190
M . A B E R C R O M B I E
sheet, which is led by an active ruffled membrane (Harrison, 1910),
must as a rule stop when it meets a similar cell in the opposing edge,
since the two sheets become confluent and the previous mass cellular
movement that produced the collision stops (Howes, 1943; Lash, 1955).
The mere existence of unstratified epithelia is in accord with the idea
that all the cells in such a sheet are showing contact inhibition to each
other, which prevents superimposition.
The precise cytological analysis of epithelial behaviour requires tissue
culture, and the investigation has yet to be made on normal cells.
Observations by Trevan and Roberts (1960) on an ascites-adapted
epithelioma demonstrated that these cells do not become superimposed
when moving on a plane surface. The mechanism, however, was
different from that of contact inhibition in fibroblasts. When the ruffled
membranes of two cells met, one of them lost its activity and withdrew,
while the other continued. This was interpreted as the result of an
undercutting of one membrane by the other, breaking its adhesion to
the substrate.
Macrophages, monocytes, lymphocytes and polymorphs, although
they show no discernible contact inhibition by fibroblasts (Oldfield,
1963), do not overlap each other when they are moving on a plane
surface. The interpretation is complicated by the possible existence of a
mutual negative chemotaxis (see above). But collisions are nevertheless
fairly frequent, so it is likely that a contact reaction is also involved.
Behaviourally this would be the same as the reaction between fibroblasts. It involves very little mutual adhesion however, and the locomotory mechanisms seem so disparate that the cellular processes in the
reaction may well be different too.
Contact inhibition as it occurs in fibroblasts, even though it may act
through mutual adhesion, usually involves a redirection of the cell away
from the point of contact, and certainly stops further approach. There
is no doubt that there exists a large and important class of contactdependent phenomena in which the cells are stopped from moving
away from the point of contact, or are actually drawn towards the point
of contact. The effect of these responses is then in one respect the
opposite of the effect of contact inhibition. Lack of information makes
it impossible as yet to analyse the cell reactions involved, and hence to
relate these phenomena clearly to each other, but a number of them
will be mentioned when associative behaviour of cell populations is
discussed (p. 194).
(c) Electrical stimuli. One other quite different stimulus, resulting from
the application of electrical currents to cultures, has been the subject
of experimental work. Ingvar (1920) reported positive results, but
Weiss (1934) was unable to confirm them, and pointed out how
M . A B E R C R O M B I E
sheet, which is led by an active ruffled membrane (Harrison, 1910),
must as a rule stop when it meets a similar cell in the opposing edge,
since the two sheets become confluent and the previous mass cellular
movement that produced the collision stops (Howes, 1943; Lash, 1955).
The mere existence of unstratified epithelia is in accord with the idea
that all the cells in such a sheet are showing contact inhibition to each
other, which prevents superimposition.
The precise cytological analysis of epithelial behaviour requires tissue
culture, and the investigation has yet to be made on normal cells.
Observations by Trevan and Roberts (1960) on an ascites-adapted
epithelioma demonstrated that these cells do not become superimposed
when moving on a plane surface. The mechanism, however, was
different from that of contact inhibition in fibroblasts. When the ruffled
membranes of two cells met, one of them lost its activity and withdrew,
while the other continued. This was interpreted as the result of an
undercutting of one membrane by the other, breaking its adhesion to
the substrate.
Macrophages, monocytes, lymphocytes and polymorphs, although
they show no discernible contact inhibition by fibroblasts (Oldfield,
1963), do not overlap each other when they are moving on a plane
surface. The interpretation is complicated by the possible existence of a
mutual negative chemotaxis (see above). But collisions are nevertheless
fairly frequent, so it is likely that a contact reaction is also involved.
Behaviourally this would be the same as the reaction between fibroblasts. It involves very little mutual adhesion however, and the locomotory mechanisms seem so disparate that the cellular processes in the
reaction may well be different too.
Contact inhibition as it occurs in fibroblasts, even though it may act
through mutual adhesion, usually involves a redirection of the cell away
from the point of contact, and certainly stops further approach. There
is no doubt that there exists a large and important class of contactdependent phenomena in which the cells are stopped from moving
away from the point of contact, or are actually drawn towards the point
of contact. The effect of these responses is then in one respect the
opposite of the effect of contact inhibition. Lack of information makes
it impossible as yet to analyse the cell reactions involved, and hence to
relate these phenomena clearly to each other, but a number of them
will be mentioned when associative behaviour of cell populations is
discussed (p. 194).
(c) Electrical stimuli. One other quite different stimulus, resulting from
the application of electrical currents to cultures, has been the subject
of experimental work. Ingvar (1920) reported positive results, but
Weiss (1934) was unable to confirm them, and pointed out how