Connectivity of cone photoreceptor telodendria in the zebrafish retina
Telodendria were first observed in the human retina, and noted to be a common feature of photoreceptors in the 19th century (Cajal, 1893; Schultze, 1866). Since then, photoreceptor telodendria have been reported in many vertebrates, such as reptiles (Baylor, Fuortes, & O'Bryan, 1971; Copenhagen & Owen, 1976a; Detwiler & Hodgkin, 1979; Firsov & Green, 1998; Goede & Kolb, 1995; Kolb & Jones, 1984; Normann, Perlman, Kolb, Jones, & Daly, 1984; Ohtsuka & Kawamata, 1990; Owen, 1985), amphibians (Custer, 1973; Gold & Dowling, 1979; Zhang & Wu, 2004), fish (Hess, Melzer, & Smola, 2002; Kraft & Burkhardt, 1986; Li et al., 2009; O'Brien et al., 2004; Stell, 1972), and mammals (Ahnelt, Keri, & Kolb, 1990; DeVries, Qi, Smith, Makous, & Sterling, 2002; Kolb, 1977; Kolb & West, 1977; Smith, Freed, & Sterling, 1986), including non‐human primates (Hornstein, Verweij, Li, & Schnapf, 2005; O'Brien et al., 2012; Raviola & Gilula, 1975). It was noted that these photoreceptor projections were forming gap junctions with their targets based on inferences from ultrastructural analysis, the presence of connexin, and electrophysiological assessment, suggesting that photoreceptors are indeed using these processes to communicate with other photoreceptors (Baylor et al., 1971; Copenhagen & Owen, 1976a; Detwiler & Hodgkin, 1979; DeVries et al., 2002; Kolb, 1977; Kolb & Jones, 1984; Kolb & West, 1977; Kraft & Burkhardt, 1986; Li et al., 2009; Normann, Perlman, & Daly, 1985; Normann et al., 1984; O'Brien et al., 2004; Owen, 1985; Raviola & Gilula, 1975; Smith et al., 1986; Zhang & Wu, 2004). The function of telodendrial connections remains ambiguous, though they are speculatively thought to improve cone sensitivity and play a role in specific light situations (such as crepuscular vision) to reduce background and increase visual acuity (DeVries et al., 2002; Hornstein et al., 2005; O'Brien et al., 2012; Smith et al., 1986) and reduce signal‐to‐noise (Attwell, Wilson, & Wu, 1985; Gold & Dowling, 1979; Lamb & Simon, 1976).