Fig. 2

Ultrastructure of photoreceptors from the retinas of 97 dpf sablefish and 15 dpf zebrafish with emphasis on double cone partitions and junction-like regions. (A) Tangential electron micrograph of the juvenile sablefish retina showing the elliptical and circular profiles of double cones (dc) and single cones (sc), respectively. Black arrowheads point to partitioning membranes of double cones. The area within the white rectangle is magnified in (B). (B) Double cone ellipsoid showing “decorated” plasma membranes with minute electron-dense globular structures on either side. The area within the pink rectangle shows part of the partition that is similar to the junction-like processes identified in zebrafish²²; electron-dense filaments (dark extensions between the two membranes) appear to link them together. (C) Radial view of a multi-layer double cone partition in the central retina of 15 dpf zebrafish. (D) Radial view of a “paired cone” from the peripheral retina of 15 dpf zebrafish near the circumferential growth zone showing two singular membranes apposed together (opposing white arrowheads). The area within the white rectangle is magnified in (E). (E) The apposing membranes are not “decorated” or show any signs of structural connectivity. (F) Radial micrograph of multiple cones from the central retina spanning the nuclear to myoid/proximal ellipsoid region of 15 dpf zebrafish. Black arrowheads point to multi-layered double cone partitions. Müller glia form electron-dense junctions with photoreceptors at the proximal myoid region constituting the outer limiting membrane (olm); these junctions appear to form a continuous membrane under light microscopy, hence the historical misnomer. The area within the white rectangle is magnified in (G). (G) Magnified view of two junctions between Müller glia (Mg) and cone photoreceptors. er endoplasmic reticulum, m mitochondria, n nucleus, os outer segment, pg pigment granule. The magnification bar on the bottom right of each electron micrograph = 0.5 μm.