The Anatomy of Hair

The structure and growth cycle of hair is a fascinating process and in this post we explore the key stage of hair development.

Hair inside the skin is generated from the hair follicle or root bulb. Hairs vary considerably around the body, and in some areas such as the eyelids they do not project beyond the follicles containing them.

On the scalp the profile or cross-section of individual hair-shafts results in different hair types. For example, a round cross-section usually results in straight hair while an oval or flattened cross-section usually results in wavy or curly hair. It is the shape, size and curve of the follicle that determines the profile of each hair.

The follicle is an extension of the epidermis into the dermis layer of the skin. When broken down it consists of an outer dermic layer and an inner epidermic layer. The base of the hair follicle is moulded onto the papilla from which it grows. Some cells that occur at this level contain the pigment granules, which give hair its natural color, and these are called melanocytes. This area of cell production is often called the hair matrix.

1. Slightly above, toward the top of the root bulb, is an area known as the keratinization zone. Here, keratin is ‘injected’ into the cells of the hair, ultimately making them hard and dry.

2. The outer dermic coat is broken down further into three separate layers; the most external of these is a thick layer of connective tissue in which blood vessels and nerve endings are found. The middle layer consists of a compact layer of spindle-shaped cells; finally the inner layer is almost membrane-like and is glassy in appearance.

3. The papilla is a small cone-shaped elevation at the base of the hair follicle. Attached to the papilla are arteries and veins, which deliver blood and nutrients and remove waste products. The hair bulb fits around the papilla.

The epidermic layer surrounds the hair itself and is called the root sheath. This is divided into an outer sheath, and an inner sheath that sits next to the delicate hair cuticle. The cells of the inner sheath cuticle are layered downwards, fitting closely over the upward layered scales of the hair cuticle. The inner root sheath comprises two layers: Huxley’s layer made up of horn shaped, flat, nucleated cells and Henle’s layer, made up of oblong cells with no visible nuclei.

The hair itself has a central core called the medulla. Within the skin layer this is liquid in form and frequently contains air bubbles. Surrounding the medulla is the cortex, elongated cells which form the main fibrous structure and strength of the hair. Pigment granules can be found in small gaps between these cells. As the cortex cells move outwards toward the external layer of hair, they become thin and scale-like, forming the cuticle of the hair strand.