which consists of a layer of flattened, overlapping cuticle cells that cover an elongated polyhedral cortical core. The healthy, smooth cuticle reflects light and minimises friction between the core and the strand, giving it its shiny appearance. It also allows for the flow of moisture in and out of the cortex, ensuring that your hair remains hydrated and flexible.
Shed strands of fine hair are often translucent or even transparent when held up to light, and they feel like silk threads. Medium hair feels thick and is easily identifiable by its sheen, and coarse strands are dense and hard to the touch.
The hair follicles that produce straight hair are roughly circular in cross-section, while those that create curly hair are more curved. When a person has curly hair, their strands are less prone to breakage because they are more tightly wound. In addition, the sheen in their hair is typically lighter than that of straight hair because the strands are coiled more tightly, which makes them more reflective of light.