Skeletons of the Eye: An Evolutionary and Developmental Perspective

Abstract

The ocular skeleton, composed of the scleral cartilage and scleral ossicles, is present in many vertebrates. The morphology of the scleral cartilage and ossicles varies within different extant reptiles (including birds) and also varies dramatically from the morphology in extant teleosts. This incredible range of diverse morphologies is the result of millions of years of evolution. Both the position of these elements within the eye and the timing of development vary amongst different vertebrates. While the development of both the scleral cartilage and scleral ossicles is somewhat understood in reptiles and in teleosts, the functional advantage of these elements is still debated. Most reptiles have a multi-component scleral ossicle ring composed of a series of flat bone plates and a scleral cartilage cup lining the retina, some sharks have calcified cartilage plates, and some teleosts have two bones while most others only have a ring of scleral cartilage. The data presented shows that different vertebrates have adapted to similar selective pressures in different ways. However, the reason why sarcopterygians have a series of overlapping bones in the sclera remains unclear. A better understanding of the ocular skeletal diversity in Reptilia as well as a better understanding of the mechanisms of vision within different environments (i.e., air vs. water) and that used by secondarily aquatic organisms is needed. This review discusses the observed variation in morphology and development of the ocular skeleton in the context of evolution and highlights our knowledge gaps in these areas. Anat Rec, 303:100-109, 2020.

A heterochronic shift in skeletal development in the barn owl (Tyto furcata): A description of the ocular skeleton and tubular eye shape formation

“…The scleral cartilage cup provides structural support to the delicate retina. The scleral ossicles either assist in visual accommodation by preventing the eyeball shape from distorting as the cornea changes shape to focus light onto the retina or it also provides structural support (Reference ; reviewed in Reference ). The temporal difference in ocular system development between the barn owl and chicken likely relates to differences in the degree of development of each bird at the time of hatching.…”

“…This is intriguing as the optic cup of all birds develops into a globular‐shaped eyeball in early development. The sclera, which extends from the corneal edge to the back of the eye, houses the annulus ossicularis sclerae (ring of scleral ossicles) anteriorly and a scleral cartilage cup posteriorly . The shape of the eyeball determines the size of the scleral ossicle ring and the scleral cartilage cup .…”

“…The sclera, which extends from the corneal edge to the back of the eye, houses the annulus ossicularis sclerae (ring of scleral ossicles) anteriorly and a scleral cartilage cup posteriorly . The shape of the eyeball determines the size of the scleral ossicle ring and the scleral cartilage cup . The scleral ossicles are longer in birds with longer axial eye lengths (e.g., owls) than in those with shorter axial lengths (e.g., the domestic chicken) as they are positioned at the junction of the cornea and sclera (the scleral‐corneal limbus) …”

“…8,9 The shape of the eyeball determines the size of the scleral ossicle ring and the scleral cartilage cup. 9 The scleral ossicles are longer in birds with longer axial eye lengths (e.g., owls) than in those with shorter axial lengths (e.g., the domestic chicken) as they are positioned at the junction of the cornea and sclera (the scleral-corneal limbus). 9 The development of the ocular skeleton of birds is intriguing.…”

“…9 The scleral ossicles are longer in birds with longer axial eye lengths (e.g., owls) than in those with shorter axial lengths (e.g., the domestic chicken) as they are positioned at the junction of the cornea and sclera (the scleral-corneal limbus). 9 The development of the ocular skeleton of birds is intriguing. The only in-depth studies that have been conducted are in the domestic chicken, Gallus gallus (e.g., References 8,[10][11][12][13][14]etc.).…”