Figure 7  The rd7 Mutant Retina Contains a Morphologically Hybrid Photoreceptor Cell Type in Addition to Supernumerary S-Opsin–Positive Cones
(A and B) Toluidine blue-stained semi-thin sections of the outer nuclear layer (scleral edge oriented up).
(C and D) Hand-drawn diagrams of the cells in (A) and (B), respectively. Cells with the nuclear features of cones are highlighted in blue. Note that the number of such cells is greater in the mutant, and their cell bodies are scattered throughout the outer nuclear layer. In addition, the overall columnar architecture of the outer nuclear layer seen in the wild type is disrupted in this portion of the mutant retina. Other portions of the mutant retina with fewer supernumerary cone cells, however, retain the normal columnar appearance (unpublished data).
(E and F) Images of the outer nuclear layer (scleral edge up) stained by in situ hybridization for S-opsin. Note the typical pattern of staining at the scleral edge of the outer nuclear layer in the wild type. The rd7 mutant retina shows supernumerary S-opsin–positive cells scattered throughout the outer nuclear layer in a distribution very similar to the supernumerary cone cells seen in (B). Since images (E) and (F) derive from different retinas than those depicted in (A) and (B), the location of the individual cells do not correspond.
(G and H) Electron micrographs of the outer nuclear layer (10,000× magnification). Note the uniform distribution of rod cell bodies in the wild type (G). The cell bodies are nearly round and consist almost exclusively of a nucleus with a single, dense mass of heterochromatin. In the rd7 mutant (H), two types of cell are shown. The ovoid one with a lesser quantity of heterochromatin, paler euchromatin, and two juxtanuclear mitochondria (yellow arrow) represents a typical cone cell body. The adjacent cell with a more “rod-like” mass of heterochromatin and a single juxtanuclear mitochondrion (red arrow) represents one of the hybrid photoreceptors discussed more fully in the main text.