Hair Development, Growth, and Loss 58 percentage of telogen hair and hair loss peaks in late summer and autumn.72 Hair disorders are common problems with diverse etiologies and may bring large psychological impact to affected subjects.73 In addition to cosmetic concerns, hair loss may be associated with underlying diseases, which need to be carefully identified and treated. Based upon the clinical and pathological features, alopecia can be categorized as acute vs. chronic, focal vs. diffuse, pattern vs. non-pattern, and scarring vs. non-scarring. Abnormalities in the HS can be another cause of hair loss. Signs that may indicate abnormal hair loss: • Increased number of shed hairs exceeding personal average or 100 hairs per day • Recession of frontal hair line • Widening of the hair parting line • Decreasing caliber of the hair shaft or the ponytail • Diffuse or focal thinning of hair • Excessive hair shedding by gentle hair pull • Disrupted hair shafts with a break point or abnormal morphology Pattern Hair Loss Pattern hair loss refers to hair thinning at the frontal and vertex scalp sparing the temporal and occipital area. In male pattern hair loss (also referred as androgenetic alopecia or common baldness), recession of the bitemporal hair line in an “M” shaped pattern is another hallmark, while in female pattern hair loss the hair line is usually preserved. Miniaturized hair follicles are common features with terminal hair gradually turning into intermediate or vellus hair. Male androgenetic alopecia: Male androgenetic alopecia (AGA) is the most common hair loss disorder. Moderate to severe AGA affects 50% of the Caucasian male population and 20% of the Asian male population by age 50.74 The onset age can range from adolescence to young or middle adulthood (12–40 years). AGA is characterized by