The structure of a hair

The hair is composed mainly of the protein keratin and therefore is a dead structure. Longitudinally, the hair is divided into three parts:

• hair shaft — the part of the hair lying above the surface of the skin

• hair root — the part found below the surface of the skin

• hair bulb — the enlarged part at the base of the hair root.

Internally, the hair has three layers which all develop from the matrix (the active, growing part of the hair).

Hair layer

Location

Description

Function

Cuticle

Outer layer

Made up of transparent protective scales which overlap one another

Protects the cortex and gives the hair its elasticity

Cortex

Middle layer

Made up of tightly packed keratinised cells containing the pigment melanin, which gives the hair its colour

Helps to give strength to the hair

Medulla

Inner layer

Made up of loosely connected keratinised cells and tiny air spaces

Determines the sheen and colour of hair due to the reflection of light through the air spaces

Hair colour is due to the presence of melanin in the cortex and medulla of the hair shaft. In addition to the standard black colour, the melanocytes in the hair bulb produce two colour variations of melanin: brown and yellow. Blond, light-coloured and red hair has a high proportion of the yellow variant. Brown and black hair possesses more of the brown and black melanin.

The structure of a hair in its follicle

Fig 2.4 A hair in its follicle

The individual parts of a hair's structure are shown below.

Structure

Location

Function

Connective tissue sheath

Surrounds hair follicle and sebaceous gland

Supplies follicle with nerves and blood Main source of sustenance for the follicle

Outer root sheath

Forms the follicle wall and is continuous with the basal cell layer of the epidermis

Provides a permanent source of growing cells (hair germ cells) to enable the follicle to grow and renew cells during its life cycle

Dermal papilla

Elevation at the base of the hair bulb, which contains a rich blood supply

Crucial source of nourishment for hair, providing the hair cells with food and oxygen

(Continued )

Structure

Location

Function

Inner root sheath

Originates from the dermal papilla at the base of the follicle and grows upwards with the hair (it ceases to grow when level with the sebaceous gland)

Shapes and contours the hair, helping to anchor it into the follicle

Hair bulb

Enlarged part at the base of the hair root

Area where the cells grow and divide by the process of mitosis

Matrix

Lower part of the hair bulb

Area of mitotic activity of the hair cells

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