Mucus cell and ionocyte precursors migrate between epithelial cells to disperse through the zebrafish epidermis

Mucus-secreting cells and ionocytes play critical roles in many organs. Both cell types are usually distributed as scattered, solitary cells within an epithelium, a pattern presumably optimal for their function. To investigate how they attain their dispersed distributions, we imaged mucus cell and ionocyte precursors in the epidermis of developing zebrafish. Previous reports found that precursors of both cell types are first detected in the ventral epidermis (covering the yolk) before spreading dorsally, but the mechanism driving this progression was unknown. Photoconverting basal epidermal cells in the ventral embryo revealed that some cells actively migrate away from this area to populate the rest of the epidermis. These cells lose their basal cell identity when they start migrating and begin expressing markers of mature mucus cells or ionocytes during migration. These cells travel entirely between the two epithelial layers of the epidermis, occasionally pause migration to divide, and repel one another through contact inhibition of locomotion, behaviors that likely aid in their dispersal. After migrating for about a day, mucus cell and ionocyte precursors intercalate into the superficial epithelial layer of the epidermis, where they complete differentiation. These observations reveal how mucus cells and ionocytes achieve their scattered distributions in the zebrafish epidermis, suggesting that similar processes promote their distribution in other mucosal organs.