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Fenestrated epithelium of the pelvic recess wall on the lateral parenchymal tissue in the dog kidney.

The epithelium of the pelvic recess separating the urine and the lateral parenchym of the dog kidney was examined by scanning and transmission electron microscopy (SEM and TEM) to try and identify the epithelial structures permitting filtration of urine substances. In cross sections through of the renal hilus, recesses were found in the pelvic space extending along the internal surface of the lateral parenchym. Light microscopy (LM) showed the external wall of the recess facing the parenchymal tissue to be a thin squamous epithelium underlined with a layer of smooth muscle cells. The cytoplasm of epithelial cells observed by TEM was extremely thin in comparison to the epithelia on other parts of the pelvic wall. Their peripheral cytoplasm usually demonstrated interdigitations or showed overlapping of cytoplasm of neighboring cells in two or three laminae. Distinct junctional structures were observed but no distension of the intercellular space was seen. The lumenal cell surface was relatively smooth, but the peripheral cytoplasm was equipped with either angular or stumped processes and deep cavae. In addition to a small number of cell organellae, numerous vesicles and vacuoles of varying sizes were conspicuously observed. The vacuoles contained flucculent materials and were fenestrated by pores closed by a diaphragm at fused potions of their limiting membrane with the adluminal and abluminal plasma membrane. The openings of the vacuoles were also seen at the basal cytoplasm. Similar fenestrations also occurred between the neighboring vacuoles in chains or conglomerations. Attenuated epithelial cytoplasm in close association with urinary tubules was also fenestrated by uniformly sized pores closed by thin diaphragm with punctuate central thickening. The fenestrations through the cytoplasm and between the chained vacuoles may thus be structures which have yet to be previously identified, and this finding seems to indicate that the filtration of urine substances occurs across the epithelial lining of the pelvic wall. Layers of thin smooth muscle cells with mutual connections are seen to underline the epithelium. The innermost muscle cells penetrated the basal lamina and thus come in contact with epithelial cells. However, their significance could not be clearly determined.