Anatomical correlations between soma size, axon diameter, and intraretinal length for the alpha ganglion cells of the cat retina

Abstract

Retinal ganglion cells within the same region of the retina may have different lengths of axon before reaching the optic disc depending on the route they take with respect to the temporal raphe. We have investigated whether there is a correlation between soma and intraretinal axon diameter and how these parameters relate to intraretinal axon length on both sides of the cat temporal raphe. Retinas were wholemounted and alpha-cell somata and fibers stained with a modified neurofibrillar method. Moving peripherally from the area centralis along the raphe there was a progressively increasing difference between the intraretinal axon lengths for nearly adjacent cells across the raphe, which reached a maximum of 4-5 mm at the retinal periphery. Cells on the nasal aspect of the raphe had shorter axons than did adjacent cells on the temporal aspect of the raphe. Comparison of soma diameter samples across the raphe showed there was no clear trend between soma diameter and intraretinal length. Replotting the raphe and sample areas on a cell density map indicated that differences in soma diameter could be attributed to ganglion-cell density differences between the sampled areas. Examination of the stained cells revealed that within the initial length of the axon there was a region showing a reduction of axon diameter (diameter less than 1 micron), which varied in length from cell to cell. The axon was, therefore, divided into three segments: the portion of axon prior to thinning (A), the thin segment itself (B), and the part of the axon after the thin segment (C). The diameter of each segment (A,B,C) and the lengths of the first and second segments (A,B) were significantly correlated with soma diameter (P less than 0.001). From measurements of the axon diameter of segment C, it was concluded that alpha-cell axons continue to increase in diameter along their path towards the optic disc. The present report indicates that alpha-cell soma size, when going from the area centralis to the periphery along the raphe, reaches a plateau and then declines within more peripheral retinal locations in spite of increasing intraretinal axon length. Thus, there is no positive correlation between soma or axon diameter and intraretinal axon length. The anatomical findings are discussed in relation to previous reports of retinal development and complementary conduction times within intraretinal and extraretinal visual pathways.