On the mechanism of sebaceous secretion.

Following the studies of Kligman, most investigators now believe that sebaceous glands function continuously in excreting sebum to the skin surface [7, 8]. Populations of differentiating cells are maintained by mitotic activity both in the peripheral cells of the sebaceous lobules and in aggregations of undifferentiated cells which extend through the body of the lobules. Once formed, and as long as maintained by circulating hormones, each lobule continues to produce a stream of differentiating cells which accumulate sebum as they move towards the sebaceous duct and finally disrupt to release their contents into the pilosebaceous canal. After intradermal injections of 3H-thymidine to label germinative cells during DNA replication, up to 28 days elapse before all labeled cells disappear from the glands. When differentiating cells are labeled with 3H-amino acids, much of the label is lost in 7 days. Likewise, when lipids are labeled with 14C-acetate, the average excretion time for the labeled sebum is 8 days. To this time may be added the renewal time of undifferentiated cells to give an average sebaceous cell transition time of 14 days [15]. From knowledge of the time between synthesis and excretion of sebum, sebum production rates were calculated from the sebum content of punch biopsies. The transit time of sebum in the follicular canals was estimated to be 14 h. Production rates determined in this way agree with those measured by long-term absorption of sebum at the skin surface.