Evidence for endocytotic mechanisms in the epidermal Langerhans cells.

When the Langerhans cells (LC) are prepared forelectron microscopy with an osmotically balanced prefixative,they display their classical criteria, but in additionthey are provided with numerous microvilli and containpinocytotic vesicles. In cross-sections the specific discshapedgranules are visualized as rod-like profiles limited bya trilaminar membrane similar to the cell membrane. Inface-on projection the electron density of a single granule istoo low to permit its observation. It ts suggested that theracquet-shaped profile bounded by a non-trilaminar membrane,regarded as classical, is a fixation artifact. The viewis advanced that the Birbeck granules are formed by a processimplying a superimposition of membrane folds. Theoccurrence of phagocytosed melanosomes seems to be anormal phenomenon.The LC are highly activated in contact allergic dermatitis,as expressed by an increase in number, a polarizationof dendrites towards the surface and the appearance ofmembrane active zones characterized by intense membranefolding, combined with a formation of pinocytotic vesiclesand specific granules. Challenging sensitized skin withnickel leads to the occurrence of dark vesicles which probablylack a limiting membrane. It is concluded that the LCshow endocytotic activity and that they constitute animportant defense system in the epidermis, i.e., capable ofcapturing extracellular (endogenous or exogenous) materialvia different routes