Based on serial sectioning, focused ion beam scanning electron microscopy (FIB/SEM), and electron tomography, we depict in detail the highly unusual anatomy of the marine hyperthermophilic crenarchaeon, Ignicoccus hospitalis. Our data support a complex and dynamic endomembrane system consisting of cytoplasmic protrusions, and with secretory function. Moreover, we reveal that the cytoplasm of the putative archaeal ectoparasite Nanoarchaeum equitans can get in direct contact with this endomembrane system, complementing and explaining recent proteomic, transcriptomic and metabolomic data on this inter-archaeal relationship. In addition, we identified a matrix of filamentous structures and/or tethers in the voluminous inter-membrane compartment (IMC) of I. hospitalis, which might be responsible for membrane dynamics. Overall, this unusual cellular compartmentalization, ultrastructure and dynamics in an archaeon that belongs to the recently proposed TACK superphylum prompts speculation that the eukaryotic endomembrane system might originate from Archaea.

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. podocyte PODOCYTES REPRESENT THE OUTERMOST cell layer of the glomerular filtration barrier. Based on results obtained with conventional imaging techniques, the current model pictures the podocyte cell body as floating in Bowman's space while being anchored to the glomerular basement membrane via its arborizing cellular extensions. The finest branches, the foot processes, are arranged in an interdigitating pattern on the outer surface of the basement membrane (11). Neighboring foot processes are connected by a slit diaphragm which supposedly contains the integral membrane proteins nephrin, Neph1, FAT-1, and Pcadherin. These proteins are linked to the actin cytoskeleton through the adapter proteins podocin, CD2AP, and zonula occludens (ZO)-1 (17). It has been suggested that the urinary space between the podocyte and the glomerular basement membrane, the subpodocyte space, impedes the flow of the primary filtrate into Bowman's space and that therefore the subpodocyte space contributes to the filtration properties of the renal glomerulus (8, 9, 13). Hereditary and acquired podocytopathies lead to the destruction of the intricate cytoarchitecture of podocytes and consequently to a failure of the renal filter. The hallmark of such podocytopathies is the disappearance of foot processes and ultimately a detachment of affected podocytes into the urine, thus causing albuminuria (7, 15).The current view on the structural basis of the renal filtration barrier was derived from classic transmission and scanning electron microscopy. While these techniques have provided valuable information, they also suffer from intrinsic limitations and therefore leave open fundamental questions: 1) What is the dimension of a single podocyte? 2) How are podocytes arranged on the glomerular basement membrane? and 3) Does a podocyte form slit diaphragms between its own foot processes? To answer these questions, we applied novel electron microscopic techniques, i.e., focused ion beam-scanning electron microsco...