Towards understanding the dose and timing effect of hydrocortisone treatment on the scleral ossicle system within the chicken eye

The elusive scleral cartilages: Comparative anatomy and development in teleosts and avians

The sclera of all vertebrate eyes is comprised of connective tissue, with some organisms developing cartilage within this tissue. A review of the cartilages that have been described in the vertebrate sclera and their anatomical relationships is discussed together with their potential homology. Incorrect terminology erroneously implies similarity in location, development, morphology, and evolution, which may lead some scientists to assume all cartilages in orbit are the same elements when reading the literature. Therefore, new terminology to distinguish the different types of cartilage associated with the vertebrate eye is proposed. The scleral cartilages that are likely homologous to one another and which are situated in the sclera, should be termed scleral cartilages sensu stricto, while other cartilages in the sclera should be termed ocular cartilages. Some of the cartilages also ossify, and these bones should be distinguished from the scleral ossicles. The plasticity of the scleral tissue layer and its range of morphologies from fibrous to cartilaginous connective tissue across different vertebrate lineages are also described. This review also highlights several gaps in our understanding of the vertebrate scleral cartilages, in particular.

Quantitative gene expression dynamics of key placode signalling factors in the embryonic chicken scleral ossicle system

Development of the scleral ossicles, a ring of bony elements within the sclera, is directed by a series of papillae that arise from placodes in the conjunctival epithelium over a 1.5-day induction period in the chicken embryo. The regular spacing of the papillae around the corneal-scleral limbus suggests that their induction may be regulated by a reaction-diffusion mechanism, similar to other epithelial appendages. Some key placode signalling molecules, including β-catenin, are known to be expressed throughout the induction period. However, others have been studied only at certain stages or have not been successfully detected. Here we use qPCR to study the gene expression patterns of the wingless integration (WNT)/β-catenin, bone morphogenetic protein (BMP), ectodysplasin (EDA), fibroblast growth factor (FGF) and hedgehog (HH) signalling families in discrete regions of the eye throughout the complete conjunctival placode and papillae induction period. This comprehensive analysis revealed a variable level of gene expression within specific eye regions, with some genes exhibiting high, moderate or low changes. Most genes exhibited an initial increase in gene expression, followed by a decrease or plateau as development proceeded, suggesting that some genes are important for a brief initial period whilst the sustained elevated expression level of other genes is needed for developmental progression. The timing or magnitude of these changes, and/or the overall gene expression trend differed in the temporal, nasal and/or dorsal eye regions for some, but not all genes, demonstrating that gene expression may vary across different eye regions. Temporal and nasal EDA receptor (EDAR) had the greatest number of strong correlations (r > 0.700) with other genes and β-catenin had the greatest number of moderate correlations (r = 0.400-0.700), while EDA had the greatest range in correlation strengths. Among the strongly correlated genes, two distinct signalling modules were identified, connected by some intermediate genes. The dynamic gene expression patterns of the five signalling pathways studied here from conjunctival placode formation through to papillae development is consistent with other epithelial appendages and confirms the presence of a conserved induction and patterning signalling network. Two unique gene expression patterns and corresponding gene interaction modules suggest functionally distinct roles throughout placode development. Furthermore, spatial differences in gene expression patterns among the temporal, nasal and dorsal regions of the eye may indicate that the expression of certain genes is influenced by mechanical forces exerted throughout development. Therefore, this study identifies key placode signalling factors and their interactions, as well as some potential region-specific features of gene expression in the scleral ossicle system and provides a basis for further exploration of the spatial expression of these genes and the patterning mechanism(s) active throughout conjunctival placode and papillae formation.

The Forgotten Skeletogenic Condensations: A Comparison of Early Skeletal Development Amongst Vertebrates

The development of a skeletogenic condensation is perhaps the most critical yet considerably overlooked stage of skeletogenesis. Described in this comprehensive review are the mechanisms that facilitate skeletogenic condensation formation, growth, and maintenance to allow for overt differentiation into a skeletal element. This review discusses the current knowledge of gene regulation and characterization of skeletogenic condensations in the chicken, mouse, zebrafish, and other developmental models. We limited our scope to condensations that give rise to the bones and cartilages of the vertebrate skeleton, with a particular focus on craniofacial and limb bud regions. While many of the skeletogenic processes are similar among vertebrate lineages, differences are apparent in the site and timing of the initial epithelial⁻mesenchymal interactions as well as in whether the condensation has an osteogenic or chondrogenic fate, both within and among species. Further comparative studies are needed to clarify and broaden the existing knowledge of this intricate phenomenon.

Skeletons of the Eye: An Evolutionary and Developmental Perspective

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, 2018. © 2018 American Association for Anatomy.