Non-uniform postnatal growth of the cat retina

The distributions of alpha-type ganglion cells in 3-week-old and adult cats were used to measure the increase in the distances between existing cells and thus the amount of growth in various regions of the retina. Growth shows two major non-uniformities. (1) The area centralis is at the point of minimum growth; its area increases by only about 3% while regions near the retinal margin increase in area by about 80%. (2) The retina grows about half as much in linear extent as does the radius of the eye and thus comes to occupy a smaller fraction of the globe. Measurements of retinal dimensions indicate that both non-uniformities also occur from birth to 3 weeks. These non-uniformities have the following implications. (1) They would tend to elongate dendritic fields radially, in the direction of the area centralis, in central retina but perpendicular to this direction in peripheral retina. However, these asymmetries are probably not the primary reason why ganglion cells throughout the retina tend to have radially oriented dendritic fields (Leventhal and Schall, '83). (2) Greater growth in the periphery could contribute to the gradient of increasing dendritic field size from central to peripheral retina if the dendritic fields of ganglion cells passively stretched as the retina expanded. Passive stretching is not the primary determinant of dendritic extent, however, because the dendritic fields of beta-type ganglion cells were found to grow 70% more from 3 weeks to adulthood than can be accounted for by passive stretching. (3) Greater peripheral growth steepens the central-to-peripheral gradient of decreasing ganglion cell density; if this trend also occurs prenatally, it could be the major factor in producing the final adult gradient.