Scalp hair loss is not random across follicular units: A new insight into human hair ageing

OBJECTIVES: Scalp hair has the greatest number of hairs (typically 1-5) per follicular unit but is also the most susceptible body site to hair loss with age. Hence, we set-out to determine the degree to which scalp hair parameters change with age in women and men, any sex differences thereof and whether hair loss is random across follicular units. METHODS: A retrospective cross-sectional study of 200 Chinese men and 200 Chinese women (30-69 years). Image analysis and manual counting methods were used to measure occipital located hair parameters from 6 × 8 mm shaved scalp photographs and plucked hair microscopy images. RESULTS: Of the five hair parameters, the number of hairs per follicular unit had the greatest (negative) correlation with age in both men and women. Men had a greater number of hairs and follicular units than women on average but had a greater decrease in the number of hairs per follicular unit with age, particularly for the loss of multi-hair (3+) follicular units. The loss of hairs with age was significantly different to that expected by a random loss of hairs across follicular units and better described by a model of increased hair loss risk the greater number of hairs per follicular unit. CONCLUSIONS: We have found evidence of hair loss preferentially occurring in multi-hair follicular units, which was more pronounced in men. These data suggest that part of the reason scalp hair is more susceptible to hair loss than on other body sites is due to the greater presence of multi-hair follicular units on the scalp.