LARGE RETINAL GANGLION-CELLS IN THE PIPID FROG XENOPUS-LAEVIS FORM INDEPENDENT, REGULAR MOSAICS RESEMBLING THOSE OF TELEOST FISHES

Population-based studies of retinal neurons have helped to reveal thei r natural types in mammals and teleost fishes. In this, the first such study in a frog, labeled ganglion cells of the mesobatrachian Xenopus laevis were examined in flatmounts. Cells with large somata and thick dendrites could be divided into three mosaic-forming types, each with its own characteristic stratification pattern. These are named alpha( a), alpha(ab), and alpha(c), following a scheme recently used for tele osts. Cells of the alpha(a) mosaic (similar to 0.4% of all ganglion ce lls) had very large somata and trees, arborizing diffusely within subl amina a (the most sclerad). Their distal dendrites were sparsely branc hed but achieved consistent coverage by intersecting those of their ne ighbors. Displaced and orthotopic cells belonged to the same mosaic, a s did cells with symmetric and asymmetric trees. Cells of the alpha(ab ) mosaic (similar to 1.2%) had large somata, somewhat smaller trees th at appeared bistratified at low magnification, and dendrites that bran ched extensively. Their distal dendrites arborized throughout sublamin a b and the vitread part of a, tessellating with their neighbors. All were orthotopic; most were symmetric. Cells of the alpha(c) mosaic (si milar to 0.5%) had large somata and very large, sparse, flat, overlapp ing trees, predominantly in sublamina c. All were orthotopic; some wer e asymmetric. Nearest-neighbor analyses and spatial correlograms confi rmed that each mosaic was regular and independent, and that spacings w ere reduced in juvenile frogs. Densities, proportions, sizes, and mosa ic statistics are tabulated for all three types, which are compared wi th types defined previously by size and symmetry in Xenopus and potent ially homologous mosaic-forming types in teleosts. Our results reveal strong organizational similarities between the large ganglion cells of teleosts and frogs. They also demonstrate the value of introducing mo saic analysis at an early stage to help identify characters that are u seful markers for natural types and that distinguish between within-ty pe and between-type variation in neuronal populations.