Cytomembrane-derived Birbeck granules transport horseradish peroxidase to the endosomal compartment in the human Langerhans cells.

It has been previously shown that the cytomembrane of human Langerhans cells (LC) has the capacity to fold upon itself, thereby forming Birbeck granules (BG), which then internalize. I confirmed this by exposing LC in vitro to horseradish peroxidase (HRP) at +8 degrees C and +37 degrees C. On incubation at +37 degrees C the label appeared not only in BG, but also in tubular structures and vesicles of different shapes and sizes. Interconnections between these labeled endosomal structures were common. The LC cytomembrane could form BG at +8 degrees C and, moreover, the BG was the only organelle that was labeled and internalized at this temperature. Thus, cytomembrane-derived BG are endocytotic in nature and link the exterior of the cell and the endosomal compartment. The membrane interlinking of the BG may eventually dissolve and the BG then transform into an endosomal vesicle.