Electron microscopy of the tubular cells of the kidney cortex

An Electron Microscopic Study of the Ductuli Efferentes and Rete Testis of the Guinea Pig

The ductuli efferentes and rete testis of the guinea pig were isolated by micro dissection, fixed in cold buffered osmium tetroxide, and sectioned for examination with the light and electron microscopes. Proximal and distal segments of the ductuli efferentes were identified and their respective cytological organizations characterized. The cytological components of the rete testis are briefly described and figured. Non-ciliated and ciliated cells are found in both segments of the ductuli efferentes. The non-ciliated cells have a microvillous border, mitochondria, a Golgi complex, an ubiquitous endoplasmic reticulum, and numerous cytoplasmic vacuoles. The ciliated cells contain more mitochondria, an endoplasmic reticulum with a relatively sparse distribution, and few, if any, cytoplasmic vacuoles. A regional difference exists in proximal and distal segments based on the distribution, size, number, and electron opacity of the cytoplasmic vacuoles. Attention was paid to the disposition of the endoplasmic reticulum and its relation to the system of cytoplasmic vacuoles. These findings are interpreted as suggesting that the continuity of the vacuolar system with elements of the endoplasmic reticulum represents a pathway for transfer of large quantities of fluid, an activity which has long been ascribed to the epithelium of the ductuli efferentes. Periductular capillaries possess pore-like apertures in their endothelia similar to those in other tissues known to engage in fluid transfer.

Cellular differentiation in the kidneys of newborn mice studies with the electron microscope

The structure of the kidney of the Swiss albino mouse changes progressively during the first 2 weeks after birth. Cells proliferate to form new nephrons, cells differentiate by acquiring specialized membranous components, and certain cytological features which are present at birth diminish in abundance or disappear. The differentiation of the cells of the cortical tubules has been studied using the light and electron microscopes. The tubules are partially and variably differentiated at birth. During the first 2 weeks after birth the brush border develops in the proximal tubules by the accumulation of numerous microvilli on the apical cell margins. Basal striations develop in proximal and distal tubules as an alignment of mitochondria, the result of what appears to be progressive interlocking of adjacent fluted cells. The mitochondria increase in number and size, accumulate homogeneous matrix, and acquire small, very dense granules. The collecting ducts develop tight pleating of the basal cell membranes, and dark cells containing numerous small cytoplasmic vesicles and microvilli appear. At birth there are dense irregular cytoplasmic inclusions presumed to be lipide in renal cells, the cytoplasmic granules of Palade are abundant, and there are large round bodies in the cells of the proximal tubules. The lipide inclusions disappear a few days after birth, and the cytoplasmic granules of Palade diminish in abundance as the cells differentiate. The large round bodies in the proximal tubules consist of an amorphous material and contain concentrically lamellar structures and mitochondria. They resemble the cytoplasmic droplets produced in the proximal tubules of adult rats and mice by the administration of proteins. The large round bodies disappear from the proximal tubules of infant mice during the first week after birth, but the concentric lamellar structures may be found in adult mice.

The electron microscopy of the choroid plexus

1. The choroid plexus of the rat has been studied in detail by electron microscopy. Samples from the frog, rabbit, and cat have also been examined without noting significant differences. 2. The surface of the ependymal epithelium is covered by pedicels of variable size. There is reason for thinking of these structures as labile. They may actually pinch off and contribute to the secretory product. In any case, the surface area is vastly increased by their presence. Polypoid border seems an apt term to apply to this type of surface. 3. There is also a great expansion of the basal surface of ependymal cells. In the vicinity of cell junctions this surface is deeply infolded, and continuous with elaborate interdigitations of the lateral intercellular surfaces. Analogous infolding of the basal cell surface is known to exist in other epithelia also noted for their water transport (kidney tubules, salivary gland, and ciliary body). 4. Pretreatment of rats with diamox, an agent known to block cerebro-spinal fluid production, did not produce an important morphological change in the features of the ependyma, or any other part of the choroid plexus. 5. Capillaries of the choroid plexus have a very attenuated endothelium. This is seen to be fenestrated. It is thought this probably represents the condition in life, and is not simply a fixation artefact. 6. Pial cells tend to interpose sheets of cytoplasm between the capillaries and ependyma. The sheets are not continuous, however, and so would not constitute a serious diffusion barrier. These cells belong to the reticuloendothelial system, and undergo shape changes, and probably increase in number, when the system is stimulated by the repeated injection of trypan blue.

Infolded basal plasma membranes found in epithelia noted for their water transport

Epithelia noted for their water transport have been studied by electron microscopy with particular emphasis upon basal specializations. Epithelia of the submaxillary gland, choroid plexus, and ciliary body are described in this article, and compared with previous observations on the kidney. The basal surface of all these epithelia is tremendously expanded by folds which penetrate deeply into the cytoplasm. In the submaxillary gland this is particularly notable in cells of the serous alveoli and in the secretory ducts. In these instances the folds have a fairly regular distribution and have a marked tendency to turn back upon themselves and so form repeating S-shaped patterns. In the choroid plexus the penetrating basal folds are limited to the lateral regions of each ependymal cell where they blend with the intercellular membranes that are also folded. In the epithelium of the ciliary body it is the inner layer that is specialized. The surface adjacent to the cavity of the eye penetrates irregularly, nearly through the full depth of the cell layer. The exposed surface is, in a fundamental sense, the basal surface of this epithelial layer. It is apparent that the pattern of folding is quite distinctive in the different epithelia. Therefore, the specializations should be regarded as analogous rather than homologous. Topographic considerations presumably limit the manner in which basal cell surfaces might be expanded. Penetrating folds would seem to represent almost the only possible solution.

Nature of fluids which functionally distend the kidney

The nature of the fluid draining from the kidney, after its artery was occluded, was investigated. Samples of systemic arterial blood, renal venous blood and urine were also analyzed. It was found that the fluid draining from the kidney after occlusion is a mixture of vascular blood and another fluid designated as "diluting fluid," each contributing half to the composite mixture. In volume the mixture is 26 per cent of the functionally distended kidney. With the assumption that the renal extracellular fluid can be considered a simple mixture of blood plasma and a cell-free fluid, the composition of the "diluting fluid" was deduced from the known compositions of vascular blood and total fluid draining. The ratios of its content in a given substance to that in systemic (or renal venous) plasma are: for Na and Ca, 1.0; K, 1.5; Cl, 1.2; PO(4), 2.0; urea, 1.8; plasma protein, 0.3; albumin, 0.4; glucose, 0.4, and osmolarity, 1.2. The fluid bears little or no relation to urine, especially since the urine varied considerably between individual dogs whereas the "diluting fluid" was relatively constant in composition. It was also found that the hematocrit of the fluid draining after arterial occlusion progressively decreased as it flowed out, until the last portion contained only 5 per cent red cells. It is concluded that since renal lymph has approximately the same composition in protein, urea, glucose, and inulin as does "diluting fluid," the latter is, in all probability, renal interstitial fluid. Under the conditions of the experiment, it drains out of the kidney slowly relative to blood drainage. It is large in volume, particularly when compared with the capillaries that nourish it. Its high protein content explains the observation that the kidney is apparently naturally distended with a fluid disproportionately rich in plasma protein.