Diseases on hair follicles leading to hair loss part I: nonscarring alopecias.

Hair follicles are unique structures with exceptional regenerative potential. They are believed to be crucial for epidermal homeostasis and reepithelialization after damage to human skin. Like other, more active and quickly proliferating organ systems, hair follicles may be easily disturbed in their normal growth cycle by systemic and local influences, including specific skin diseases. This may lead to hair loss, a very common complaint in men and women. The difficulties in reviewing the diseases of hair follicles lay in the long list of different etiologic factors (infectious, autoimmune, inflammatory, neoplastic, physical, chemical, congenital) and a still missing classification system according to etiopathogenetic principles. In this article (Part I of II), the structure and function of hair follicles, the diagnostic approach to diseases causing hair loss, and the most common nonscarring alopecias (telogen effluvium, anagen effluvium, androgenetic alopecia, and alopecia areata) are reviewed in regard to pathogenesis, clinical findings, and current options of treatment. Part II will focus on scarring alopecias.