Open and closed comedones were manually expressed and examined for the presence of vellus hairs in potassium hydroxide crush mounts. The mean number of hairs was 5.0 (range, 2 to 16) for open and 1.45 (range, one to three) for closed comedones.
The hairs were of typical vellus type and were always in telogen, with mean lengths of 1.8 and 1.27 mm for open and closed comedones, respectively.
It was concluded that (1) the filling up of sebaceous follicles with horny squamae does not interfere with the cyclic generation of hairs, (2) closed comedones have fewer hairs because they are an earlier stage, (3) larger open comedones may have longevities of two to three years, and (4) sequestration of hairs into the dermis following rupture of the comedones may contribute to the chronicity of acne lesions.
Leyden JJ, Kligman AM. Hairs in Acne Comedones. Arch Dermatol. 1972;106(6):851-853. doi:10.1001/archderm.1972.01620150037012