Advanced electron microscopic techniques provide a deeper insight into the peculiar features of podocytes

Podocytes constitute the outer layer of the glomerular filtration barrier, where they form an intricate network of interdigitating foot processes which are connected by slit diaphragms. A hitherto unanswered puzzle concerns the question of whether slit diaphragms are established between foot processes of the same podocyte or between foot processes of different podocytes. By employing focused ion beam-scanning electron microscopy (FIB-SEM), we provide unequivocal evidence that slit diaphragms are formed between foot processes of different podocytes. We extended our investigations of the filtration slit by using dual-axis electron tomography of human and mouse podocytes as well as of Drosophila melanogaster nephrocytes. Using this technique, we not only find a single slit diaphragm which spans the filtration slit around the whole periphery of the foot processes but additional punctate filamentous contacts between adjacent foot processes. Future work will be necessary to determine the proteins constituting the two types of cell-cell contacts.