The origin and migration of the earliest-developing sensory neurons in the peripheral nervous system of amphioxus

In amphioxus embryos, the early development of the type 1 sensory neurons was followed by Tlx gene expression and scanning electron microscopy. At the early neurula stage, AmphiTlx transcription is first detectable in a ventral zone of epidermis. In later embryos, expression is also seen in differentiating type 1 receptor cells of the peripheral nervous system, in a few cells in the central nervous system (CNS), and in anterior notochord cells. The type 1 receptors originate from the ventral epidermis by delaminating individually into the subepidermal space, where they can conveniently be visualized because of their strong expression of AmphiTlx. Within the subepidermal space, the apex of each delaminated receptor loses its cilium and microvilli whereas the base of the cell extends lamellate pseudopodia on the surface of the deeper tissues. The pseudopodia become oriented toward the dorsal side of the embryo, the direction of cell migration. After reaching the flanks of the embryo, the receptor cell stops migrating, loses its pseudopodia, and produces an axon that extends to the CNS. As the receptor finishes differentiating, it downregulates Tlx expression and evidently re-inserts its perikaryon into the epidermis. The discussion considers the possible homology of the type 1 receptor neurons of amphioxus with migrating lateral line primordia and hair cells of vertebrates.