Abstract
A well-developed visual system can provide significant sensory information to guide motor behavior, especially in fruit-eating bats, which usually use echolocation to navigate at high speed through cluttered environments during foraging. Relatively few studies have been performed to elucidate the organization of the visual system in bats. The present work provides an extensive morphological description of the retinal projections in the subcortical visual nuclei in the flat-faced fruit-eating bat (Artibeus planirostris) using anterograde transport of the eye-injected cholera toxin B subunit (CTb), followed by morphometrical and stereological analyses. Regarding the cytoarchitecture, the dorsal lateral geniculate nucleus (dLGN) was homogeneous, with no evident lamination. However, the retinal projection contained two layers that had significantly different marking intensities and a massive contralateral input. The superior colliculus (SC) was identified as a laminar structure composed of seven layers, and the retinal input was only observed on the contralateral side, targeting two most superficial layers. The medial pretectal nucleus (MPT), olivary pretectal nucleus (OPT), anterior pretectal nucleus (APT), posterior pretectal nucleus (PPT) and nucleus of the optic tract (NOT) were comprised the pretectal nuclear complex (PNT). Only the APT lacked a retinal input, which was predominantly contralateral in all other nuclei. Our results showed the morphometrical and stereological features of a bat species for the first time.
Highlights
The order Chiroptera is the second most diverse taxon among the class Mammalia, comprising approximately 1,100 living species (Simmons, 2005)
To provide a comparative morphometric analysis between neurons in the various subdivisions of the studied nuclei, we considered large neurons to be those with areas ranging between 150 μm2 and 100 μm2
Medium neurons were considered to be those with areas between 99 μm2 and 50 μm2, and small neurons were considered to be those with areas under 50 μm2
Summary
The order Chiroptera is the second most diverse taxon among the class Mammalia, comprising approximately 1,100 living species (Simmons, 2005). Functional studies have shown evidence of the importance of visual cues under specific environmental conditions, such as luminosity, foraging behavior, predator avoidance and long-range navigation, in bats (Chase, 1981; Greif et al, 2014; Gutierrez et al, 2014), suggesting congruent functions between echolocation and visual information to mediate goal-directed orienting movements (Hoffmann et al, 2016). These recently identified novelties in bat visual functions have created excitement in the scientific community and generated an interest in better understanding the neural framework that underlies the visual system in bats (Melin et al, 2014; Butz et al, 2015; Scalia et al, 2015). Several features of the neural arrangement have been neglected in the bat visual neuroanatomy, e.g., regarding the morphologies of the retinal fibers and varicosities that project to a given downstream region (see Sherman and Guillery, 2011) because slight morphological variations in retinal fibers can dramatically alter visual functions (Gauvain and Murphy, 2015)
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