Splitting Hairs
Pluck a single strand of hair from your head and you've lost what scientists call the hair shaft. The shaft is made of three layers, each inside the other. The outer casing is the cuticle. Under an electron microscope, the cuticle reveals itself as a series of overlapping layers, something like shingles on a roof. Inside the cuticle lies the cortex, a column of cells containing keratin, the same protein that hardens tooth enamel and fingernails. The central core of the hair is the medulla. Also called the pith, it is made of small, hardened cells snared in a web of fine filaments.
What you left behind when you pulled that hair was the follicle, a tiny pouch below the scalp's surface that manufactures hair. You were born with all the hair follicles you'll ever have. You lose quite a few as you grow older, and some will change what they do, but you will never get any new ones. Furthermore, the follicles get farther apart as you grow. On the average, a newborn baby has about 1,135 hair follicles per square centimeter of scalp area. By the time the baby is an adult, the number is nearly half at 615. An adult male has about 5 million hair follicles on his entire body.
The average scalp contains between 80,000 and 120,000 hair follicles. You probably have the higher number if you're a blond, the lower number if you are a redhead. Brunettes usually fall in the middle at about 100,000. The number of hairs may be deceptive however. Red hair is usually thicker and coarser than blond hair so it appears fuller. Hair is thickest between the ages of 15 and 30. Measured in diameter, the finest hairs vary from 0.017 to 0.050 mm, the coarsest from 0.064 to 0.181 mm.
