Fig. 7

Development of the double cone partition membrane complex in the central retina of precocial fishes, and comparison with peripheral retina. (A–K) Electron micrographs from embryonic [two days (A, B) and one day (C, D) prior to hatching] and yolk sac embryo (15 days post hatching) rainbow trout. White rectangles in (A, C,J) are magnified in (B, D,K), respectively. Prior to outer segment formation, some apposing membranes between single cones become multi-layered (black arrowhead, B) and square mosaic formation is visible (C). Following outer segment appearance (E–K), cone patterning can vary from the square mosaic (e.g., 6 double cones surrounding a single cone, E) as double cone recruitment of sub-surface cisternae continues (double white arrowheads, F). The square mosaic unit contains both centre (blue asterisk) and corner (pink asterisk) single cones (G, J) with prominent intrusions of the plasma membrane between inner segment ellipsoids (black circles, H,I) and filopodial (f) contacts between myoids of neighbouring cones and Müller glia (Mg) (K). (L–P) Micrographs from threespine stickleback at hatching (L, M) and upon yolk absorption (N–P). The white rectangle in (L) is magnified in (M). (L, M) Primordial double cones and square mosaic units prior to outer segment formation. (N–P) Variable mosaic patterns, including 5 double cones surrounding a single cone (white rectangle, O), among square mosaic units with only centre cones (blue asterisk, N). (P) Incorporation of sub-surface cisternae (double white arrowheads) at different contact sites, from multi-layered (black arrowhead) to devoid of cisternae (opposite white arrowheads), between members of a double cone. (Q–AA) Micrographs from zebrafish larvae at 8 days post-fertilization (dpf, Q-W) and 15 dpf (X–AA). Short single (UV) cones (shcos) are vitreally displaced with respect to other cone types at 8 dpf (Q, V). Most cones abut plasma membranes lacking sub-surface cisternae (opposite white arrowheads, S) and cog-like cytoplasmic protrusions at regions of contact are common (black circles, R), perhaps due to interlocking calyceal processes (white rectangle shows calyceal processes in a no contact zone, R). Some cones start developing multi-layer partition membranes (black arrowheads, T,U) by amalgamation of sub-surface cisternae (double white arrowheads, W) becoming double cones (U, V). At 15 dpf, the mosaic remains hexagonal-like (X) and long single cones are displaced vitreally with respect to double cones (Y), whose partitions continue to incorporate sub-surface cisternae (double white arrowheads, Z) as opposed to contacts involving single cones (opposite white arrowheads, AA). (A’–K’) Micrographs from the peripheral growth zone in juvenile/adult retina. The ora serrata of juvenile (post-metamorphic) Atlantic halibut shows cone nuclei in hexagonal formation (blue hexagon, A’) whereas nearby differentiated cones are arranged in a square mosaic with varying numbers of corner cones (pink asterisks, B’). Deviation from a square lattice is common (C’) and single cones link with neighbours by forming multi-layered partitions (D’,E’) to become double cones (F’). Adult Atlantic halibut shows double cones with similar partition characteristics (G’). The peripheral retinas of juvenile European anchovy (H’), rainbow trout (I’) and 15 dpf zebrafish (J’,K’) show but single cones without subsurface lamellae near the undifferentiated ora serrata. Rods appear after the first cones (J’). The white rectangle in (J') is magnified in (K'). c connecting cilium, cos cone outer segment, dcos double cone outer segment, er endoplasmic reticulum, lcos long cone outer segment, m mitochondria, n nucleus, olm outer limiting membrane, os outer segment, pg pigment granule, rpe retinal pigment epithelium, r rod, ros rod outer segment, sc single cone, shcos short cone outer segment. The magnification bar on the bottom right of each electron micrograph = 0.5 μm.