Locomotion ofFundulusDeep Cells during Gastrulation

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Abstract
SYNOPSIS. 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 μ/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 by retraction fibers, suggesting strong punctate adhesions. 3) Locomotion by shortening of a long lobopodium is really a form of spreading movement; the tip of a lobopodium always spreads. Also, since speed of shortening decreases with continuance, it may depend primarily on elastic recoil rather than active contraction. 4) Fundulus deep cells appear to move in two ways: a) protrusion of blebs, followed by much cytoplasmic flow; b) protrusion of lamellipodia, accompanied by filopodia and frequent cell shortening. 5) Filopodia were not found except at the leading edge of a spreading lamellipodium and often spread themselves; perhaps filopodia and lamellipodia are interconvertible. 6) A lamellipodial margin may form undulations in vivo that move backward like ruffles in vitro. 7) At all times, whether stationary or moving, the surface of deep cells is smooth, raising unanswered questions concerning the source of surface for their rapid protrusive activity.

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