Fig. 1. Polarity along the animal-vegetal (a-v) axis of the ascidian oocyte (see Movies 1-3 in supplementary material). (A1,A2) Confocal equatorial section (30 µm from surface) showing the distribution of the ER network (A1) and the 7-8 µm thick mitochondria-rich subcortical myoplasm (m) (A2). (A3,A4) Enlarged views of the ER-poor myoplasm. (B) Electron-microscopy section of the vegetal-pole region showing the ER network (red), ER microdomains (ER), cER near the plasma membrane (arrowheads), mitochondria (m, green) and yolk platelets (YP, blue). (C1,C2) Confocal subcortical section (3 µm from surface) passing through the myoplasm (m). (C3,C4) Enlarged views of the transition zone at the edge of the myoplasm (m). (D1,D2) High-magnification confocal cortical sections (1 µm from surface) of the cER network in the vegetal (D1) and animal hemispheres (D2). (E) ER around the meiotic spindle (ms, arrowhead) at the animal pole. (F1,F2) Equatorial (F1, 30 µm from the surface) and cortical (F2, 1 µm from surface) confocal sections of fluorescent in situ localization of Ci-PEM1 RNAs (arrowheads). (F3) Confocal cortical view (1 µm from surface) of the vegetal hemisphere at higher magnification. Ci-PEM1 RNA signal appears as a reticulated network. (a) Animal pole. (v) Vegetal pole. All confocal acquisitions are made on Ciona intestinalis oocytes, the electron micrograph is from a section of a Phallusia mammillata oocyte.