Comedones  represent  pathological  changes of  the pilosebaceous ductal contents. They are described as being either open or closed. Open comedones The open comedo (blackhead) presents clinically as an obvious black lesion, 0.1-3.0  mm in diameter ,  which usually takes several  weeks or more to develop. The larger comedones can be expressed with  a comedo  extractor, producing  as  extruded material a worm-like grey-white greasy structure. The black tip of the  comedo was once thought to be caused by oxidation of the surface fats but  it is now  known to be due to melanin.4 Although many clinicians regard blackheads as the hallmark of acne,  their absence by no means negates the diagnosis. Indeed, there are many acne patients who have few or no blackheads. Closed comedones The closed  comedo (whitehead) represents  a pilosebaceous duct distended with inspissated ductal material, the orifice  of which is hardly visible to the naked eye .  It presents as a small lesion, usually just palpable, 0.1-3.0 mm in diameter. Of typical whiteheads, 25 per cent resolve within 3-4 days; 75 per cent  develop into inflamed  lesions. The larger closed comedones last many weeks or months. Closed  comedones  are  potentially  the  most  proinflammatory of comedones.

For  many years, dermatologists have  taught their students that it is uncommon to see an ugly inflammatory lesion at the site of a blackhead. However, Orentreich and Durr, albeit from a prolonged  study of one individual, have shown that open comedones can also develop into inflammatory lesions.5 Not all comedones are clearly open or closed: some show a black melanized centre with a white halo.  Such lesions are common constituting up to 40 per cent of comedones - but they have no official name.  Perhaps the term 'intermediate noninflamed  lesions'6  or  even 'blighthead' could be used.