Lipid synthesis in chick epidermis

Lipid synthesis in newborn chick epidermis was studied by electron microscopic autoradiography after injection of tritiated palmitate. The labeled lipid product in the tissue was identified as mostly triglyceride. At the earliest time after injection (6 hr), the radioactive precursor was taken up by all viable cells of the epidermis. Grain density was heaviest over basal cells, moderate over spinous cells, and slight over granular cells; thus lipid incorporation is highest in the basal and spinous regions of the chick epidermis. As time after injection progressed, the increasing amounts of grains over the granular and horny cells and decreasing amounts over the basal and spinous cells reflected the continuous upward displacement of cells from one layer into the next. From the distribution of silver grains within the epidermal cells, it has been concluded that, with the passage of time, triglycerides synthesized by the epidermal cells were mainly located in lipid droplets. The numerous grains associated with the elements of the endoplasmic reticulum indicated that this organelle is involved in aggregating triglyceride molecules into lipid droplets. The fact that grains were seen within the horny cells indicated that part of the horny cell consists of lipid probably derived from the lipid droplets retained by the cells during keratinization.