4 RETINAL GANGLION-CELL TYPES THAT PROJECT TO THE SUPERIOR COLLICULUSIN THE 13-LINED GROUND-SQUIRREL (SPERMOPHILUS-TRIDECEMLINEATUS)

The morphology of retinal ganglion cells projecting to the superior co lliculus (SC) of the thirteen-lined ground squirrel (Spermophilus trid ecemlineatus) was studied after retrogradely labeling the cells with c holera toxin subunit B. On the basis of previous reports, labeled cell s were classified as small (6-10 mu m in soma diameter), medium (11-14 mu m), or large (>14 mu m). A total of 3,427 cells were studied. Smal l cells constituted 78% of the population, 21% were medium cells, and only 1% were classified as large. The morphology of medium-sized cells was studied in more detail because large cells were few in number and the staining of the dendritic tree of small cells was not optimal. Th e best labeled medium-sized cells were classified on the basis of the shape and size of their dendritic tree and the pattern of dendritic ra mification. Four types were identified among the medium-sized ganglion cells. Two types were classified as symmetric delta-like and asymmetr ic delta-like cells considering the relative symmetric or asymmetric d istribution of their dendritic branches and their similarities with th e delta type of the cat. Approximately 52% of all the medium-sized cel ls studied were symmetrical delta-like, and 19% were classified as asy mmetrical delta-like. These cells were also very similar to the symmet rical and asymmetrical directionally selective ganglion cells describe d in rabbit retina. Other cells were termed beta-like. They had the sm allest dendritic tree diameter, and their tree size seemed to be relat ed to retinal eccentricity. Medium beta-like cells comprised approxima tely 21% of all cells projecting to the SC. The fourth type was termed ''acute angle'' because most of their dendritic branches were relativ ely straight and formed acute angles (10-45 degrees) at their branchin g points. These cells were few in number (approximately 8% of all medi um-sized cells studied) and did not resemble any reported previously i n cats. Thus, a variety of morphological types of retinal ganglion cel ls projected to the SC. Of these, the symmetrical and asymmetrical del ta-like cells appeared to correspond to the directionally selective ty pe described in the ground squirrel (Michael, C.R. [1968] J. Neurophys iol. 31:257-267) and reported in the rabbit retina. (C) 1998 Wiley-Lis s, Inc.