Hair
Anatomy of the Hair
What is hair?
Hair is one of the three accessory organs, or appendages, of the skin. Hair is composed mostly of dead, keratinized cells and hard keratin-unlike the skin, which is made up of soft keratin. A hair is also known as a pilus, or in plural form, pili. A hair is a slender fiber that grows from a hair follicle, which is an oblique tube in the skin. Hair can be found almost everywhere on the body. The only places that are exempt from hair are the palms, soles, palmar surfaces, plantar surfaces, lateral surfaces, certain segments of the fingers and toes, lips, nipples, and some parts of the genitals.
There are three types of hair. |
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Types of Hair
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1. Lanugo: Fine, downy, unpigmented hair that appears on the fetus in the last three months of development.
![Picture](/uploads/2/9/2/7/29270985/204901209.jpg?285)
2. Vellus: Similarly fine, pale hair that replaces lanugo hair by birth. Constitutes about:
- 2/3 of the hair of women.
- 1/10 of the hair of men.
- All of the hair of children, except for eyebrows, eyelashes, and scalp.
![Picture](/uploads/2/9/2/7/29270985/436962885.jpg?295)
3. Terminal: Longer, coarser, and usually more heavily pigmented.
- Forms eyebrows and eyelashes and covers the scalp.
- Forms axillary and public hair, male facial hair, and some trunk/limb hair.
Structure of the Hair
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1. Bulb: A swelling at the base where the hair originates in the dermis or the hypodermis.
- The bulb grows around the dermal papilla.
- The dermal papilla provides the hair with its only source of nutrition.
![Picture](/uploads/2/9/2/7/29270985/391246330.jpg?307)
3. Shaft: Portion above the skin surface.
Structure of the Hair Follicle
The follicle is a diagonal tube that dips deeply into the dermis and sometimes extends into the hypodermis. It has two layers.
Nerve and muscle fibers are associated with the hair follicle. Nerve follicles called hair receptors interweave with each follicle and respond to hair movements. Each hair also has a pilierector muscle, which is a bundle of smooth muscle cells. These cells extend from dermal collagen fibers and go all the way to the connective tissue root sheath of the hair follicle. Pilierector muscles pull the follicles into a vertical position and cause "goose bumps," but other than that, they don't serve a real purpose.
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Video
This video covers a topic that wasn't covered in this anatomy page: hair formation.