Abstract
We used immunocytochemistry to determine the presence and topographical density distributions of rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) in the four-striped field mouse (Rhabdomys pumilio) and the Namaqua rock mouse (Micaelamys namaquensis). Both species possessed duplex retinas that were rod dominated. In R. pumilio, the density of both cones and rods were high (cone to rod ratio: 1:1.23) and reflected the species’ fundamentally diurnal, but largely crepuscular lifestyle. Similarly, the ratio of cones to rods in M. namaquensis (1:12.4) reflected its nocturnal lifestyle. Similar rod density peaks were observed (R. pumilio: ~84467/mm2; M. namaquensis: ~81088/mm2), but a density gradient yielded higher values in the central (~56618/mm2) rather than in the peripheral retinal region (~32689/mm2) in R. pumilio. Two separate cone types (S-cones and M/L-cones) were identified implying dichromatic color vision in the study species. In M. namaquensis, both cone populations showed a centro-peripheral density gradient and a consistent S- to M/L-cone ratio (~1:7.8). In R. pumilio, S cones showed a centro-peripheral gradient (S- to M/L-cone ratio; central: 1:7.8; peripheral: 1:6.8) which appeared to form a visual streak, and a specialized area of M/L-cones (S- to M/L-cone ratio: 1:15) was observed inferior to the optic nerve. The number of photoreceptors per linear degree of visual angle, estimated from peak photoreceptor densities and eye size, were four cones and 15 rods per degree in M. namaquensis and 11 cones and 12 rods per degree in R. pumilio. Thus, in nocturnal M. namaquensis rods provide much finer image sampling than cones, whereas in diurnal/crepuscular R. pumilio both photoreceptor types provide fine image sampling. IpRGCs were comparably sparse in R. pumilio (total = 1012) and M. namaquensis (total = 862), but were homogeneously distributed in M. namaquensis and densest in the dorso-nasal quadrant in R. pumilio. The adaptive significance of the latter needs further investigation.
Highlights
The vertebrate eye and in particular the retina, acts as a mediator between the light environment and the brain of an animal, converting external light stimuli into biological signals
We present and compare the size of the eyes, the density and topographical distributions of rods, cones and intrinsically photosensitive retinal ganglion cells (ipRGCs) in two murid rodents that inhabit different photoenvironments; the diurnally active Rhabdomys pumilio and the nocturnally active Micaelamys namaquensis
The presence and spatial distribution of rods, cones and ipRGCs in R. pumilio and M. namaquensis retinae are described in the present study
Summary
The vertebrate eye and in particular the retina, acts as a mediator between the light environment and the brain of an animal, converting external light stimuli into biological signals (neural impulses). The recognition of the ipRGCs as distinct photoreceptors was first suggested by studies showing normal photoentrainment of the circadian system in mice that lacked all functional rods and cones (rd/rd cl mice) [1,6,7]. As a result, it has been concluded in the past that conventional photoreceptors do not contribute to non-image forming vision. Studies have since shown that ipRGCs receive input from rods and cones for non-imageforming processes, but that they are involved in image-forming vision [8,9,10,11]
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