158
B. M. SHAFFER
may be left in contact with aggregating cells without seriously affecting
them, though in the same concentration (1 mg/ml, pH 7) ribonuclease is
highly toxic to vegetative cells, perhaps simply because the enzyme can
more easily gain access to their cytoplasm.
Since Herbst's work on sea-urchin blastulae at the turn of the
century, lowering of the calcium concentration has often been employed
in attempts to separate cells. DeHaan (1959) found that ethylenediamine-tetra-acetate could reduce a migrating grex to a ragged mass of
violently and independently active cells within a few minutes, and within hours could reduce it to a layer of separate bubbling cells. However,
this provides little specific information about adhesion, for, as he
pointed out, not only can EDTA have at least five biochemical actions,
but as it affects vegetative cells similarly, it probably disperses aggregates primarily by increasing surface instability. Further, though, a few
years ago, I observed this effect on the cell surface, I found that aggregates were rather resistant to EDTA: even in a higher concentration
than DeHaan used, and at a higher pH (which in itself leads to dissociation of some tissues and of EDTA too, making this more effective), a
grex did not completely separate into its constituents, though it
writhed for hours; it was remarkable that small groups at its highly
irregular surface, attached to the main mass perhaps by single cells,
could be flung about so vigorously in all directions without falling off.
I am indebted to Dr. G. R. E. Naylor of the Department of Pathology,
Cambridge University, for suggesting a mammalian red-blood-cell suspension as a tool for studying adhesiveness. If this is poured over
amoebae crawling on glass and then washed off, se\
r eral red cells remain
stuck to the majority of the 'nonadhesive' non-social cells, but very
few to the 'strongly adhesive' aggregating ones. This is not easily explained by a theory of nonspecific adhesion supplemented by secondary
sorting mechanisms. It might well be possible to determine what surface
component is involved.
Aggregates covered in slime sheath are known to be strongly resistant
to wetting. But even naked aggregations tend to float up on to the water
surface when an agar plate is flooded, and at first I thought that the
cells had become more hydrofuge. However, sufficiently young and small
aggregations, or parts of them, often remain adherent to the agar,
whereas a proportion of the separate cells are in fact lifted off it. An
important determinant of behaviour could be merely flatness—the area
of attachment to the agar relative to volume.
Weiss (1947) and Tyler (1947) suggested that metazoan cells were held
together, at least initially, by a reaction between surface molecules
resembling antigens and antibodies, or, to put it more generally, any
complementary molecules. Weiss (1958) himself now regards this as too
B. M. SHAFFER
may be left in contact with aggregating cells without seriously affecting
them, though in the same concentration (1 mg/ml, pH 7) ribonuclease is
highly toxic to vegetative cells, perhaps simply because the enzyme can
more easily gain access to their cytoplasm.
Since Herbst's work on sea-urchin blastulae at the turn of the
century, lowering of the calcium concentration has often been employed
in attempts to separate cells. DeHaan (1959) found that ethylenediamine-tetra-acetate could reduce a migrating grex to a ragged mass of
violently and independently active cells within a few minutes, and within hours could reduce it to a layer of separate bubbling cells. However,
this provides little specific information about adhesion, for, as he
pointed out, not only can EDTA have at least five biochemical actions,
but as it affects vegetative cells similarly, it probably disperses aggregates primarily by increasing surface instability. Further, though, a few
years ago, I observed this effect on the cell surface, I found that aggregates were rather resistant to EDTA: even in a higher concentration
than DeHaan used, and at a higher pH (which in itself leads to dissociation of some tissues and of EDTA too, making this more effective), a
grex did not completely separate into its constituents, though it
writhed for hours; it was remarkable that small groups at its highly
irregular surface, attached to the main mass perhaps by single cells,
could be flung about so vigorously in all directions without falling off.
I am indebted to Dr. G. R. E. Naylor of the Department of Pathology,
Cambridge University, for suggesting a mammalian red-blood-cell suspension as a tool for studying adhesiveness. If this is poured over
amoebae crawling on glass and then washed off, se\
r eral red cells remain
stuck to the majority of the 'nonadhesive' non-social cells, but very
few to the 'strongly adhesive' aggregating ones. This is not easily explained by a theory of nonspecific adhesion supplemented by secondary
sorting mechanisms. It might well be possible to determine what surface
component is involved.
Aggregates covered in slime sheath are known to be strongly resistant
to wetting. But even naked aggregations tend to float up on to the water
surface when an agar plate is flooded, and at first I thought that the
cells had become more hydrofuge. However, sufficiently young and small
aggregations, or parts of them, often remain adherent to the agar,
whereas a proportion of the separate cells are in fact lifted off it. An
important determinant of behaviour could be merely flatness—the area
of attachment to the agar relative to volume.
Weiss (1947) and Tyler (1947) suggested that metazoan cells were held
together, at least initially, by a reaction between surface molecules
resembling antigens and antibodies, or, to put it more generally, any
complementary molecules. Weiss (1958) himself now regards this as too