Hair growth occurs in a cyclic pattern that keeps repeating itself until follicles stop producing hair altogether, resulting in baldness. Hair actively grows for two to eight years, followed by brief periods of transition and rest, according to "Milady's Standard Textbook of Cosmetology." Hair on the arms and legs remains short because it has growth periods of only a few months. People who can grow extremely long head hair have longer growth periods in that region than others.
Building Blocks
Hair is made up of a dead, hard protein called keratin. Keratinocytes, living cells of keratin, divide and die at the follicular level, resulting in hair growth, according to Hair Biology. Energy needed for this activity is supplied via blood-rich structures located deep within hair follicles called dermal papilla.
Follicles
The follicle, a tiny sheath-like structure located in the skin, makes hair growth possible. Around the dermal papilla, located at the very bottom of the follicle, keratinocytes divide and die, creating the hair shaft. The shaft remains attached to the papilla while it is actively growing, even though it does not receive blood through the connection. The follicle also houses a sebaceous gland, an arrector pili muscle, an inner and outer root sheath, sweat glands and nerves.
Hair-shaft Structures
The hair shaft consists of two to three layers. The outside cuticle, or protective, layer of the hair consists of flat keratin cells that overlap to create a tight, protective barrier. A healthy cuticle layer appears shiny, sleek and smooth. The cortex, or middle layer of the hair, houses all of the main structures of the hair that lend it strength, shape and color. Chemical treatments such as perms, color and relaxers alter this layer to make the hair look different. Not everyone posseses three layers of hair. The medulla, or inner layer, serves no purpose and is missing from hair of fine and medium diameters, according to the keratin.com website.
Growth Cycles
Hair growth occurs in phases of active growth, change and rest. Active growth occurs during the anagen phase. During this phase, cells reproduce, divide and die at the base of the hair follicle, giving rise, or growth, to the hair shaft of approximately 1/2 inch per month, according to "Milady's Standard Textbook of Cosmetology." The catagen phase involves change. Structures located in the follicle die, and the follicle becomes more superficially rooted in the skin. At this point, the hair detaches from the papilla and remains in place due to the bulb-like shape of the hair root. The telogen phase is one of rest in which the hair is shed and the follicle lies dormant.
Repair
Since hair is essentially dead, it does not repair or regenerate itself once it has sprouted from the head. External repair options include hair-care products that fill in damaged portions, smooth the cuticle and add shine and manageability. Protein-based products and conditioners penetrate the hair shaft and lend strength and suppleness to cortex structures that have been undermined by damage. Once damage occurs, however, it never truly goes away. A haircut is the only way to completely eliminate damaged hair.
References
- Teens Health: Hair Removal
- Hair Biology: Hair Follicle Anatomy
- Keratin.com: Hair Follicle Anatomy
- "Milady's Standard Textbook of Cosmetology"; Milady, Diane Carol Bailey and Margrit Attenburg; 2008
Member Comments