A and B, electron micrographs of E60 fetal sheep choroid plexus. Alcian blue in Krebs solution was injected i.v. 10 min prior to fixation. Alcian blue is electron dense and binds to plasma albumin. Particulate precipitate (P in A) is visible within tubular endoplasmic reticulum (TER; dark arrows in B), which extends close to the lateral cell membrane (LM). JC, junctional complex separating the lateral intercellular space from lateral ventricular CSF. Curved arrows in A indicate precipitated Alcian blue on apical cell membrane; open arrows indicate close contact of TER with apical cell membrane, exposed to CSF. From Møllgård & Saunders (1975). C, high voltage double impregnation thick section EM of E60 fetal sheep choroid plexus. Note the extensive network of TER with contacts to CSF surface (uppermost in micrograph) and close association of TER with Golgi complex (G) and mitochondria (M). From Møllgård & Saunders (1977). D, electron micrograph of ultracryosection from E60 fetal sheep choroid plexus immunolabelled for human (HA) 6 nm gold particles and sheep albumin (SA) 12 nm gold particles (arrows). Gold particles labelling each of the albumins are colocalized within the same TER‐cistern (star) and a multivesicular body (MVB; inset). Scale bar, 0.2 μm. From Balslev et al. (1997a). E and F, fetal sheep choroid plexus (E60) double impregnation technique. Profiles of rough endoplasmic reticulum (RER) and TER system. Note TER termination on apical plasma membrane via a caveola (arrowhead in F). MVB, multivesicular body; M, mitochondrion. From Møllgård & Saunders (1977).