By labelling DNA-synthesizing cells with tritiated thymidine, one can measure the proliferative activity of the adjacent epidermis, and the infrainfundibulum.

Plewig et al found no difference in the cell renewal rate in the epidermis and infrainfundibulum of acne and non-acne subjects. They found an increased proliferation in the facial ducts compared to those on the back. Proliferative activity was also greater in a comedo than in a normal sebaceous follicle.

In closed comedones 21 per cent of the basal cells were labelled. The mean was lower for open comedones, being of the order of 14 per cent. Thus, despite the pressure of accumulating horn there is no decrease in the mitotic activity of the basal cells. On the contrary, the basal cells are more actively dividing..

Another study by Plewig examined the migration of horny cells into the comedo by labelling the fibrous proteins with tritiated glycine and histidine. There was no difference in the movement of the cells between uninvolved skin of acne and normal subjects. However, they demonstrated that a well-defined band of labelled horny material was present around the periphery of the infrainfundibulum a few days after injection.

The grain density of the epithelial cells of a comedo was greatly increased over that of normal follicular epithelium, a clear indication of intensified metabolic activity. By ten days the leading edge of the band was well into the substance of the comedo. Cell movement was surprisingly rapid..

Because escape was cut off, a tightening ring of labelled horny cells formed in closed comedones and was still intact after two months.

By contrast, a U-shaped band formed in open comedones, the upper arms of which were already sloughing by ten days . Thus, in functional terms the words 'open' and 'closed' comedo are appropriate; the solid contents of open comedones are continuously being extruded whereas there is complete retention in closed lesions.

Presumably other products - lipids and bacterial metabolites - are also swept rapidly out of open comedones whereas in closed units the microscopic size of the pore would certainly limit their outward dispersion. Retention of toxic materials is possibly one of the reasons why closed comedones have a greater propensity for rupturing than open ones.

These are the 'time bombs' so aptly named by Strauss and Kligman..

Inevitably, with a high rate of generation of horny cells and their rapid migration into a sealed-off cavity, one of two events must occur. Either the wall of the comedo ruptures and creates an inflammatory lesion, or the pressure of the expanding mass dilates the orifice and the horny core begins to extrude, thus transforming a closed comedo into an open one.

Both happenings have been observed in spontaneous and experimentally induced acne vulgaris. However, lesion counts show that most acne patients usually have many more closed than open comedones and so presumably most closed comedones remain as such, or resolve..

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The dynamics of comedo formation .-::34095::-.