14. C E L L A G G R E G A T E S
521
not only in aggregative ability but also in tendency to group selectively.
Thus, when monolayer-modified skin-cells are included in aggregates
of freshly dissociated cells from a different tissue, they remain dispersed
throughout the composite cluster (Moscona, 1960). The inhibitory
effect on cell aggregation of suppressors of protein synthesis (Moscona
and Moscona, 1963) is clearly relevant here. Even more pertinent may
be the recent isolation of materials with selective cell-binding properties
from dissociated living cells (Moscona, 1962b, 1963). The intriguing
analogy between decrease in mutual adhesiveness and contact selectivity of monolayer-grown cells and decline in specificity of virus uptake
in such cells was mentioned before, and the attribution of the latter to
changes in cell-surface materials in response to conditions of dispersal,
pointed out.
T w o classes of phenomena represent the most striking aspect of cell
behavior in aggregates: selective grouping of cells, and regional
localization of cell groupings. These phenomena may not be entirely
separate in that the formation of homogeneous groupings by cells of a
given kind may be aided by their tendency to converge inside or at the
surface of the aggregates. On the other hand, it is by no means definite
that localization of cells in aggregates is due, in all cases, to individual
cell properties, rather than to properties of emergent cell groupings in
relation to each other.
Selective sorting out of cells and their grouping in distinct regions
involve cell migrations that in many respects resemble morphogenetic
movements in the embryo (DeHaan, 1958). Motility phenomena in the
grouping of cells fall into three classes: (a) movements of individual cells
in relation to or in contact with neighbouring cells or their extracellular
products; (b) movements of cells as a group which maintains a degree
of integrity while it shifts position; (c) translocations of cohesive cell
layers in which the cells retain their relative positions. It is very likely
that sorting-out movements in aggregates belong to all these classes.
Individual cell movements in initial aggregates are progressively superseded by movements of small cell groups, as cells of matching types make
and maintain contact. The merger of smaller into larger groups and
their movements towards the inside or the outside of the cluster must
play a major role in forming the ultimate internal pattern of the
aggregates. The merger of cell groups within the aggregates may be due
to passive encounter, to the "pooling" of small groups by way of
cellular bridges or extracellular products, or to active attraction of
smaller groups and cells to larger groups, perhaps by way of tracks of
extracellular material. There is no critical evidence to favor one of these
possibilities against the others.
While it is characteristic of cells in heterotypic aggregates to become
s*
521
not only in aggregative ability but also in tendency to group selectively.
Thus, when monolayer-modified skin-cells are included in aggregates
of freshly dissociated cells from a different tissue, they remain dispersed
throughout the composite cluster (Moscona, 1960). The inhibitory
effect on cell aggregation of suppressors of protein synthesis (Moscona
and Moscona, 1963) is clearly relevant here. Even more pertinent may
be the recent isolation of materials with selective cell-binding properties
from dissociated living cells (Moscona, 1962b, 1963). The intriguing
analogy between decrease in mutual adhesiveness and contact selectivity of monolayer-grown cells and decline in specificity of virus uptake
in such cells was mentioned before, and the attribution of the latter to
changes in cell-surface materials in response to conditions of dispersal,
pointed out.
T w o classes of phenomena represent the most striking aspect of cell
behavior in aggregates: selective grouping of cells, and regional
localization of cell groupings. These phenomena may not be entirely
separate in that the formation of homogeneous groupings by cells of a
given kind may be aided by their tendency to converge inside or at the
surface of the aggregates. On the other hand, it is by no means definite
that localization of cells in aggregates is due, in all cases, to individual
cell properties, rather than to properties of emergent cell groupings in
relation to each other.
Selective sorting out of cells and their grouping in distinct regions
involve cell migrations that in many respects resemble morphogenetic
movements in the embryo (DeHaan, 1958). Motility phenomena in the
grouping of cells fall into three classes: (a) movements of individual cells
in relation to or in contact with neighbouring cells or their extracellular
products; (b) movements of cells as a group which maintains a degree
of integrity while it shifts position; (c) translocations of cohesive cell
layers in which the cells retain their relative positions. It is very likely
that sorting-out movements in aggregates belong to all these classes.
Individual cell movements in initial aggregates are progressively superseded by movements of small cell groups, as cells of matching types make
and maintain contact. The merger of smaller into larger groups and
their movements towards the inside or the outside of the cluster must
play a major role in forming the ultimate internal pattern of the
aggregates. The merger of cell groups within the aggregates may be due
to passive encounter, to the "pooling" of small groups by way of
cellular bridges or extracellular products, or to active attraction of
smaller groups and cells to larger groups, perhaps by way of tracks of
extracellular material. There is no critical evidence to favor one of these
possibilities against the others.
While it is characteristic of cells in heterotypic aggregates to become
s*