DEVELOPMENT OF THE RABBIT RETINA .5. THE QUESTION OF COLUMNAR UNITS

A qualitative and quantitative description of the columnar units in the mammalian retina, and a discussion of their ontogeny and putative functions is given. Columnar arrangements of cells exist in the developing retina which can be observed by means of scanning electron microscopy. In the adult retina, each Muller cell ensheaths a columnar group of neuronal cells. Counting the number of cells in radial H/E stained sections at various developmental stages reveals a constant ratio of neuronal cells per Muller cell, independent of the developmental stage (after postnatal day 9), and independent of the retinal topography. Such groups of cells always consist of one Muller cell, 11 rod photoreceptor cells, about 2 bipolar cells, and 1 to 2 amacrine cells. Retinal ganglion cells, cone photoreceptor cells, and horizontal cells are more sparsely distributed in the retina than these units; since they are known to arise earlier in the ontogenesis than other cell types they are considered to exist independently of the columnar units. It is suggested that the units arise by migration of groups of preneurons along a common Muller (precursor) cell; these preneurons and the corresponding Muller cell may be clonally related. In the adult retina, such columns might constitute metabolic and functional units.