Directional cell movement during early development of the teleost Blennius pholis: II. Transformation of the cells of epithelial clusters into dendritic melanocytes, their dissociation from each other, and their migration to and invasion of the pectoral fin buds.

After clusters of pigmented epithelial cells have rested immobile in the yolk sac of Blennius pholis for 2-4 days (Trinkaus, '88), their constituent cells transform into mesenchymal, dendritic melanocytes. Then these melanocytes dissociate from one another and migrate directionally toward the developing pectoral fin bud (PFB) on either side. Each of these changes takes place in a proximodistal sequence, starting with the epithelial cluster closest to each PFB. Even individual clusters conform to this sequence, the proximal side dissociating first. Eventually, all melanocytes reach and invade the PFB. This is a 100% efficient morphogenetic cell movement. At the completion of this developmental sequence, each PFB is filled with melanocytes arranged in an arc with their filopodia extending outward and the yolk sac is bereft of pigment cells. The form and surface activity of these cells in relation to their motility and to their rate of movement are considered in detail. Attempts to understand the forces involved in giving directionality to these cell migrations are described. Finally, the significance of these observations for morphogenetic cell movements generally and for the relation between epithelial and mesenchymal cells is discussed briefly.