Protrusive activity, mode and rate of locomotion, and pattern of adhesion of Fundulus deep cells during gastrulation

The mechanism of locomotion of deep cells of Fundulus heteroclitus was studied in vivo during gastrulation with the aid of time‐lapse cinemicrography (Nomarski differential interference contrast optics), scanning electron microscopy of cells known to be moving at the time of fixation, and cell culture. These are our findings: (1) Deep cells usually move rapidly, at about 10–15 μm/min, regardless of whether they move by blebbing or spreading. Evidence suggests that this high speed is associated with weak adhesion of the trailing edge: it remains rounded, without large retraction fibers, and it advances continuously with advance of the leading edge, not sporadically, as it would if it adhered strongly. (2) In contrast, when stationary cells in close contact separate, they remain connected at first by large retraction fibers, suggesting strong punctate adhesions. (3) Fundulus deep cells appear to move in two ways: a) protrusion of blebs, followed by much cytoplasmic flow; b) spreading of lamellipodia, preceded by filopodia and frequent cell shortening. Locomotion by shortening of a long lobopodium is really a form of spreading movement, since the tip of a lobopodium always spreads. (4) Filopodia were not found except at the leading edge of a spreading lamellipodium and often spread themselves; perhaps filopodia and lamellipodia are interconvertible. (5) A lamellipodial margin may form undulations in vivo that move backward like ruffles in vitro. (6) At all times, whether stationary or moving, the surface of any given deep cell is smooth, raising unanswered questions concerning the source of surface for their rapid protrusive activity.