Understanding the spacing of placodes in the eye: A comparative study across age and species

The conjunctival placodes of the avian eye form in an intriguing and conserved sequence in a circular annulus around the cornea. These 13-16 placodes develop into papillae that are essential for inducing underlying intramembranous flat bones, known as scleral ossicles, which form an important part of the ocular skeleton. The numbers of papillae present are used in almost all bird staging tables as an important anatomical feature for accurate staging. Despite this, we are lacking knowledge on how these papillae are induced and how the regular spacing between these placodes/papillae is patterned, both within chicken embryos and in other bird species. In this study, we conduct a comparative assessment of the spacing of conjunctival placodes during ontogeny in chicken embryos and compare this to the spacing in quail and ducks, which develop at different rates and have different body sizes. Our results indicate that the average spacing between papillae changes over development in chicken embryos, but that no change in spacing occurs in the first set of induced placodes (group 1). We further show that placode spacing differs even in birds with similar eye diameters and placode/papillae number (quail vs. duck). The position of the placode ring with respect to the cornea also varies in these species. Interestingly, despite different incubation durations, all three species develop placodes at 28%-33% of their incubation period. Overall, our data indicate that mathematical models describing the patterning of these regularly spaced placodes should consider both eye growth and papillae ring position.