Hair Loss in Children
James E. Rasmussen MD1
1 Associate Professor, Departments of Dermatology and Pediatrics, University of Michigan Medical School, Ann Arbor
PHYSIOLOGY OF NORMAL HAIR1,2
Unlike the hair of most animals, human hair usually follows a random pattern of growth, rest, and shedding, followed by the growth and emergence of a new hair from the follicle. This cycle is related to histologic changes in the follicle: The anagen phase involves active growth (long follicle); the telogen phase is resting (short follicle); and the catagen phase is a transition between anagen and telogen.
The duration of the growing phase (anagen) determines the ultimate length of the hair: normally longest on the scalp, followed in descending order of length by the hairs of the beard, pubis, axillae, body, eyebrows, and eyelid margins. Longer hair is associated with a higher ratio (10:1) of anagen to telogen follicles. Hair plucking is the most rapid, convenient path to examine this relationship: anagen hairs have a glistening, cylindrical proximal sheath, approximately 3 mm long (Fig 1) whereas the telogen follicle yields a hair with a short, 1-mm knob, appropriately called a club (Fig 2).
The shape and color of hair are usually determined genetically. Straight hair is round in cross section, with curly and kinky (spiral) varieties becoming progressively more ellipsoid. Hair color may normally darken and shape change for the first five to ten years of life; this tendency is most commonly seen in the transformation of a curly haired blond child into a tow-headed or brunette adolescent.
