Figure 5.

Serial electron micrographs through 4 retracted spherules in 7-day detached retina. Scale bar (in A) for A-D represents 2 μm. A: Spherules 1 and 2 contain synaptic ribbons (arrows), each apposing the top of their respective synaptic invaginations. Only mitochondria are seen in spherules 3 and 4. Arrowheads line the interfaces of apposing Müller cells. Swollen HCat processes (*) that lack polysomes can be differentiated from Müller cell processes (M) that have many. B: In the next section, the opposing lobes at each synaptic ribbon in spherules 1 and 2 are visible. They contain vesicles as do the swollen HCat processes (*) from which they arise. The hilus of spherule 1 can be seen. Another HCat process directly apposes spherule 3. C: Two sections later the openings to the synaptic invaginations of spherules 1 and 2 are obvious. The hilus of spherule 2 actually faces the outer nuclear layer instead of the outer plexiform layer. The horizontal cell (HC) lateral elements innervating spherules 1 and 2 connect with numerous swollen HCat processes (*) running along apposing Müller cell surfaces. Synaptic ribbons (arrows) now can be seen in spherules 3 and 4, both of which directly appose HCat processes. D: Two sections later, one lobe of the synaptic invagination can be seen in spherule 3 while spherule 4 makes open contact with its innervating HCat processes (*). The innervation of spherule 1 by HC lateral elements is still evident, while that of spherule 2 is not. There is no clear evidence that any of these 4 spherules have contact with rod bipolar dendrites.