Although all human hair has the same basic structure, it varies considerably in size, shape, and density, depending in its location and stage of development. Hair shafts are mainly composed of fibrous alpha-keratin proteins. Two types of hair shafts are recognized:
- Vellus hairs are soft, hypopigmented, unmedullated, almost invisible hair seen on the forehead. These hairs are less than 0.03 mm in diameter and less than 1cm in length. Vellus hairs spread over the body surface and are difficult to see without appropriate magnification.
2. Terminal hairs are longer, coarser, and of variable pigment. They characterize the adult years and exceed 0.06 mm in diameter and 1 cm in length. Subtypes of terminal hair are found on the scalp, eyebrows, upper lip, chin, axillae, chest, and pubis.