When provided with a suitable solid substrate, tissue cells undergo a
rapid conversion from the spherical form expressed in suspension culture to
a characteristic flattened morphology. As a result of this conversion,
called cell spreading, the cell nucleus and organelles come to occupy a
central region of “deep cytoplasm” which slopes steeply into a peripheral
“lamellar” region less than 1 pm thick at its outer edge and generally free
of cell organelles. Cell spreading is accomplished by a continuous outward
repositioning of the lamellar margins. Cell translocation on the substrate
results when the activity of the lamellae on one side of the cell become
dominant. When this occurs, the cell is “polarized” and moves in the
direction of the “leading lamellae”. Careful analysis of tissue cell
locomotion by time-lapse microphotography (1) has shown that the
deformational movements of the leading lamellae occur in a repeating cycle
of advance and retreat in the direction of cell movement and that the rate
of such deformations are positively correlated with the speed of cell
movement. In the present study, the physical basis for these movements of
the cell margin has been examined by comparative light microscopy of living
cells with whole-mount electron microscopy of fixed cells. Ultrastructural
observations were made on tissue cells grown on Formvar-coated grids, fixed
with glutaraldehyde, further processed by critical-point drying, and then
photographed in the High Voltage Electron Microscope. This processing and
imaging system maintains the 3-dimensional organization of the whole cell,
the relationship of the cell to the substrate, and affords a large sample
size which facilitates quantitative analysis. Comparative analysis of film
records of living cells with the whole-cell micrographs revealed that
specific patterns of microfilament organization consistently accompany
recognizable stages of lamellar formation and movement. The margins of
spreading cells and the leading lamellae of locomoting cells showed a
similar pattern of MF repositionings (Figs. 1-4). These results will be
discussed in terms of a working model for the mechanics of lamellar motility
which includes the following major features: (a) lamellar protrusion results
when an intracellular force is exerted at a locally weak area of the cell
periphery; (b) the association of cortical MFs with one another determines
the local resistance to this force; (c) where MF-to-MF association is weak,
the cell periphery expands and some cortical MFs are dragged passively
forward; (d) contact of the expanded area with the substrate then triggers
the lateral association and reorientation of these cortical MFs into MF
bundles parallel to the direction of the expansion; and (e) an active
interaction between these MF bundles associated with the cortex of the
expanded lamellae and the cortical MFs which remained in the sub-lamellar
region then pulls the latter MFs forward toward the expanded area. Thus, the
advance of the cell periphery on the substrate occurs in two stages: a
passive phase in which some cortical MFs are dragged outward by the force
acting to expand the cell periphery, and an active phase in which additional
cortical MFs are pulled forward by interaction with the first set.
Subsequent interactions between peripheral microfilament bundles and
filaments in the deeper cytoplasm could then transmit the advance gained by
lamellar expansion to the bulk of the cytoplasm.
