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ANATOMY OF THE HAIR FOLLICLE

Embryologically speaking hair derives from the epithelium (external layer) and from the mesenchyma (mid layer of the embryo).

Epithelial component: hair matrix. Keratinoblasts in the hair matrix will give rise to the hair shaft which is composed of three parts going from the inside to the outside: medulla, cortex and cuticle. The matrix also contains melanocytes from which hair pigment will develop. This structure forms the internal epithelial sheath.

Mesenchymal component: the mesenchymal component contains the dermic papilla together with its vessels and nerves and the conjunctive sheath surrounding the follicle.

The Hair Follicle Can Be Divided Into Different Structures:

Hair bulb: the hair bulb is a, onion-shaped structure formed by the matrix and the dermal papilla. It is located in the lower portion of the hair follicle (in dermis-hypodermis).

Matrix: the matrix contains cells with large nuclei with a great number of organelles and with a very high replacement rate. These cells go upwards and are different to cells in the medulla, cortex and cuticle. In other words, the matrix gives rise to the hair shaft.

Dermal papilla: it is a sail-shaped structure composed of round-shaped fiberblasts, macrophages, mastocytes and endothelial cells.

Hair shaft: the hair shaft contains the actual hair and is composed of three parts:

Medulla: the medulla gives rise to the upper cells of the dermal papilla and will form the central portion of the hair shaft. This structure does not exist in very fine hair or lanugo.

Cortex: as cells from the matrix ascend towards the follicle they become elongated and keratinized. As the amount of keratin increases, the number of organelles decreases and the nucleus disappears.

Cuticle: this structure is formed by 6 to 10 layers of keratinized, thin cells that are arranged as fish scales. These cells in the cuticle give support to the follicle anchoring it to the external epithelial sheath.

Internal epithelial sheath: this structure also contains three layers.

Internal epithelial sheath cuticle: this cuticle only contains one layer of keratinized cells.

Huxley’s layer and Henle’s layer: these layers appear to be the shaft mold and where the keratinisation process starts.

External epithelial sheath: the external epithelial sheath is the continuation of the epidermis as it invaginates and surrounds the follicle.

Protuberance: the protuberance is located in the upper third of the follicle at the level of insertion into the erector muscle. The protuberance is made of follicular pluripotent cells that give rise to a new follicle after the catagen stage. It appears that these cells need to be activated by the cells in the papilla during the late telogen phase. This explains the upward movement of the papilla which permits both structures to come into contact.

There Are Also Two Other Important Structures:

MELANOCYTES: melanocytes are responsible for hair colour as they transfer melanin granules through their dendrites (prolongations) into the cells in the cortex and medulla.
Melanocytes in the bulb transfer melanin granules to the cells in the cortex and medulla.

There are two types of melanin:

EUMELANIN: large and elliptic melanosomes in brown and black hair.

PHEOMELANIN: small and sferic melanosomes in blond and red hair.

White hair has no melanin.

SEBACEOUS GLAND: this gland is present in all hair follicles. However, its degree of development is inversely proportional to hair size.