56 VISION 



cells form the lining of the primitive neural tube and ultimately the ventricu- 

 lar spaces of the CNS. This agrees precisely with the location of photo- 

 receptors, which line and project into the remnant of the ventricular space 

 created by the embryological outpocketing of the brain during formation of 

 the eye. The ciliary origin of vertebrate photoreceptors is further suggested 

 by the connective junction between the outer and inner segments. First 

 described by Sjostrand (1953), the connective has a microstructure similar to 

 that of motile cilia. A pair of ciliary stalks arises from rather typical cen- 

 trioles located in the inner segment. The cilia, composed of nine pairs of 

 microtubules, are arranged in a circle and connect the inner segment with the 

 outer segment. The cilia continue into the outer segment, running about 

 halfway along its length. Stell observed that the outer and inner segments of 

 Squalus are connected not only by a cilium but also by a direct cytoplasmic 

 bridge. He also observed calyceal processes— microvillous structures that 

 extend from the inner segment a short distance over the external part of the 

 outer segments of both rods and cones. Dunn (1973) presented excellent 

 micrographs of the 9 + ciliary connectives in the photoreceptors of 

 Urolophus and Rhinobatos. 



Photoreceptors terminate with specialized synaptic structures differing 

 considerably in rods and cones. For example, the cone synaptic pedicle is 

 broadly conical, somewhat larger, and contains relatively more invaginating 

 contacts than its rod counterpart. The rod pedicle is roughly spherical and 

 on account of this is often termed the "rod spherule." Stell (19726) 

 described both cone pedicles and rod spherules from the retina of Squalus, 

 noting that each contains presynaptic lamellae (synaptic ribbons), as well as 

 synaptic vesicles 400-500 A in diameter (Figure 11). Typically, vertebrate 

 photoreceptors make four types of contacts: (1) receptor-receptor contacts 

 via direct intercommunication between synaptic pedicles; (2) receptor- 

 receptor contacts via cytoplasmic processes between pedicles; (3) basal sur- 

 face contacts from dendrites of horizontal and bipolar cells; and perhaps 

 most important, (4) contacts by dentritic processes of horizontal and bipolar 

 cells that invaginate into the synaptic pedicle (Dunn 1973). Stell described in 

 detail contacts within the rod spherules of Squalus. Invaginating horizontal 

 cell terminals about 0.25 jum in diameter make at least three specialized 

 types of contacts, along with broad unspecialized apposition. Other invagina- 

 ting dendrites, most of which are probably unspecialized bipolar neurons, are 

 granule- and vesicle-free and form characteristic "basal junctions," first 

 described by Lasansky (1969). Specialized contacts between bipolar and 

 horizontal cell dendrites were not observed. Because of their relatively low 

 number, Stell's observations on cone pedicles were fragmentary. However, 

 the synaptic structure of cones was rather similar to that found in rod 

 pedicles. Both rod and cone pedicles extend telodendria to the distal layer of 

 horizontal cells. Telodendria were also found within the external plexiform 

 layer. In Squalus, the synaptic relations of rods and cones appear similar to 

 those of amphibians, reptiles, and birds rather than of fish and mammals. 

 Stell and Witkovsky (19736) also reported, in the retina of the smooth 

 dogfish Mustelus canis, the presence of cones which resembled those of 



