Coated Langerhans cell granules in histiocytosis X cells

Langerin/CD207 sheds light on formation of birbeck granules and their possible function in Langerhans cells

Langerhans cells (LCs) are immature dendritic cells of epidermis and epithelia, playing a sentinel role through their specialized function in antigen capture, and their capacity to migrate to secondary lymphoid tissue to initiate specific immunity. A unique feature of LCs is the presence of Birbeck granules (BGs), which are disks of two limiting membranes, separated by leaflets with periodic "zipperlike" striations. The recent identification of Langerin/CD207 has allowed researchers to decipher the mechanism of BG formation and approach an understanding of their function. Langerin is a type II lectin with mannose specificity expressed by LCs in epidermis and epithelia. Remarkably, transfection of Langerin cDNA into fibroblasts creates a dense network of membrane structures with features typical of BGs. Furthermore, mutated and deleted forms of Langerin have been engineered to map the functional domains essential for BG formation. Langerin is a potent LC-specific regulator of membrane superimposition and zippering, representing a key molecule to trace LCs and to probe BG function.

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.

Electron microscopy of large cell undifferentiated and giant cell tumors

Electron microscopy is a valuable morphologic method for the diagnostic evaluation of undifferentiated tumors composed of polygonal or oval large cells and mononuclear or multinucleated giant cells. Although few ultrastructural details are pathognomonic, electron microscopic findings may add significantly to the formulation of the final diagnosis if used in the context of other pathomorphologic and clinical data. Contributions of electron microscopy to tumor diagnosis are summarized and illustrated with appropriate examples from personal experience in a routine university hospital laboratory.