Hunting needles in a haystack: Migrating precursors of epidermal sensory neurons in amphioxus found with serial blockface scanning electron microscopy (SBSEM)

Migrating precursors of epidermal sensory neurons are a relatively rare cell type in developing amphioxus. Serial blockface scanning electron microscopy (SBSEM) was used here to locate these cells and reconstruct them in three dimensions in the context of the surrounding tissues. The reconstructions showed that, in contrast to some earlier reports, these cells are not located in an appreciable subepidermal space while they are crawling dorsally up the flank of the animal. Instead, they migrate while tightly sandwiched between overlying epidermal cells and the underlying basal lamina. The cytoplasmic region at the leading edge of the crawling cell has some characteristics of a large podosome, an F‐actin‐rich organelle with prominent roles in adhesion, matrix degradation and cell migration. The discussion includes a comparison of the precursors of epidermal sensory cells of amphioxus with migrating neurogenic cells from vertebrate placodes.