Intermediate hair follicles: a new more clinically relevant model for hair growth investigations

Alopecia causes widespread psychological distress, but is relatively poorly controlled. The development of new treatments is hampered by the lack of suitable human hair follicle models. Although intermediate and vellus hair follicles are the main clinical targets for pharmacological therapy, terminal hair follicles are more frequently studied as smaller hair follicles are more difficult to obtain.

This investigation was designed to quantify in vivo morphological and in vitro behavioural differences in organ culture between matched intermediate and terminal hair follicles, in order to develop a new clinically relevant model system.

Microdissected terminal and intermediate hair follicles, from the same individuals, were analysed morphometrically (250 follicles; five individuals), or observed and measured over 9 days of organ culture (210 follicles; six individuals).

Intermediate hair follicles were less pigmented and smaller, penetrating less below the skin surface (mean +/- SEM) (2.59 +/- 0.07 vs. 3.52 +/- 0.10 mm; P = 0.02), with smaller fibre (0.03 +/- 0.002 vs. 0.07 +/- 0.002 mm), connective tissue sheath (0.24 +/- 0.01 mm vs. 0.33 +/- 0.01 mm), bulb (0.19 +/- 0.01 vs. 0.31 +/- 0.01 mm) and dermal papilla (0.06 +/- 0.002 vs. 0.12 +/- 0.01 mm) diameters (P < 0.001). Intermediate hair follicle bulbs appeared 'tubular', unlike their 'bulbous' terminal follicle counterparts. In organ culture they also grew more slowly (0.044 +/- 0.002 vs. 0.067 +/- 0.003 mm per day; P < 0.001), remained in anagen longer (84 +/- 0.03% vs. 74 +/- 0.03% at day 9; P = 0.012) and produced less hair fibre (0.36 +/- 0.02 vs. 0.50 +/- 0.03 mm; P < 0.001) than terminal follicles.

Smaller intermediate hair follicles showed major morphological differences from terminal follicles in vivo and retained significant, biologically relevant differences in vitro in organ culture. Therefore, intermediate hair follicles offer a novel, exciting, more clinically relevant, albeit technically difficult, model for future investigations into hair growth. This should be particularly important for developing new therapies.