Development of the lateral line system in juvenile Siberian sturgeon ( Acipenser baerii )

The Siberian sturgeon ( Acipenser baerii Brandt), a chondrostean, occupies an important position in the evolution of the electroreceptor. In order to more fully understanding the evolution of these receptors, we examined the development of the lateral line system during early ontogeny of the Siberian sturgeon by using light and scanning electron microscopes. We detected four major events in this process: the lateral line placodal development, the sensory ridge formation, the receptor formation and the canal formation. On day 1 of post hatching, all six lateral line placodes are present and the posterior lateral line placode starts actively migrating posteriorly along the mid-line of the trunk, depositing neuromasts at intervals on the way of migration. The other lateral line placodes elongate to form sensory ridges according to its destination line pattern over the head, all containing primordial neuromasts. By day 7, ampullary organs rise from the lateral zones of the ventral of the head, though this may lag up to one week behind of that of the neuromasts. On day 9, the epidermis under the neuromast slowly invaginates, and the bony lateral line canals begin to form. Towards day 29, the epidermal cells surrounded some single openings of the ampullary organs at the ventral surface of the head, begin to migrate, and then transform into 3 to 4 aggregate openings. By this point, abundant microvilli are visible on the surface of the receptor epithelium, similar to the structure in elasmobranches and other sturgeons. On the day 57 of post hatching, the trunk canal is fully embedded into the lateral scutes. By then, the majority of ampullary organs are highly concentrated on the ventral rostrum, arranged in clusters of 3−4, distributing closely such as the shape of quincunx, thus completing the formation of the lateral line system