14.
C E L L
A G G R E G A T E S
493
become attached singly and proliferate. These represent cells which
were either unable to recohere when dissociated or whose cohesive
competence became progressively attenuated in response to the conditions of dispersal.
The behavior of self-aggregating cells reflects a complex interplay
between cell-dependent and environmental factors. Rate of formation,
number and size of aggregates and their composition vary with cell
concentration and are affected by differences in cell mobilities, constituents of the culture medium, and properties of the mediumsubstratum interface (Moscona, 1960). It is therefore impossible to
speak of typical behavior of aggregating cells or to compare results,
without exact specifications of the experimental circumstances. The
conditions at the interface between the medium and the substrate must
be critically examined for two reasons. On the one hand, they affect
cell movement and cell attachment (Garber, 1953; L. Weiss, 1960;
Saxen and Penttinen, 1961; Earle, 1962), and thus cell aggregation; on
the other, they are variable, ill-defined, difficult to control, and not
always directly relevant to the problem under investigation. The nature
of the interface is determined by the structural and chemical characteristics of the solid and the liquid phases, their inhomogeneities and reaction products, and by the contributions of cells to it. The process of
self-aggregation is modulated by these conditions and thus reflects
diverse, second-order effects. Abercrombie (1961) found that the nature
of the substrate upon which cells migrate affects their interactions,
particularly
4
'contact inhibition", a phenomenon which may be of
importance in self-aggregation. A. C. Taylor (1961) found that the
presence of serum in the culture medium retarded adhesion of dissociated cells to glass, presumably by altering the conditions at the
liquid-glass interface. Rutstein, Ingenito, Craig and Martinelli (1958)
by using different proportions of cholesterol and lipoproteins in the
medium have altered the appearance and behavior of cells in tissue
cultures; since sera vary in their lipid content, the adsorption of lipids
on to cells or at the glass-liquid interface may be a significant variable
in aggregation.
That the cells themselves contribute to the interface within which they
function may be inferred from the presence of exudates or ' 'trails" from
migrating cells. The
4
'ground-mats" and colloidal exudates from cells
in culture described by Weiss (1945), the materials deposited by aggregating cells (Moscona, 1960), the "microexudates" (Rosenberg, 1960)
and "microruptures" (L. Weiss, 1961) of cells probably represent
closely related phenomena. Relevant here is the finding of Ambrose and
Jones (1961) who detected by surface-contact microscopy an intervening
space between cells and the glass surface upon which they migrated;
C E L L
A G G R E G A T E S
493
become attached singly and proliferate. These represent cells which
were either unable to recohere when dissociated or whose cohesive
competence became progressively attenuated in response to the conditions of dispersal.
The behavior of self-aggregating cells reflects a complex interplay
between cell-dependent and environmental factors. Rate of formation,
number and size of aggregates and their composition vary with cell
concentration and are affected by differences in cell mobilities, constituents of the culture medium, and properties of the mediumsubstratum interface (Moscona, 1960). It is therefore impossible to
speak of typical behavior of aggregating cells or to compare results,
without exact specifications of the experimental circumstances. The
conditions at the interface between the medium and the substrate must
be critically examined for two reasons. On the one hand, they affect
cell movement and cell attachment (Garber, 1953; L. Weiss, 1960;
Saxen and Penttinen, 1961; Earle, 1962), and thus cell aggregation; on
the other, they are variable, ill-defined, difficult to control, and not
always directly relevant to the problem under investigation. The nature
of the interface is determined by the structural and chemical characteristics of the solid and the liquid phases, their inhomogeneities and reaction products, and by the contributions of cells to it. The process of
self-aggregation is modulated by these conditions and thus reflects
diverse, second-order effects. Abercrombie (1961) found that the nature
of the substrate upon which cells migrate affects their interactions,
particularly
4
'contact inhibition", a phenomenon which may be of
importance in self-aggregation. A. C. Taylor (1961) found that the
presence of serum in the culture medium retarded adhesion of dissociated cells to glass, presumably by altering the conditions at the
liquid-glass interface. Rutstein, Ingenito, Craig and Martinelli (1958)
by using different proportions of cholesterol and lipoproteins in the
medium have altered the appearance and behavior of cells in tissue
cultures; since sera vary in their lipid content, the adsorption of lipids
on to cells or at the glass-liquid interface may be a significant variable
in aggregation.
That the cells themselves contribute to the interface within which they
function may be inferred from the presence of exudates or ' 'trails" from
migrating cells. The
4
'ground-mats" and colloidal exudates from cells
in culture described by Weiss (1945), the materials deposited by aggregating cells (Moscona, 1960), the "microexudates" (Rosenberg, 1960)
and "microruptures" (L. Weiss, 1961) of cells probably represent
closely related phenomena. Relevant here is the finding of Ambrose and
Jones (1961) who detected by surface-contact microscopy an intervening
space between cells and the glass surface upon which they migrated;