Searching for the basic pattern of morphogenesis in the inner earstudy in a basal vertebrate, the shark Scyliorhinus canicula

Resumo

The inner ear is the organ responsible for the detection of sound, balance and acceleration. Its origin dates back to the early evolution of the vertebrates, about 600 million years ago, and it exists in all the craniate vertebrates, from agnathans (hagfishes and lampreys) to gnathostomes (jawed vertebrates). The inner ear comprises a dorsal region with a variable number of semicircular canals, each one with a sensory crista, and a ventral region, with different chambers containing sensory otolithic and non-otolithic maculae. The inner ear of all the vertebrates shares a common origin and similar developmental characterisitcs. It originates from the otic placodes, a pair of thickenings of the ectoderm adjacent to the hindbrain. The otic placode invaginates (in lampreys, chondrichthyans and non-reptile tetrapods) or cavitates (in teleosts and reptiles) and becomes the otic vesicle, which enlarges and develops evaginations that originate the ventral chambers and internal protrusions that delimitate the semicircular canals. Despite the common origin and developmental processes shared by all the craniates, the inner ear presents certain particularities in some vertebrate groups, like the chondrichthyans. Chondrichthyans belong to the oldest radiation of the jawed vertebrates. They present a key phylogenetic position in the basis of the gnathostome and closer to the ancestral condition. They present certain special characteristics of its inner ear respect to other gnathostomes, like the almost circularized posterior semicircular canal, the presence of an utricular recess or the maintenance of the open endolymphatic duct in the adulthood. These characteristics make them an ideal model to study the origin, development and evolution of the inner ear, in spite of which studies about the development of its inner ear are scarce. Among chondrichthyans, the catshark Scyliorhinus canicula, a small shark recognized as an emerging model in evolutionary developmental studies, is an ideal subject for morphogenetic studies due to its availability of obtaining embryos, its lengthy development and relative large size of embryos and its easy maintenance under laboratory conditions. In this study we have developed a simple but efficient protocol for contrast-enhancing and scanning samples of the catshark through micro-computed tomography (micro-CT), a 3D imaging technique which provides high-resolution images. This protocol permitted us to obtain highresolution two-dimensional and three-dimensional images of embryos and juveniles of S. canicula at different developmental stages, which allowed us to make a precise and realistic description of the morphogenesis of the inner ear and to establish the three main periods of its development. The three-dimensional description also allowed a better understanding of the spatial expression patterns of different molecular markers studied through immunohistochemistry and in situ hybridization along the development of the catshark. The obtained results increase our knowledge about the development of the inner ear of this model species and also allow the comparison with other model organisms for a better comprehension of the evolution of the vertebrate inner ear.