186
M. A B E R C R O M B I E
the response involves diminished rate of turning, but no increase of
speed, when the cell is moving in the favoured direction. This behaviour
of white blood cells will not be further considered here since it has often
been fully reviewed (see McCutcheon, 1946; Harris, 1954, 1961).
Another instance of chemotaxis is the well-known reaction of the
amoeboid form of the cellular slime-moulds to concentration gradients
of acrasin (Bonner, 1959).
The converse reaction, the movement of cells down a diffusion
gradient, away from its source, is in much the same case as positive
chemotaxis. It has been a favourite hypothesis amongst tissue culturists
to explain the movement of cells away from an explant. Once again
actual evidence is lacking, and there are other available authenticated
mechanisms capable of producing the movement (contact inhibition
and contact guidance). There is, however, a type of cell, the amphibian
neural-crest cell before its differentiation, particularly the melanoblast,
for which there is evidence of negative chemotaxis, obtained by Twitty
(1945) and Twitty and Niu (1948, 1954). The response is to a substance
produced by the crest cells themselves, so that its effect is a mutual
repulsion. Twitty and Niu showed that culture of pairs or of small
groups of cells within a capillary tube, where disturbance by convection
currents is minimized, leads to retreat of the cells from each other in a
strongly directional way, and at distances well beyond those bridgeable
by contact. When they reach a certain distance apart the cells go into
random oscillation, like wholly isolated crest cells which, though constantly changing shape, rarely translocate more than a few cell diameters. A chemotactic effect was also detectable where the coelomicfluid-filled capillary tube opened into a reservoir of saline; cells tended
to move away from the opening in so far as this did not conflict with their
mutual repulsion. These reactions are believed to involve both speed
and rate of turning. It is possible too that white blood cells, besides the
positive chemotaxis they are known to show, may be negatively
chemotactic to each other. They have a peculiarity of mass behaviour
(the "no-man's-land" between colonies described by Carrel and
Ebeling, 1922), which is most easily explained in this way. Oldfield
(1963) has concluded that the explanation is probably correct for
polymorphs, though the chemotaxis seems to be rather ineffective in
inducing repulsion between individual cells; it requires the massive
influence of a whole colony to be detectable.
2. Oriented Responses Due to Contact Stimuli
(a) Non-living objects as contact stimuli. Because of the necessity of
providing an artificial substrate for cell movement, tissue culture has
focused much attention on this kind of stimulus, the effects of which are
M. A B E R C R O M B I E
the response involves diminished rate of turning, but no increase of
speed, when the cell is moving in the favoured direction. This behaviour
of white blood cells will not be further considered here since it has often
been fully reviewed (see McCutcheon, 1946; Harris, 1954, 1961).
Another instance of chemotaxis is the well-known reaction of the
amoeboid form of the cellular slime-moulds to concentration gradients
of acrasin (Bonner, 1959).
The converse reaction, the movement of cells down a diffusion
gradient, away from its source, is in much the same case as positive
chemotaxis. It has been a favourite hypothesis amongst tissue culturists
to explain the movement of cells away from an explant. Once again
actual evidence is lacking, and there are other available authenticated
mechanisms capable of producing the movement (contact inhibition
and contact guidance). There is, however, a type of cell, the amphibian
neural-crest cell before its differentiation, particularly the melanoblast,
for which there is evidence of negative chemotaxis, obtained by Twitty
(1945) and Twitty and Niu (1948, 1954). The response is to a substance
produced by the crest cells themselves, so that its effect is a mutual
repulsion. Twitty and Niu showed that culture of pairs or of small
groups of cells within a capillary tube, where disturbance by convection
currents is minimized, leads to retreat of the cells from each other in a
strongly directional way, and at distances well beyond those bridgeable
by contact. When they reach a certain distance apart the cells go into
random oscillation, like wholly isolated crest cells which, though constantly changing shape, rarely translocate more than a few cell diameters. A chemotactic effect was also detectable where the coelomicfluid-filled capillary tube opened into a reservoir of saline; cells tended
to move away from the opening in so far as this did not conflict with their
mutual repulsion. These reactions are believed to involve both speed
and rate of turning. It is possible too that white blood cells, besides the
positive chemotaxis they are known to show, may be negatively
chemotactic to each other. They have a peculiarity of mass behaviour
(the "no-man's-land" between colonies described by Carrel and
Ebeling, 1922), which is most easily explained in this way. Oldfield
(1963) has concluded that the explanation is probably correct for
polymorphs, though the chemotaxis seems to be rather ineffective in
inducing repulsion between individual cells; it requires the massive
influence of a whole colony to be detectable.
2. Oriented Responses Due to Contact Stimuli
(a) Non-living objects as contact stimuli. Because of the necessity of
providing an artificial substrate for cell movement, tissue culture has
focused much attention on this kind of stimulus, the effects of which are