Retinal photoreceptor fine structure in the domestic sheep.

The structure of the photoreceptors of the domestic sheep has been investigated by light and electron microscopy. In the sheep retina the photoreceptors are readily differentiated and adequately described by the classical terminology of rods and cones. Rods are more numerous then cones. They appear as tall slender cells measuring 45-50 microns in length and 2-3 microns in diameter with inner and outer segments of the same diameter. Rod nuclei are located at all levels of the outer nuclear layer. The rod synaptic ending is small and rounded with two to three invaginated (ribbon) synaptic sites. Cone cells are shorter than rods, measuring only 30-35 micron in total length. The cone inner segment is wider than the outer segment which tapers distally. Cone nuclei are located adjacent to the external limiting membrane and are more vesicular than rod nuclei. Cone synaptic endings are larger spherules with ten to twelve (ribbon) synaptic sites. Both rods and cones also display superficial synaptic regions. An unusual form of apparent degeneration is noted in cone photoreceptors. This involves a disorganization and loss of the outer segment and a progressive distal movement of the nucleus through the inner segment towards the retinal epithelial layer. The structure and ratio of rods and cones remains similar in both the tapetal and nontapetal regions of the fundus.