Morphological characteristics of the developing cranial nerves and mesodermal head cavities in sturgeon embryos from early pharyngula to late larval stages

As sturgeons are considered to represent a basal group of Osteichthyes, it is necessary to evaluate their developmental features to understand the evolution, not only of bony fishes, but also of tetrapods in general. Using Besters, commercially established hybrid sturgeons, the neural crest cell distribution pattern, mesodermal epithelium, and peripheral nerves were observed based on whole-mount immunostained and -sectioned embryos, from the pre-hatching embryonic stage to a late swimming larval stage. At the early pharyngula stage, the hindbrain exhibits at least six rhombomeres. These have a typical arrangement of neuroepithelial cells, and segmentally distributed cephalic crest cell populations associated with even-numbered rhombomeres medially, and single pharyngeal arches laterally. The head cavities first arise as a pair of epithelial primordia in the prechordal region. Secondarily, the cavity is subdivided mediolaterally into the premandibular and mandibular cavities. These mesodermal components never affect the segmental pattern of cranial nerve roots as seen in the shark embryo (Kuratani and Horigome, 2000), probably due to the early degeneration of the cavities. The hyoid cavity never appears. As observed in several teleosts, the newly hatched Bester larva possesses extensive neurites in the epidermis, originating from both trigeminal placodes and Rohon-Beard cells. This neurite network diminishes during development, in concordance with the appearance of lateral line nerves. All the epibranchial placodes are seen as focal, HNK-1-positive epidermal thickenings and give rise to inferior ganglia of the branchiomeric nerves. Metameric morphology of the branchiomeric nerve innervation is secondarily disturbed through modification of the head region, involving the expansion of the operculum and modification of the jaw.