In vivo studies of cutaneous lipid biosynthesis

The epidermis and the sebaceous glands are both characterized as terminally differentiating tissues in which cells produced by replication in the germinative layer progress during several weeks through a programmed series of biochemical and morphological changes that end in cell death and eventual sloughing of the final metabolic products. Because of these slow changes and the presence of successive anabolic and catabolic regimes in the same tissue, classic methods of biochemistry using cell homogenates have not been widely successful. Much of what is known of the lipid metabolic pathways has therefore been obtained from experiments with biopsy materials or in intact animals. Radiotracer experiments in intact animals have been successful largely because the lipid products into which the radiolabel is incorporated are not lost to the general circulation during the several weeks in which they are retained in the tissue. Nor are they subject to continual degradation and resynthesis that would be the case in most other tissues, but progress steadily through the series of biochemical transformations that occur along with the morphologic changes characteristic of the epidermal and sebaceous tissues. Furthermore, of the radiolabel that enters the general circulation, only an insignificant proportion returns to the cutaneous tissues to obscure the local metabolic activities.