Accessory structures that originate from photoreceptor bundles in the retina of the teleost Notopterus notopterus (Pallas)
Introduction
In certain groups of bony fishes (teleosts), the retinal organisation is peculiar in the sense that the photoreceptor cells (rods and cones) are grouped together into bundles. The individual photoreceptor bundle (PB) is ensheathed by a reflective, retinal tapetum lucidum formed from a part of the modified retinal pigment epithelium (RPE; McEwan, 1938; Ali and Anctil, 1976; Wagner and Ali, 1978; Braekevelt, 1982; Nag, 1993, 2004; Landsberger et al., 2008; Kreysing et al., 2012; Pusch et al., 2013a,b; Francke et al., 2014). Teleosts that possess bundled retinal photoreceptors are generally found to live in dim-light or murky aquatic environments (Nicol, 1989). It has been suggested that bundled photoreceptors with a prominent retinal tapetum lucidum are efficient morphological specialisations of the retina to function in extremely low light condition (Wagner et al., 1998; Landsberger et al., 2008; Kreysing et al., 2012; Francke et al., 2014).
The fine structure of the RPE and PB in teleosts with grouped photoreceptors has been described from time to time by several of workers (references cited above). In those teleosts, in light-adapted retina, the rod outer segments are located at the outer half, and the cone outer segments occupy the inner half of the PB. Individual PB are optically isolated from each other by the RPE, though this may not be the situation with the innermost portion of the PB, where RPE processes do not reach. The present paper reports some accessory structures originating from PB in the retina of Notopterus notopterus, a species known to have grouped retinal photoreceptors (Ali and Anctil, 1976; Nag, 1993, 2004; Francke et al., 2014).
Section snippets
Materials and methods
Healthy specimens of N. notopterus (Teleostei: Osteoglossiformes) were procured from local dealers in Kolkata, eastern India. Their standard length ranged from 12 to 18.7 cm, n = 4). They were reared in a glass aquarium for 3 days under 12 h light: 12 h dark photoperiod (light intensity: 300 lx) and fed with dry fish pellets and worms. The light-adapted specimens were mildly anesthetized with MS 222 (Sigma-Aldrich Corporation, MO, USA) dissolved in water and sacrificed during mid-day (12 noon)
Results and discussion
Light micrographs of light-adapted retina of N. notopterus are shown in Fig. 1, Fig. 2. The photoreceptor cells are arranged into bundles (PB), showing an outer and an inner portion (Fig. 1A). The RPE has few melanin granules that are located towards its basal (not shown) and apical aspect (Figs. 1C, D, 2 D); the bulk of the RPE is filled with crystals that form the retinal tapetum lucidum (Figs. 1B–D, 2 B–D). The fine, apical processes of the RPE is intimately associated with the apical end of
Conflict of interest
None.
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