Anatomy of Hair
Hair is a filamentous, usually pigmented outgrowth from the skin found only on mammals. Hair can be found on all areas of the skin except the lips, fingertips, palms, soles, glans penis and labia minora). These thin strands grown from hair follicles in the deeper portion of the skin (dermis) and protrude the outer skin layer (epidermis). Each fiber is composed of non-living cells primarily made of the protein keratin.
Types of Hair
Humans have three different types of hair:
- Lanugo, very thin, soft usually unpigmented hair that covers the unborn or newborn human.
- Vellus hair, the short, fine, “peach fuzz” body hair that grows in most places on the human body in both sexes. It covers most of the body during childhood
- Terminal hair, longer, coarser, thicker and darker fully developed hairs. The conversion from vellus to terminal is stimulated by androgenic hormones during puberty
Scalp Hair
Humans have about 5 million hair follicles over their body with approximately 100,000 on their scalp. (approximately 130 scalp hair follicles per cm2). This equates to about 300 – 500 hair shafts per cm2. Each follicle can grow about 20 individual hairs over one’s lifetime.1 The average daily hair loss is about 100 strands.
Hair density is related to both race and hair color. Caucasians have the highest hair density, with an average growth rate, while Asians have the lowest density but fastest growing hair, and Africans have medium density with the slowest growing hair. Hair density and hair thickness varies by natural hair color (see Table).
1About. 2007. About: Hair loss. About.com, a part of The New York Times company. Retrieved March 2, 2007
Scalp Hair Characteristics by Color
|
Hair Color |
Number of Follicles |
Average Hair Shaft Diameter |
|
Blonde |
146,000 |
17-51 µ |
|
Black |
110,000 |
64 – 100 µ |
|
Brown |
100,000 |
38 – 50 µ |
|
Red |
86,000 |
38 – 50 µ |
Adopted from Stevens, C. 2007. Hair: An introduction. The Trichological Society. Retrieved March 2, 2007. View Source
Structure of Hair
A hair follicle is a cavity in the skin which contains the root of a hair (see photo). At the base of the follicle is the papilla, which is made of connective tissue and small blood vessels. Around the papilla is the hair matrix which are a collection of epithelial cells and interspersed melanocytes. Cell division within the matrix results in formation of the hair fiber. The hair matrix epithelium is one of the fastest growing cell populations in the body. Therefore, some forms of chemotherapy and radiation therapy which kill rapidly dividing cells can lead to temporary baldness.
Attached to the hair follicle is the sebaceous gland which produces oil (sebum) alongside the hair shaft. Sebaceous glands are not found on the palms, lips and soles of feet. The arrector pili is a tiny muscle attached to the hair follicle. When it contracts, the hair becomes more perpendicular commonly known as “goose bumps.”
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