184
M. A B E R C R O M B I E
number of neighbours with which a cell is in contact is significantly
related to a decrease in its speed. Apart from this relation, the act of
making or breaking a contact (especially the latter) produces a brief
acceleration. Their interpretation was in terms of the mutual interference produced by contacts between cells which are not moving consistently in direction or speed; the contacts are firm though temporary
adhesions. They also suggest that the prohibition of movement in the
direction of any of the contacts (contact inhibition) could contribute to
the effect. Abercrombie and Gitlin (1964) have investigated whether a
completely isolated cell may on the contrary be stimulated in its movement by contact with another cell, but with negative results.
2. Rate of Change of Direction
How cells achieve their changes of direction depends upon their
degree of intrinsic polarization. As already remarked, when an isolated
cell is moving it seems as a rule to be led by an adhesive pseudopodium
terminating in, if not largely consisting of, a ruffled membrane, or
moving filamentous projections. In some cells the pseudopodium is a
relatively permanent structure, and turning then depends on its activity
becoming asymmetrical. The lymphocytes described by De Bruyn
(1945) are such cells during their active phase: they are able to turn
fairly freely (Lewis and Webster, 1921), but when they do so the whole
cell turns, the original polarization being maintained. During their
inactive phase they lose their polarity. A Schwann cell when extended
behaves similarly, though often there is a persistent pseudopodial membrane at both ends (Lubinska, 1961). Amphibian early embryonic
cells are strongly polarized (Holtfreter, 1946; Townes and Holtfreter,
1953). In contrast, a fibroblast has a much less strongly imprinted
polarity. Though it may turn by an asymmetrical action of its leading
ruffled membrane, it often turns by the suppression of the leading
membrane and the development of a quite distinct new polarity. A
macrophage seems to be completely depolarized (De Bruyn, 1945) and
moves in a wildly irregular course when unoriented by its environment,
pseudopodia appearing and disappearing around its periphery.
Obviously these categories of polarization grade into each other, and
cells no doubt change in this respect.
Successful progression by a relatively depolarized cell depends on one
pseudopodium at a time dominating the locomotory mechanism, and
this implies some form of co-ordination within the cell. A fibroblast on
a plane surface, for instance, seems rarely to develop a new ruffled
membrane to an important extent before the old one has ceased its
activity. Weiss and Garber (1952) suggested that the pseudopodia of
fibroblasts compete with each other, and some form of mutual suppres-
M. A B E R C R O M B I E
number of neighbours with which a cell is in contact is significantly
related to a decrease in its speed. Apart from this relation, the act of
making or breaking a contact (especially the latter) produces a brief
acceleration. Their interpretation was in terms of the mutual interference produced by contacts between cells which are not moving consistently in direction or speed; the contacts are firm though temporary
adhesions. They also suggest that the prohibition of movement in the
direction of any of the contacts (contact inhibition) could contribute to
the effect. Abercrombie and Gitlin (1964) have investigated whether a
completely isolated cell may on the contrary be stimulated in its movement by contact with another cell, but with negative results.
2. Rate of Change of Direction
How cells achieve their changes of direction depends upon their
degree of intrinsic polarization. As already remarked, when an isolated
cell is moving it seems as a rule to be led by an adhesive pseudopodium
terminating in, if not largely consisting of, a ruffled membrane, or
moving filamentous projections. In some cells the pseudopodium is a
relatively permanent structure, and turning then depends on its activity
becoming asymmetrical. The lymphocytes described by De Bruyn
(1945) are such cells during their active phase: they are able to turn
fairly freely (Lewis and Webster, 1921), but when they do so the whole
cell turns, the original polarization being maintained. During their
inactive phase they lose their polarity. A Schwann cell when extended
behaves similarly, though often there is a persistent pseudopodial membrane at both ends (Lubinska, 1961). Amphibian early embryonic
cells are strongly polarized (Holtfreter, 1946; Townes and Holtfreter,
1953). In contrast, a fibroblast has a much less strongly imprinted
polarity. Though it may turn by an asymmetrical action of its leading
ruffled membrane, it often turns by the suppression of the leading
membrane and the development of a quite distinct new polarity. A
macrophage seems to be completely depolarized (De Bruyn, 1945) and
moves in a wildly irregular course when unoriented by its environment,
pseudopodia appearing and disappearing around its periphery.
Obviously these categories of polarization grade into each other, and
cells no doubt change in this respect.
Successful progression by a relatively depolarized cell depends on one
pseudopodium at a time dominating the locomotory mechanism, and
this implies some form of co-ordination within the cell. A fibroblast on
a plane surface, for instance, seems rarely to develop a new ruffled
membrane to an important extent before the old one has ceased its
activity. Weiss and Garber (1952) suggested that the pseudopodia of
fibroblasts compete with each other, and some form of mutual suppres-