The hair follicle can be divided into three parts:
The inferior segment includes the deepest part of the hair follicle and extends to the bulge (the part just below the arrector pili muscle attachment). It contains the hair bulb or matrix with its ger minative matrix cells and melanocytes. The hair bulb surrounds a highly vascularised connective tissue called the dermal papilla. The cells in the matrix divide and form the hair. The cells divide
rapidly – about every 23 to 72 hours. This is one of the fastest division rates in the body. The isthmus extends from the bulge to the opening of the sebaceous duct.
The infundibulum extends from the sebaceous duct opening to the epidermal surface. Note: as the hair follicle is angled the bulge lies on the deeper aspect of the follicle.
The hair follicle is composed of the external and internal root sheaths and the hair shaft. When looking at a vertical section of the hair follicle it will display an external sheath, an internal sheath and a hair shaft.
The external sheath is formed by the basal and spinosum layers of the epidermis creating a downward structure. Towards the top of sheath, all the epidermal layers are present, whereas towards the bottom of the sheath, only the stratum basal is present. At the level of the infundibu-lum this external sheath is continuous with the epidermis. This layer never totally regresses and gives rise to the new hair in the next growth cycle. It contains melanocytes, Langerhan cells, mast cells, Merkel cells and neuronal stem cells. All these cells operate within the hair follicle and form a reservoir from which the epidermis regenerates after injury.
The internal sheath provides rigid support for the growing hair shaft. It tapers off above the bulge and does not grow beyond this level. The growing hair is shaped and curved by the internal sheath – this is the part that determines the shape of the hair.
The hair shaft is the part of the hair that we can see growing above the skin and which reaches down into the hair sheath. There are three main parts to the terminal hair shaft – the medulla, the cortex and the cuticle.
The medulla is the innermost part of the hair shaft when looking at a cross section. It is made up of rows of polyhedral cells. These contain eleiden (an early form of keratin) and air spaces. You will remember that eleiden is a component of the stratum lucidum in the epidermis. Eleiden is translu cent. The medulla may be absent in fine hair.
The cortex is the middle and major part of the hair. The cells in the cortex are elongated cells con taining melanin in dark hair. In white hair these cells contain air. This layer gives hair its colour.
The cuticle is the outermost layer of the hair and contains a single layer of thin flat cells which are keratinised. The arrangement of the cells is like that of shingles or fish scales with the edge of the cell pointing up. This layer protects the hair. The innermost layer of the internal sheath is also shingled and this helps keep the cuticle in place.
Try holding a single hair and moving fingers away from the scalp and then towards the scalp – you will notice a difference.