Ultrastructure of choroid epithelium. CP from a lateral ventricle of an untreated adult Sprague-Dawley rat was fixed for electron microscopy with OsO4. There is a profusion of apical membrane (CSF-facing) microvilli (Mv) and many intracellular mitochondria (M). J refers to the tight junction welding two cells at their apical poles. C = centriole. G and ER, Golgi apparatus and endoplasmic reticulum. Nucleus (Nu) is oval and has a nucleolus. Arrowheads point to basal lamina at the plasma face of the epithelial cell; the basal lamina separates the choroid cell above from the interstitial fluid below. Basal labyrinth (BL) is the intertwining of basolateral membranes of adjacent cells. Choroidal morphology resembles proximal tubule, consistent with both cell types rapidly turning over fluid. Scale bar = 2 μm.