From axolotl to zebrafish: a comparative approach to the study of thyroid involvement in ocular development.

While discussion of interactions between the thyroid gland and the eye mostly focus on thyroid eye disease, the involvement of the thyroid and its hormones on development of the eye before birth is also an important perspective that should not be forgotten. Experimental models involving amphibians and fish are valuable both because the developing larvae are not constrained within the adult but can be observed and manipulated as they grow autonomously and also because they metamorphose with substantial morphological changes driven by the hypothalamo-pituitary-thyroid axis occurring as they develop from eggs to adults. In this paper we will first discuss changes seen in the axolotl, a naturally occurring neotonous hypothyroid salamander and the Xenopus toad a widely used laboratory amphibian. Secondly we will document ocular changes in flatfish which exhibit remarkable anatomical alterations during their change from a pelagic to benthic lifestyle. Finally we will evaluate ocular changes in developing zebrafish when subjected to thyroid-disrupting chemicals. These experiments show the influence of the thyroid on ocular development when thyroid function is altered which may be of value in determining changes in human hypothyroid infants but are also important to note as these chemicals are widely used in plastics and also as flame retardants used to control wildfires. Run-off into water courses can damage both wildlife and humans consuming contaminated water and thyroid disruption may have significant effects on reproduction and development, although influences on ocular morphology have yet to be investigated.