Evolution and development of epithelial appendages in the jawed vertebrates

Epithelial appendages of the integument comprise a remarkably diverse group of structures that have facilitated the evolutionary adaptation of animal life to wide-ranging ecological niches. They include numerous appendages adorning the skin, such as scales, spines, feathers, hair and teeth. In many cases, for example with feathers and hair, they constitute clade-defining characteristics. This thesis aims to examine how alterations to the shared developmental mechanisms underpinning the formation of these structures can explain how their incredible evolutionary diversity has arisen. Previous research into epithelial appendage development has broadly concerned the appendages of two classic vertebrate models: feathers of the chicken embryo and hair of the mouse embryo. I aim to compliment this research through developmental comparisons with the epithelial appendages of an emerging model cartilaginous fish, the small-spotted catshark (Scyliorhinus canicula). Sharks have epithelial appendages known as dermal denticles which cover the body. These units are structurally homologous to vertebrate teeth. Denticle-like structures have been observed in the fossil record from as long as 450 million years ago. They facilitate a plethora of functions, including the provision of drag reduction and protective armour. Here, I compare aspects of shark denticle patterning, initiation and morphogenesis to avian epithelial appendage development. This enables conclusions to be drawn regarding both the conservation and divergence of different aspects of epithelial appendage development throughout the gnathostomes (jawed vertebrates). Overall, I demonstrate that small alterations to broadly conserved developmental systems and genetic circuitry contribute to the incredible diversity of epithelial appendages we observe in nature.