Scalp sebums of 64 young men were applied once daily for ten times to the external ear canal of the rabbit. In every instance follicular hyperkeratosis resulted.
This finding supports the conception that sebum contains substances that directly stimulates the follicle to form a comedo, the elementary lesion in acne vulgaris.
Kligman AM, Katz AG. Pathogenesis of Acne VulgarisI. Comedogenic Properties of Human Sebum in External Ear Canal of the Rabbit. Arch Dermatol. 1968;98(1):53-57. doi:10.1001/archderm.1968.01610130059013