Fig. 4. Prominin-1-containing protrusions on the apical plasma membrane of neuroepithelial cells at various stages of embryonic mouse brain development. (a-d) Prominin-1 immunofluorescence at the apical surface of neuroepithelial cells at E8.5 (a,c; future hindbrain) and E11.5 (b,d; dorsal telencephalon), viewed from the lateral (a,b) and lumenal (c,d) side of the neuroepithelium. Scale bar in c: 10 µm. (e-n) Prominin-1 immunogold EM of the apical plasma membrane of neuroepithelial cells at E8.5 (e-g) and E11.5 (h-n) (h,i,k,l,n: midbrain; j,m: forebrain). Scale bars: 300 nm. Some of the gold particles (12 nm) are indicated by arrowheads and junctional complexes by asterisks. (e,f) Microvilli. (g-k) Large pleiomorphic protuberances; note the thin stalks (g,j), the vicinity of the junctional complexes (j,k), the abundant prominin-1 immunoreactivity (h), and the small prominin-1-labelled membrane buds (arrows) in (k); (i) shows the prominin-1-labelled cup-shape surface structure of (n) at higher magnification. (l) Short, thin protrusions; note the concentration of prominin-1 immunoreactivity at the tips (arrows). (m,n) Overviews of the apical plasma membrane of a single (m) and several (n) neuroepithelial cells; note the prominin-1-labelled protuberance in m and the cilium (arrow, note the basal body), but lack of microvilli, in n. (o,p) Scanning EM of the apical plasma membrane of E11.5 neuroepithelial cells in the forebrain. The boxed region in o indicates the area shown at higher magnification in p. Open arrow, cilium; double-headed arrow, cauliflower-like protuberance; triple-headed arrow, cup-shape protuberance. Scale bar: 5 µm (o); 1 µm (p). (q-t) Floorplate of the E11.5 spinal cord. (q) Prominin-1 immunofluorescence at relatively low exposure to show the more intense immunoreactivity of the floorplate cells (FP). Scale bar: 10 µm. (r) Overview of prominin-1 immunogold EM; note the microvilli of the floorplate cells (FP) and the pleiomorphic protuberances but lack of microvilli on the neuroepithelial cells adjacent to the floor plate (double-asterisks); the bracket indicates the area shown at higher magnification in s. Scale bar: 300 nm. (s) Prominin-1-labelled microvilli of the floorplate (12-nm gold). Scale bar: 300 nm. (t) Scanning EM of the apical plasma membrane. Note the bending of the lumenal surface, resulting in a lateral view of the microvilli-bearing floorplate cells and a lumenal view of the adjacent neuroepithelial cells. The black triangle and white bar at the left margin indicate the transition from floorplate to adjacent neuroepithelial cells and an apparent boundary between neuroepithelial cells with abundant vs. sparse protrusions, respectively. Bar, 10 µm.