The microanatomy of the distal tubules, collecting tubules and collecting ducts of the starling kidney.

Light and electron microscopic techniques were employed to characterise the various cell types found in the distal tubules, collecting tubules and collecting ducts in the kidneys of wild starlings (Sturnus vulgaris). There was a gradual change in cell characteristics along the length of the distal tubule. A short transition zone was identified, between the distal tubule and the intralobular collecting tubule, which contained cell types that were characteristic of both segments. The end of the distal tubule was marked by the appearance of scattered dark cells and occasional mucin-secreting cells. The dark cells were electron-dense and usually possessed many microvesicles in the subapical region; there were also numerous, stubby apical microprocesses in many cases. In the basal regions of these cells were many, short, regular infoldings of the plasma membrane. The initial part of the intralobular collecting tubule was lined by approximately equal numbers of dark and mucin-secreting cells. However, as the tubule progressed towards the lobular periphery, the number of dark cells declined. Few dark cells were observed in the perilobular collecting ducts and none in the medullary ducts which were lined by columnar mucin-secreting cells. The mucin-secreting cells possessed a prominent supranuclear Golgi apparatus and apical vacuoles containing a strongly PAS- and alcian blue-positive mucigen. Critical electrolyte concentration methods, using alcian blue in the presence of magnesium chloride, indicated that the mucigen contained large amounts of a strongly sulphated component, but was probably of a mixed nature. The dark cells were slightly PAS-positive but only very weakly alcian blue-positive at the cell apices. The roles of the various cell types were discussed in relation to the uric acid excretion habit of birds.