Retinal ganglion cells in goldfish: a qualitative classification into four morphological types, and a quantitative study of the development of one of them.

In this paper we describe the dendritic morphology of ganglion cells that have been retrogradely stained with HRP taken up by the cut optic nerve. This technique produces an extensive Golgi-like filling of the cells. From their appearance in the retinal whole-mount, they were classified as four types, according to the sizes of the soma and dendritic field, the thickness of the primary dendrites, and the density of the arbors. Each type was subdivided according to the level(s) of stratification of the dendrites within the inner and outer plexiform layer(s) to yield a total of 15 subtypes (four for three types, three for the other). The retina of the goldfish grows by a balloon-like expansion, and by the addition of new neurons, in annuli, at the margin. Therefore, a similar cell type may be examined at a variety of stages of development in the same retina, as well as in the retinae from fish of various ages. We have used a computer-assisted microscope to do so, quantitatively, for one large and easily identified subtype. In small fish (ca. 4 cm long), the number of dendritic branch points, the total dendritic length, and the dendritic field sizes of these cells are constant inside a central zone extending to 70-80% of the retinal radius. The magnitudes of all three numeric descriptors decrease closer to the margin. In large fish (ca. 14 cm long), the central zone extends to more than 90% of the retinal radius, and the same pattern holds. The area of the dendritic fields and the total dendritic lengths are both greater in the central zone of the large fish than in the small, but the number of branches is the same in both. This suggests that once a cell has achieved the "mature" number of dendritic branches, further growth is interstitial. A comparison of dendritic morphologies across the retina shows that the pattern of dendritic outgrowth in peripheral retina is initially directed parallel to the margin, and, later, toward the margin. This suggests that dendritic growth is impeded by the dendrites present in more central retina and proceeds preferentially where they are absent. Cells of the same age are at different distances from the optic disk in the small and large retinae. In some cases, they have quite different dendritic morphologies. This implies that dendritic development depends not only on the age and subtype of the cell, but on extrinsic factors as well.(ABSTRACT TRUNCATED AT 400 WORDS)