5 . T H E L O C O M O T O R Y B E H A V I O U R OF CELLS
193
I I I . C E L L P O P U L A T I O N S
Having considered locomotory responses from the point of view of
the individual cell, we may now turn to the interpretation of the
behaviour of cell populations in vitro in terms of the individual responses.
T o consider the population is to emphasize interaction between the
cells, or social behaviour, which is probably always a mixture of
oriented and unoriented responses; and this social behaviour leads to
large-scale patterns of cellular distribution and cellular movement
within the population. We will consider these patterns under three
headings: dispersive behaviour, which tends to reduce local concentrations of cells; associative behaviour, tending to do the opposite; and
directional trends of movement, correlated between many cells.
A. DISPERSIVE B E H A V I O U R
Mere random diffusive movements of cells will disperse a focus of
raised concentration of cells, such as for instance an explant, tending to
establish a similar concentration throughout the available medium.
Kineses will increase or diminish the speed of this diffusive dispersion.
Even when the population concentration is initially similar throughout
a culture, a local increase of speed will locally diminish it. A pure
kinesis therefore can have some effect on cell distribution. Oriented
responses produced by cell interaction cannot, however, be assumed to
be negligible in dispersive behaviour. Mutual interference, simply from
the fact that two cells cannot occupy the same space, will have some
importance. Possibly some sarcoma cells, such as mouse S37, may show
little more in the way of mutual orientation than this; and the crowded
and irregular arrangement of other kinds of malignant cells when dispersing from a focus in culture (Santesson, 1935; Macpherson and
Stoker, 1962) indicates the predominance of random diffusive movement in these cells.
Usually, however, a more important oriented response of cells away
from each other is obviously at work. Any such mutual reaction will
tend to give the cells a spatial distribution that is non-random in that it
is too regular. The regularity is manifested on a plane surface by monolayering. The interaction may be a response to a diffusing stimulus, as
in the Twitty-Niu reactions of melanoblasts. Such a chemotaxis tends
to drive cells apart: it acts at a distance. Or it may be in response to
contact as in fibroblasts, contact inhibition being the reaction in
question. This does not drive cells apart; it merely prevents closer
approach than contact, and hence "selects" movement apart, though
only up to the point where the cells lose contact. The highly regular
H
193
I I I . C E L L P O P U L A T I O N S
Having considered locomotory responses from the point of view of
the individual cell, we may now turn to the interpretation of the
behaviour of cell populations in vitro in terms of the individual responses.
T o consider the population is to emphasize interaction between the
cells, or social behaviour, which is probably always a mixture of
oriented and unoriented responses; and this social behaviour leads to
large-scale patterns of cellular distribution and cellular movement
within the population. We will consider these patterns under three
headings: dispersive behaviour, which tends to reduce local concentrations of cells; associative behaviour, tending to do the opposite; and
directional trends of movement, correlated between many cells.
A. DISPERSIVE B E H A V I O U R
Mere random diffusive movements of cells will disperse a focus of
raised concentration of cells, such as for instance an explant, tending to
establish a similar concentration throughout the available medium.
Kineses will increase or diminish the speed of this diffusive dispersion.
Even when the population concentration is initially similar throughout
a culture, a local increase of speed will locally diminish it. A pure
kinesis therefore can have some effect on cell distribution. Oriented
responses produced by cell interaction cannot, however, be assumed to
be negligible in dispersive behaviour. Mutual interference, simply from
the fact that two cells cannot occupy the same space, will have some
importance. Possibly some sarcoma cells, such as mouse S37, may show
little more in the way of mutual orientation than this; and the crowded
and irregular arrangement of other kinds of malignant cells when dispersing from a focus in culture (Santesson, 1935; Macpherson and
Stoker, 1962) indicates the predominance of random diffusive movement in these cells.
Usually, however, a more important oriented response of cells away
from each other is obviously at work. Any such mutual reaction will
tend to give the cells a spatial distribution that is non-random in that it
is too regular. The regularity is manifested on a plane surface by monolayering. The interaction may be a response to a diffusing stimulus, as
in the Twitty-Niu reactions of melanoblasts. Such a chemotaxis tends
to drive cells apart: it acts at a distance. Or it may be in response to
contact as in fibroblasts, contact inhibition being the reaction in
question. This does not drive cells apart; it merely prevents closer
approach than contact, and hence "selects" movement apart, though
only up to the point where the cells lose contact. The highly regular
H