Retinal ganglion cells in the crucian carp (Carassius carassius). II. Overlap, shape and tangential orientation of dendritic trees.

Ganglion cells were studied in methylene blue stained flat-mounted retinas. Three categories of cells are described: small (S) and large (L) ganglion cells in the main ganglion cell layer, and large ganglion cells (LD) with somata more or less displaced into the inner plexiform layer. These LD cells have two to four very thick primary dendrites and are identifiable as ganglion cells by their axons. An analysis of published data reveals that the large ganglion cells of the crucian carp (type L and LD) have several striking characteristics in common with the large ganglion cells of the dogfish, the frog and the cat: (1) they are selectively stained by methylene blue; (2) they comprise only 2-5% of all the ganglion cells; (3) the large cells can be divided into two or three subtypes, and within each subtype the dendritic trees usually cover the retinal surface with a two- or threefold overlap. New ganglion cells are formed from neuroblasts at the retinal margin and most dendrites first grow along this neuroblastic zone. Thus the main dendrites of the L and LD cells tend to be oriented parallel to the margin all around the periphery of a crucian carp retina. Independent of the size of the eye this parallel orientation disappears at the same relative distance from the margin (about one-third of the distance from the margin to the optic disc). If all L and LD cells are formed at the retinal margin and first develop oriented dendrites, we have to assume that the more randomly oriented dendritic trees in the central retina have undergone a reorganization.