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Home > Hair Physiology > Hair Follicle Structure
A hair follicle is a long tube structure which penetrates deep into the skin of the scalp. The follicle is the whole structure around a hair fiber from bulb deep in the skin to the opening to the skin surface. The bulb of the hair follicle is the point from which the hair grows .There are about 100,000 hairs follicles present in an adult human scalp. The hair follicle is a separate entity within the skin,and the structure and functions of the hair follicle are based on the interaction between dermal and epidermal components.
The follicular structure of the hair can be divided into three main parts:
Dermal Papilla in the hair follicle structure:
The Dermal papilla (DP) is in the form of a "pear" shape in normal and healthy hair follicles. The DP comprises a ball of cells which, during the anagen phase of the hair growth cycle, signals to the epielial component of the hair follicle to produce a keratinized hair fiber and other associated products. The dermal papilla is made up of a small group of fibroblast cells which are derived from the mesoderm. The cells are held close to the base of the epidermal cells which are responsible for the production of the hair fiber and root sheaths. So the dermal papilla itself does not make a hair fiber, but it directs the rest of the hair follicle to make one.
Though both the cell types, dermal and epithelial, are held close together they are not mixed up as there is a thin layer, called the basement membrane between the cells of dermal papilla and the epidermal cells.
The dermal papilla directs the embryonic generation of a hair follicle and it continues with this instructive function throughout the life of the hair follicle.
The thickness of the hair fiber depends on the size of dermal papilla. If the dermal papilla is bigger with the accumulation of large number of cells then the hair fiber is thicker. The DP cells are active and contain a large amount of cytoplasm when the hair fiber is produced.
But the DP cells do not multiply or increase like that of the hair producing cells that are situated above DP. When a hair follicle is not producing a fiber the DP cells lose much of their cytoplasm and become inactive.
Formation of hair fiber
The hair follicle cycle starts again when a new hair follicle is activated and grows to form a new fiber. Apart the dermal papilla the cells, matrix cells also play major role in the formation of the hair fiber. These matrix cells are situated in the epidermis just above the dermal papilla. The matrix cells are thought to be derived from the root sheath cells.
The matrix cells that locate at the center of the hair follicle become part of hair fiber and they are called cortex (cortical) cells. These cortical cells keep multiplying continuously and the new cells push the old ones upwards to proceed towards the scalp surface.
As the cortical cells move upwards there is a difference in the appearance of the cells. The cortex cells change from a round shape into a flattened appearance. They are squeezed together into layers (lamella). If the hair follicle contains melanocyte cells then melanin pigment is incorporated into the cortex cells. Then these cortex cells become keratinized and harden which stops the normal functioning of the cells and they die. The cortex cells are now part of the dead keratinized hair fiber. Read the structure of hair fiber or shaft for more details.
Hair follicle structure-the root sheaths
In the hair follicle structure, the root sheaths are the surrounding layers of the hair fiber. The main function of the root sheaths is to protect the hair fiber as it develops. Root sheaths are of two types:
Outer root sheath
The function of the Outer Root Sheath (ORS) is to cover the hair fiber and inner root sheath until deep into the dermis but not covering the bulb region containing the dermal papilla. The stem cells of the hair follicle sit in the outer root sheath bulge region and these stem cells make sure the hair follicle is supplied with enough cells to keep growing throughout life and multiple hair cycles.
Inner root sheath
The cells of the inner root sheath are differentiated and keratinized as they are pushed away from the bulb region. As keratinization occurs, the cells die and form the IRS. Its function is to layer and protect the development of the hair fiber. It also helps to anchor the hair fiber in the hair follicle so that it cannot be pulled out easily. The IRS “grips” the hair cuticle to hold the hair fiber as it grows.
Role of sebaceous glands in Inner root sheaths
The sebaceous glands present as an outgrowth of the Outer Root Sheath (ORS). The oils of the sebaceous gland are believed to help break down the Inner Root Sheaths (IRS). The IRS disintegrates at the level of the sebaceous duct opening. As the IRS break down the hair fiber comes out of the skin of the scalp.
Sebaceous glands produce a substance called sebum (a mixture of waxes and oils). The main role of sebum is to waterproof the hair and provide it with its natural sheen. But an excess of sebum is undesirable since it produces oily skin and acne. It is important to keep the scalp clean with good hair care products. Lack of sebum, which is common in middle and older age, leads to hair dryness and may make it brittle.
Collectively, all the parts of the hair follicle structure play an important role in hair growth and hair loss.
Hair is an issue people are quite sensitive about; it is important to take good care of your hair to maintain them in the healthy state.