ANIMESH SINHA, MD, PhD: Human hair is not absolutely vital for survival, but it does play several important functions. Perhaps the most important function is its protective role in protecting us from heat loss, so it's a heat insulator. As well, human hair is generally protective to the skin to minor abrasions or cuts and against ultraviolet radiation from the sun, perhaps. Specialized hairs in the eyelashes, the eyebrows are protective to the eyes, and hair in the ear canal or the nasal passages can help filter out particles and pathogens and protect our internal organs.
Additionally, hair is a tactile organ. There are many free nerve endings associated with hair follicles and they're sensitive to light touch. Finally, hair is a marker for identity. It serves for social communication as well as for sexual attraction, and we know those are important issues.
DAVID FOLK THOMAS: I guess, like you say, personal identification we don't really have that much control, putting aside plastic surgery, over the rest of our bodies, but you can allow your hair to grow and style it and do different things.
ANIMESH SINHA, MD, PhD: Sure. Our hair is a big part of our identity, what we're given naturally, and also the way
we groom it, so these are important functions for daily life.
DAVID FOLK THOMAS: What exactly is hair made out of? What are we talking about up here?
ANIMESH SINHA, MD, PhD: It's broken down, technically, into three basic parts: a lower part called the bulb, where the hair shaft originates from; a middle part called the isthmus, one region of which is probably the source of the stem cells or regenerative cells for the hair follicle; and the top part is the infundibulum, where the hair shaft exits the hair follicle through to the surface externally.
DAVID FOLK THOMAS: You talked about the phases of the hair cycle. What are those?
