Morphological study on the regeneration of the retina in the rainbow trout after ouabain-induced damage: evidence for dedifferentiation of photoreceptors.

The retina of the rainbow trout is capable of marginal regeneration after ouabain-induced degeneration (intraocular injection of 5 microliters 10(-4) M ouabain). In the central area where the pigment epithelium proliferates to a multicellular layer, the neural retina does not regenerate up to 182 days after injection of ouabain. The regeneration process begins in the marginal growth zone with an increase in the mitotic rate; the growth zone itself is not damaged after ouabain administration. The proliferate differentiates with time into a newly layered retina; this portion of the retina is called the paramarginal zone, i.e., the "first" regenerated zone. The paramarginal zone is arranged concentrically to the retinal margin. Cells surviving ouabain administration, located outside, although close to the margin and occurring mostly in the outer nuclear layer, reveal signs of dedifferentiation: loss of the outer segment, amalgamation of the presynaptic terminal with the perikaryal cytoplasm, alteration of cell shape, and mitotic activity. The area in which these dedifferentiation processes are observed is found adjacent and concentric to the paramarginal zone; it is thinner than the latter and incompletely structured ("second" regenerated zone). The third zone adjoins the second zone and is characterized by folds, which were described previously as "rosettes". Extracellular microtubule-like structures, which are found between the horizontal cells in the normal retina of the rainbow trout, regenerate only sparsely in the paramarginal zone, whereas they are lacking in the incompletely regenerated zones.