Electron microscopy of the tubular cells of the kidney cortex

Cytoplasmic sharing through apical membrane remodeling

Multiple nuclei sharing a common cytoplasm are found in diverse tissues, organisms, and diseases. Yet, multinucleation remains a poorly understood biological property. Cytoplasm sharing invariably involves plasma membrane breaches. In contrast, we discovered cytoplasm sharing without membrane breaching in highly resorptive Drosophila rectal papillae. During a six-hour developmental window, 100 individual papillar cells assemble a multinucleate cytoplasm, allowing passage of proteins of at least 62 kDa throughout papillar tissue. Papillar cytoplasm sharing does not employ canonical mechanisms such as incomplete cytokinesis or muscle fusion pore regulators. Instead, sharing requires gap junction proteins (normally associated with transport of molecules < 1 kDa), which are positioned by membrane remodeling GTPases. Our work reveals a new role for apical membrane remodeling in converting a multicellular epithelium into a giant multinucleate cytoplasm.

Aminonucleoside nephrosis. I. Electron microscopic study of the renal lesion in rats

Experimental renal disease was produced in young rats by daily subcutaneous injections of 6-dimethylamino purine, 3-amino-d-ribose (aminonucleoside). The physiologic, biochemical, and light microscopic changes were similar to those observed in human nephrosis. Electron microscopy of the glomeruli from animals which received seven or more daily injections of aminonucleoside revealed characteristic abnormalities of the epithelial cells of the glomerular capillaries. These changes consisted of swelling, coalescence, and eventual obliteration of the epithelial cell foot processes and an increase in the number and the size of epithelial cytoplasmic vacuoles. The serial development of the ultramicroscopic pathologic changes in the epithelial cells, as observed by study of animals through the course of the disease, indicated that the smudging of the foot processes occurred at the time of onset of severe proteinuria. Further changes, consisting of hypercellularity due primarily to an increase in the number of endothelial cells and an increase in the amount of basement membrane-like material, were regularly observed by the 12th day after injections were begun. These abnormalities were correlated with the development of uremia in the later stages of the disease process. The distal and proximal renal tubules were abnormal by the 7th day after injections were begun, and showed localized swelling between the cristae of the mitochondria and a decrease in over-all cytoplasmic density, described as hydropic change. Electron microscopy of glomeruli from animals which had partially recovered from aminonucleoside nephrosis revealed areas of normal epithelial cell morphology. This observation was interpreted as evidence of partial reversal of the ultramicroscopic changes in the recovery phase of the disease. These observations of the fine structure of pathologically altered glomeruli and tubules in aminonucleoside nephrosis are similar to our findings in human nephrosis as revealed by electron microscopy of serial renal biopsies.

Electron microscopy of the tapetum lucidum of the cat

The fine structure of the tapetum of the cat eye has been investigated by electron microscopy. The tapetum is made up of modified choroidal cells, seen as polygonal plates grouped around penetrating blood vessels which terminate in the anastomosing capillary network of the choriocapillaris. The tapetal cells are rectangular in cross-section, set in regular brick-like rows, and attain a depth of some thirty-five cell layers in the central region. This number is gradually reduced peripherally, and is replaced at the margin of the tapetum by normal choroidal tissue. The individual cells are packed with long slender rods 0.1 micro by 4 to 5 micro. The rods are packed in groups and with their long axes oriented roughly parallel to the plane of the retinal surface. Each cell contains several such groups. Cells at the periphery or in the outer layers of the tapetum are frequently seen to contain both tapetal rods and melanin granules, the latter typical of the choroidal melanocytes. Also melanocyte granules may have intermediate shapes. These observations plus the similar density of the two inclusions lead to the belief that the tapetal rods may be melanin derivatives. A fibrous connective tissue layer lies between the tapetum and the retina. The subretinal capillary network, the choriocapillaris, rests on this layer and is covered by the basement membrane of the retinal epithelium. The cytoplasm of the retinal epithelium exhibits marked absorptive modifications where it comes in contact with the vessels of the choriocapillaris. This fibrous layer and the basement membrane of the retinal epithelium apparently comprise the structural elements of Bruch's membrane.

Hemoglobin absorption by the cells of the proximal convoluted tubule in mouse kidney

The formation of protein absorption droplets in the cells of the proximal convolution was studied in mouse kidney. Ox hemoglobin was administered intraperitoneally and kidney specimens were collected at intervals of 30 minutes to 4 days after injection. In the lumen of the nephron, hemoglobin was concentrated to an opaque mass whose relations with the brush border and the epithelium could be easily followed. It was found that hemoglobin passes through the brush border in between the microvilli, enters the channels of tubular invaginations at the bases of the brush border, and is transported in bulk into vacuoles in the intermediate cell zone. These vacuoles increase in size and are transformed through further concentration into dense absorption droplets. Using the opaque hemoglobin content of the nephron as a tracer, functional continuity of the system of the tubular invaginations with the lumen on one side and the vacuoles on the other was demonstrated. Mitochondria lie closely apposed to vacuoles and droplets, but are not primarily involved in droplet formation. 15 hours after injection and later, ferritin and systems of layered membranes become visible in the droplets as their density decreases. These membranes are interpreted as lipoprotein membranes; similar membranes are found in the lumen of the tubuli. It is suggested that phospholipids enter into the vacuoles together with hemoglobin from the tubular lumen and form membrane systems of lipoproteins in the droplets. At 3 to 4 days the droplets contain aggregates of ferritin, and the iron reaction becomes positive in the tubule cells. No significant changes were found in the Golgi apparatus or in the microbodies during hemoglobin absorption. At all time points investigated, the terminal bars seal the intercellular spaces against penetration by hemoglobin in the proximal and distal convolutions and in the collecting ducts.

Secretion of lipoprotein particles by the cells of the kidney proximal segment in the migrating arctic lamprey, Lampetra japonica

The cells of the kidney proximal segment of the migrating arctic lamprey, Lampetra japonica, contain particles of the same size, electron-density and intracellular location as particles identified by others as very low-density lipoproteins (VLDL) in the liver and intestine of teleost fishes and lampreys. These particles are synthesized within the cisternae of the smooth endoplasmic reticulum and elements of the Golgi complex. They are transferred to the lateral intercellular space and lamina propria by way of the Golgi vesicles and an intracellular channel system. Some particles are discharged into the lumina of the sinusoidal capillaries of the lamina propria. Although the physiological role of lipoprotein secretion in the renal proximal segment cells is unknown, the present observations provide morphological evidence that the kidney of the arctic lampreys synthesizes lipoproteins and releases them into the circulation at the time when they are undertaking their anadromous migration.

Perfusion--fixation of human kidneys for ultrastructural analysis

Surgically removed human kidneys were fixed by vascular perfusion to improve the preservation of renal tissue and, in particular, the proximal tubules for electron microscopic analysis. Fifty kidneys with various pathologic changes were perfusion-fixed with glutaraldehyde under pressure control immediately after nephrectomy. A Tyrode solution containing a spasmolytic drug initiated the perfusion. The results demonstrate that large volumes of ultrastructurally well-preserved kidney tissue can be obtained in normal (nontumorous) parts of tumor kidneys as well as in kidneys with chronic nephropathy. Several factors improved the quality of fixation: (1) start of perfusion-fixation within 15 min after clamping of the renal vessels, (2) careful handling of the kidney during nephrectomy, (3) simultaneous clamping of the main renal vessels, and (4) employment of a spasmolytic drug. In comparison with tissue from immersion-fixed renal biopsies, perfusion-fixed kidneys represented an improved basis for ultrastructural investigations of normal or pathologically changed human kidney tissue.

Hydrostatic pressure changes related to paracellular shunt ultrastructure in proximal tubule

We examined the effets of changes in hydrostatic pressures on the ultrastructural geometry of the lateral intercellular space and tight junctions in proximal tubules of contrtol (C) and volume-expanded (VE) Necturus kidney. The following groups of tubules were studied: (1) C, free-flow pressure, (2) C, stopped-flow, high-luminal pressure, (3) C, stopped-flow, low-luminal pressure, (4) VE, free-flow pressure, and (5) VE, stopped-flow, high-luminal pressure. Intratubular and peritubular capillary pressures were monitored before and during standardized perfusion-fixation for electron microscopy, and complete cross-sections of all sampled tubules were subjected to morphometric analysis. Average lateral intercellular space widths decreased significantly in C and VE stopped-flow tubules with high-luminal pressures but widened greatly in C stopped-flow tubules with low-luminal pressures. The length or width of the tight junctions did not change between the five experimental conditions. The ultrastructural changes correlate with the applied transepithelial pressure gradients rather than with transepithelial volume fluxes. The narrowing of lateral intercellular spaces in high pressure tubules correlate with the previously described increase in electrical resistance expressed per unit length tubule indicating that in these conditions part of the paracellular resistance is located in the free interspaces. The geometry of the lateral intercellular space in the proximal tubule of Necturus favors models of near-isotonic transport that do not depend on long and narrow interspaces.

Formation and turnover of plasma membrane glycoproteins in kidney tubules of young rats and adult mice, as shown by radioautography after an injection of 3H-fucose

The formation and turnover of the glycoproteins of the plasma membrane have been investigated by quantitative radioautography in the kidney tubules of young rats and adult mice killed at various time intervals after an intravenous injection of 3H-fucose. In young (40 g) rats killed five to ten minutes after the injection, radioautographs of distal tubule cells show that the Golgi apparatus contained about 85% of the cell label. By 30 hours, only 8% of the label remained in this organnele, whereas 67% was in the plasma membrane, indicating that most of the label had migrated from Golgi apparatus to this membrane. Similarly, in proximal tubule cells, about 82% of the label was initially in the Golgi apparatus, but less than 2% remained at 30 hours, at which time 78% was in the plasma membrane. In the latter cells, the apical tubules and vacuoles became heavily labeled before the apical microvilli did and, therefore, may be involved in the transit of label from the Golgi apparatus to the microvillous membrane. The results are interpreted to mean that, in kidney tubule cells, the Golgi apparatus is the site of a continuous incorporation of fucose into glycoproteins and that these migrate to the plasma membrane. In fully formed cells, such a conclusion would imply a continuous turnover of plasma membrane glycoproteins. However, in the rapidly growing kidney of young rats many new cells are added daily, the growth of which might involve net addition as well as turnover of glycoproteins. Accordingly, the experiment has been repeated in adult mice, in which the cells are assumed to be fully formed. Furthermore, since turnover implies eventual decrease of incorporated label, some of the animals have been killed at longer intervals, up to 27 days after injection. In these adult mice, as in young rats, prompt Golgi uptake and subsequent migration of label to the plasma membrane were observed in distal and proximal tubules cells. With time the label content of the plasma membrane decreased gradually, and by 27 days had virtually disappeared. From grain counts, it is concluded that the mean half-life of glycoproteins in the apical membrane of distal tubule cells is about two days, whereas in both the apical and basal membranes of proximal tubule cells, it is slightly over three days.

Morphology of the ascending thick limb of Henle

The morphologic characteristics of the medullary and cortical segments of the ascending thick limb of Henle were examined in the rat utilizing light microscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The kidneys of six normal female Sprague-Dawley rats were preserved for morphological examination utilizing in vivo intravascular perfusion with 6.25% glutaraldehyde buffered in 0.1m sodium cacodylate. SEM revealed the presence of two distinctly different patterns of surface structure among the population of cells forming the epithelium of the ascending thick limb, a roughsurfaced cell with prominent microvilli (R cell) and a smoothsurfaced cell generally devoid of extensive microvilli except along the lateral cell margins (S cell). This variation in surface structure was confirmed with light microscopy and TEM. Throughout both the medullary and cortical segments of the ascending thick limb, the R cells possessed extensive lateral processes radiating form the main cell body to interdigitate with adjacent cells. Lateral processes of S cells were less common than those of the R cells in the inner stripe of the outer medulla, but increased in number and complexity in the outer stripe of the outer medulla and throughout the cortex. S cells outnumbered R cells in the inner stripe of the outer medulla. As the thick limb transversed the outer medulla and ascended toward the parent renal corpuscle, there was a marked increase in the relative number of R cells. In the cortical segment of the ascending thick limb, both the R and S cells tended to acquire a more extensive population of microvilli over their luminal surface making it increasingly difficult to establish clearcut distinctions between the two surface patterns. With the possible exception that vesicles and tubulovesicular profiles located in the apical region were more numerous in many of the S cells, additional morphological differences between the rough and smoothsurfaced cells were not evident. Whether the morphological results represent two distinct cell types or simply a variation in the structure of a single cell type, their presence must be considered in future structural-functional studies involving the ascending thick limb of Henle in the rat.