Phylogeny of the Hair Follicle: The Sebogenic Hypothesis

The "sebogenic hypothesis" proposed that hair follicles evolved from primitive multicellular oil glands, with the hair shaft initially acting as a wick to transport glandular secretions to the skin surface. It suggested that sebaceous glands predated hair follicles, as hair follicles require sebaceous glands for proper formation and function. The study emphasized the developmental link between hair follicles and sebaceous glands, where weak growth factor signals lead to sebaceous gland formation and stronger signals induce hair follicle formation. This hypothesis had implications for understanding hair follicle biology and disorders, suggesting that some issues might stem from sebaceous gland malfunctions. Additionally, it proposed that early tetrapods developed a lipid cover from primitive sebaceous glands to enhance their epidermal permeability barrier in dry land environments, with the hair shaft evolving for additional protective benefits.