Hair Follicle

1.

Hair shaft

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Hair shaft is the visible part of the hair that protrudes through the skin. It consists of layers of of fused, dead, keratinized cells.

2.

Medulla

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The innermost layer of a hair is called the medulla.
A medulla is not present in all hairs, only in large thick ones e.g. protruding from the scalp rather than from the abdomen or upper-arms (where hairs tend to be lighter and less dense).

3.

Cortex

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The middle layer of hair is the cortex.
This provides strength and determines both the colour and the texture of the hair.

4.

Cuticle

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The outermost layer of hair is the cuticle.
This is a thin colourless layer that protects the cortex.

5.

Hair root

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The hair root is the part of the hair lying below the surface of the outer-layer of skin (i.e. the epidermis layer). The hair root therefore includes many of the main structures of the hair, incl. the bulb, papilla and germinal matrix.

6.

Dermal root sheath

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The dermal root sheath, which is sometimes referred to as simply the "root sheath" is composed of two layers of epidermal cells called:

• the external epithelial root sheath (see below), and
• the internal epithelial root sheath (see below)

These two layers are surrounded by an outer sheath of connective tissue.

7.

Arrector pili muscle

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The arrector pili muscles associated with hair follicles consist of smooth muscle (as these muscles are under unconscious, rather than conscious control). The arrector pili muscle associated with each hair follicle extends from the side of the hair follicle to the outermost (i.e. towards the surface of the skin, or upper - as shown above) part of the dermis layer of the skin.

When at rest the arrector pili muscle is extended and the hair shaft emerges from the skin at a shallow angle. However, when the body is under some form of stress, e.g. due to fear or low temperatures, the nerve endings (of the autonomic nervous system) stimulate the arrector pili muscles to contract, which in turn pulls the hair follicles so that the hair shaft emerges from the surface of the skin at closer to 90^o, i.e. perpendicular to the surface of the skin. This also results in slight elevations of skin where each hair shaft emerges - as is colloquially referred to as "goose pimples" (British English) or "goose bumps" (American English).

8.

Sebaceous gland

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Sebaceous glands are accessory structures of the skin. Most sebaceous glands are connected to hair follicles. They secret an oily substance called sebum, whose functions include:

• preventing the hair from becoming too dry
• preventing surrounding skin from becoming too dry (due to evaporation of water from the skin), so helping to keep the skin soft
• inhibiting the growth and reproduction of some bacteria

9.

Hair bulb

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The base of the hair follicle is a bulb-shaped structure called the hair bulb. This includes several layers of different types of cells that extend up through the hair follicle, an indentation called the papilla (of the hair) - see below, and a region of cells called the germinal matrix.

10.

External epithelial root sheath

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The external root sheath consists of several layers of cuboid epithelial cells visible when stained with H&E stain (see histology stains for more about this and other stains).

11.

Internal epithelial root sheath

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The internal root sheath consists of three layers: Henle's layer, Huxley's layer, and an internal cuticle that is continuous with the outermost layer of the hair shaft.

12.

Germinal matrix

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The germinal matrix is sometimes referred to as simply the "matrix" when the context of hair follicles leaves no ambiguity. This is the area of cells that produces new hairs (by the mitosis process of cell division) when previous hairs are shed.

13.

Papilla

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The papilla of the hair contains many blood vessels (the diagram above is a simplified representation; in reality there is are many tiny inter-connected vessels not just the one shown). These blood vessels supply nutrients to nourish the growing hair.

14.

Hypodermis layer

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The sub-cutaneous layer (also known as the hypodermis layer) of skin consists of adipose tissue and areolar connective tissue. It's function is to attach the skin to the underlying structures of the body. As shown, this layer of the skin includes many fat cells. It acts as a storage reserve for fat cells and also contains and protects many blood vessels, including arteries and veins that supply hair follicles.

15.

Artery
(blood supply)

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Blood supplied to tissues, from the heart, travels under higher pressure than blood drained from the tissues. Oxygenated blood and other nutrients are supplied to tissues and cells in the body via the arteries within the dermis and hypodermis layers of the skin and the arterioles (tiny branches of arteries that lead to capillaries) leading from them.

16.

Vein
(blood drainage)

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Blood drained from tissues, for return to the heart, travels under lower pressure than blood supplied to the tissues. Deoxygenated blood and waste products (from the reactions of metabolism) are returned to the heart in veins within the dermis and hypodermis layers of the skin and the venules (tiny branches of veins leading from capillaries) leading to them.