Comedo

A comedo is a plug of sebum and dead skin cells in a hair follicle. It appears as an open blackhead or a closed whitehead.

Sebum: An oily substance produced by sebaceous glands that lubricates skin and hair. Excess sebum combines with dead skin cells to form the follicular plug.
Hair follicle: A skin structure that surrounds a hair shaft and connects with sebaceous glands. Comedones form when material accumulates in this structure.
Acne vulgaris: A chronic skin condition involving comedones and, in some cases, inflammatory lesions. Comedones are characteristic lesions of this common disorder.
Milia: Small keratin-filled cysts that form beneath the skin surface, often around the face. Milia resemble closed comedones but are not plugs within hair follicles.
Follicular hyperkeratinization: Excessive buildup of keratinocytes in a hair follicle that can obstruct its opening. This buildup helps trap sebum inside the follicle.
Sebaceous gland: A skin gland that secretes sebum, usually into a hair follicle. Its secretion supplies the oil that becomes part of a comedo.
Topical retinoid: A vitamin A derivative applied to skin to alter cell turnover and reduce follicular blockage. Topical retinoids are used to treat comedonal acne.
Sebaceous filament: A normal structure that channels sebum through a follicle to the skin surface. It can look like a blackhead but is a normal follicular feature.
Open comedo: A comedo with a widened follicular opening and a darkened surface. It is the subtype commonly called a blackhead.
Keratinocyte: The predominant cell type in the epidermis, producing the structural protein keratin. Dead keratinocytes contribute to the cellular material in the plug.

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