Ocular elongation and retraction in foveated reptiles

Abstract

Background: Pronounced asymmetric changes in ocular globe size during eye development have been observed in a number of species ranging from humans to lizards. In contrast, largely symmetric changes in globe size have been described for other species such as rodents. We propose that asymmetric changes in the three-dimensional structure of the developing eye correlate with the types of retinal remodeling needed to produce areas of high photoreceptor density. As a test of this idea, we systematically examined three-dimensional aspects of globe size as a function of eye development in the bifoveated brown anole, Anolis sagrei. Results: During embryonic development, the anole eye undergoes dynamic changes in ocular shape. Initially spherical, the eye elongates in the presumptive foveal regions of the retina and then proceeds through a period of retraction that returns the eye to its spherical shape. During this period of retraction, pit formation and photoreceptor cell packing are observed. We found a similar pattern of elongation and retraction associated with the single fovea of the veiled chameleon, Chamaeleo calyptratus. Conclusions: These results, together with those reported for other foveated species, support the idea that areas of high photoreceptor packing occur in regions where the ocular globe asymmetrically elongates and retracts during development.

Development and retinal remodeling in the brown anole lizard (Anolis sagrei)

“…2 of Rasys et al (2021b)). By late stage 3, an increase in the number of cells dividing in the central region of the retina is observed, and by stage 4, a slight mounding to the retina (i.e., increased neural retina thickness) is detected centrally (data not shown; see Anolis Eye Development poster from Rasys et al (2021a)). As the embryo enters stage 5 of development, the eye has grown considerably in overall size and regional differences in both morphology and the relative numbers of mitotic figures become clearly evident (Fig.…”

“…This change in length is accompanied by a regional decrease in the cellular density of the neural retina, which is noticeable as a thinning of the GCL (Rasys et al, 2021a). By stage 15 the ocular globe begins to retract, concomitant with regional increases in cellular density (Rasys et al, 2021a). By the time of hatching, the GCL is 4-6 cells deep in all retinal regions except that of the central fovea, which is devoid of ganglion cells due to lateral displacement (Fig.…”

“…We began by looking at the development of the foveae of the retina. Formation of the foveae occur during the last week of embryonic development in anoles, between stages 16-18; it is during this time period that the eye is actively undergoing ocular retraction (Rasys et al, 2021a). As the foveae form, they do so through the lateral displacement of the retina nuclear layers and the movement of photoceptor cells towards the foveal centers.…”

“…Anole embryos develop over a 30-33 day period when incubated at 28˚C. Embryos were collected from eggs at various timepoints after egg lay and removed from their egg shells following the protocol described by Rasys et al (2021a) and staged according to Sanger et al, 2008guidelines (Sanger et al, 2008b. Eyes were collected from 4 plus embryos from each developmental stage.…”

“…Therefore, we are developing the brown anole lizard (Anolis sagrei) as a foveated model system for research (Rasys et al, 2021a, Rasys et al, 2019a, Rasys et al, 2019b, Rasys et al, 2021b. This lizard has a bifoveated retina, possessing a large prominent central fovea and second, much shallower, temporal fovea (Rasys et al, 2021a, Fite and Lister, 1981, Makaretz and Levine, 1980, Sannan et al, 2018, Underwood, 1970, Walls, 1942. Remarkably, during embryonic development anole eyes undergo dynamic changes in their ocular shape (Rasys et al, 2021a).…”