Directional cell movement during early development of the teleost Blennius pholis: I. Formation of epithelial cell clusters and their pattern and mechanism of movement

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Abstract
Embryos of the teleost Blennius pholis provide exceptional material for observation of the formation and movement of cell clusters in vivo because the clusters are packed with melano-somes and migrate beneath the transparent enveloping layer. These clusters arise from two pigmented cell masses (PCM) which appear precociously on either side of the embryonic axis at 3/5 epiboly, at the future level of somites 1 and 2. As development proceeds, each PCM enlarges and spreads on its lateral margins to form an epithelial sheet. As spreading continues, the sheet fragments, forming small cell clusters that move in a distad direction in the yolk sac. The highly motile lateral marginal cells of the spreading PCM, as well as those of the marginal cells of each moving cluster, invariably protrude highly flattened lamellipodia, which terminate in long, fine, often branched filopodia. As cell clusters leave the PCM, they form long, taut retraction fibers. The rate of spreading of both the lateral edge of the PCM and the initial phase of cluster movement, is higher (1.0 μm/min or greater) than the later rate of cluster movement, apparently because at this phase, motile activity is confined to the distal borders of each. This directional migration ceases in 24 h at 16°–18°C, when the farthest clusters have reached the ventral region of the yolk sac. By then, all clusters are spaced more or less evenly, apparently due to cessation of all cluster movement at about the same time. Once movement ceases, the clusters remain immobile for 2–4 days, depending on the temperature.