An important cause of acne is ductal hyper cornification that can be seen as microcomedones histologically. Clinically they can be seen as blackheads, whiteheads and macrocomedones. There is strong correlation between acne and number & size of microcomedones.
Comedones are due to abnormal proliferation and differentiation of ductal keratinocytes . They are due to retention of hyper proliferating ductal keratinocytes . The hyper proliferation has been confirmed by an increase in H-thymidine labeling of comedones. This technique has shown an increase in ductal proliferation of keratinocytes in non-affected follicles which were shown as clinically normal, but from an area of acne. This observation correlate with microcomedones being found during histological examination of normal skin taken from adjacent areas of acne. There is also presence of keratin 6 and 16 (which are markers of hyper proliferation) in microcomedones and comedones.
Several factors are implicated in the induction of keratinocyte hyperproliferation. Those include androgens, sebaceous lipid composition, bacteria and local cytokine production.
Of the lipid compositions in acne patients, the linoleic acid levels are relevant. Examination of lipids recovered from comedones shows that it contains only 6% lenoleic acid compared to 45% in normal subjects. Linoleic acid is also reduced in sebum in patients with acne, which returns to normal after resolution of acne in those patients. Animal experiments also show that low linoleic acid produces hyper cornification. Other lipids like free fatty acids and squalene are also blamed for hypercornification .
Anti androgen drugs reduce comedones and also increase linoleic acid concentration and thus anti androgen therapy can reduce comedogenesis. Cytokines production by ductal keratinocytes is also an important factor in comedogenesis. Interleukin-1-? (a cytokine) is present in comedones at higher levels than normal subjects.
Bacteria are less important in comedogeneis. Biopsy & culture of non inflamed areas has shown that 30% of these are with out bacteria, which shows that bacteria are not needed for initiation of cornification and comedogenesis.
Comedones are temporary structures. Like hair follicles, comedones undergo cyclical growth. For example, extracted blackhead refill in 2-6 weeks. Certain chemicals used in cosmetics can contribute to comedogenesis. They include ingredients of cosmetics like, propylene glycol, isopropyl myristate, red dyes etc