Hair loss is a topic of enormous public interest and understanding the pathophysiology and treatment of various alopecias will likely make a large impact on patients’ lives. The investigation of alopecias also provides important insight in the basic sciences; for instance, the abundance of stem cell populations and regenerative cycles that characterize a hair follicle render it an excellent model for the study of stem cell biology. This review seeks to provide a concise summary of the major alopecias with regard to presentation and management, and correlate these to recent advances in relevant research on pathogenesis.
ALOPECIAS: AN INTRODUCTION Shakespeare wrote, “There’s many a man has more hair than wit” in the Comedy of Errors. However, in today’s contemporary society, some patients are so troubled by hair loss they might trade wit for more hair if given the opportunity. The study of alopecia is desperately encouraged by society given the importance of hair to most people’s identity. Therefore, a better understanding of the pathogenesis and potential treatments of alopecia will be a welcome advancement. The study of alopecias will also broaden our understanding of the basic biology of the hair follicle, likely the most complicated structure within the skin. Replete with multiple stem cell populations and an intrinsic cycle of regeneration, the hair follicle has become an attractive model within the last 20 yr to study questions about stem cell biology. As in all biological systems, the best clues to critical players are instances in which their perturbation yields functional defects. Thus, human alopecias represent a rich arena for the study of novel control points for hair follicle function. This article will cover the clinical presentations of major alopecias and delve into recent research regarding pathogenesis. Hair itself has few physical functions. These include defense against the effects of UV radiation, suppression of heat loss, and tactile sensation. The various hair types consist of terminal, intermediate, and vellus hairs. Terminal hairs fit the classic perception of hair and are the hairs of the scalp, axillae, pubic region, beard, eyebrows, and eyelashes. These are long, pigmented, and thick. Vellus hairs, on the other hand, are short and generally lack pigmentation. These cover the body. Intermediate hairs have characteristics that fall in the middle of the spectrum between terminal and vellus hairs. Loss of hair can be irreversible, causing skin to atrophy and follicular openings to vanish. Such cases are categorized as cicatricial (or scarring, permanent) alopecia. Reversible hair loss is noncicatricial (Wolff et al. 2009).
Humans are usually born with approximately 5 million follicles, and no new follicles are thought to be added after birth. The hair follicle cycle, which begins in utero, is composed of three stages: anagen, telogen, and catagen. Anagen phase is the longest, lasting an average of 3 yr and ranging from 1 to 6 yr depending on body location. It is also the most prevalent phase, with 90%–95% of all hairs existing in anagen phase at any one point in time. Anagen represents the growth period, comprising extensive mitotic activity, such that longer anagen phase means longer hair (e.g., scalp as opposed to eyebrows, eyelashes, or pubic hair). The hair then involutes during catagen phase through apoptosis of the follicular keratinocytes, leaving a club hair. Telogen is the resting period with inactivity of the organ, persisting 2 to 3 mo on the scalp or longer elsewhere. The club hair is shed and a new anagen hair grows in its place to resume the cycle (Wolff et al. 2009; Habif 2010). Given its properties of regeneration, the hair follicle is a fascinating organ. Learning about associated pathophysiologies can yield a great deal of insight about human physiology and serve as a model of regeneration for a human organ.
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