500
A . A . MOSCONA
biosynthesis by low temperatures, or changes in their essential properties
would result in the absence of aggregation. The specific function of such
cell products appears to be calcium dependent, since in the absence of
the cation the cells do not recohere effectively at optimal temperatures.
Only small and loose cell clusters form in the presence of magnesium
as the sole divalent cation; in the presence of strontium only, the cells
do not aggregate.
Serum protein is essential for effective aggregation of trypsin dissociated cells dispersed in a balanced salt solution or a synthetic
medium (Moscona, 1961b). Whether it satisfies metabolic requirements
that affect, directly or indirectly, the aggregative behavior of cells, or
provides a suitable macromolecular protective environment for the
cells, or inactivates trypsin taken up by the cells, is not clear. The
aggregation-promoting activity of serum protein cannot be replaced
either by the smaller molecules found in serum dialysates, or by such
colloids as methyl cellulose, or Polyvinylpyrrolidone (PVP). However in the presence of fraction IV-1 of adult bovine serum ("alphalipoprotein", Nutritional Biochemicals C o . ; 0-5 mg/ml) recohesion
of cells takes place. Fetuin, which promotes attachment of cells to
glass (Fisher, Puck and Sato, 1958) is less effective in supporting
cell aggregation than a-lipoprotein and requires higher concentrations.
Since these serum fractions are of heterogeneous composition (Lieberman and Ove, 1958) the precise identity of the effective constituents is
not known. It is however of interest that a material comprising lipid
and globulin, both of which are thought to contribute to the composition of cell membranes (Danielli, 1958; Willmer, 1961), aids in maintaining the functional integrity of dissociated embryonic cells.
Detailed comparative studies on requirements for aggregation of cells
dissociated by different procedures are not available; however, our
experience indicates that, in this, as in other respects, the responses of
differently treated cells can be expected to vary considerably. It is not
always possible to decide whether a given component of the medium is
prerequisite specifically for cell reattachment, or for the function of
more general processes essential to normal cell maintenance. Yet such
distinctions are necessary if more is to be learned about the nature of
cell-binding mechanisms.
Another way of approaching this problem is by studying various
aggregation-inhibiting factors (Moscona, 1962a, b ) ; interpretation of
results is qualified by the complexities involved, but under suitable conditions highly useful and indicative information can be obtained. The
inhibitory effect of sodium-metaperiodate (10~
4 M) is of interest due to
the selective—though not exclusive—reaction of this agent with carbohydrates; the reversible inhibition by periodate of aggregation of
A . A . MOSCONA
biosynthesis by low temperatures, or changes in their essential properties
would result in the absence of aggregation. The specific function of such
cell products appears to be calcium dependent, since in the absence of
the cation the cells do not recohere effectively at optimal temperatures.
Only small and loose cell clusters form in the presence of magnesium
as the sole divalent cation; in the presence of strontium only, the cells
do not aggregate.
Serum protein is essential for effective aggregation of trypsin dissociated cells dispersed in a balanced salt solution or a synthetic
medium (Moscona, 1961b). Whether it satisfies metabolic requirements
that affect, directly or indirectly, the aggregative behavior of cells, or
provides a suitable macromolecular protective environment for the
cells, or inactivates trypsin taken up by the cells, is not clear. The
aggregation-promoting activity of serum protein cannot be replaced
either by the smaller molecules found in serum dialysates, or by such
colloids as methyl cellulose, or Polyvinylpyrrolidone (PVP). However in the presence of fraction IV-1 of adult bovine serum ("alphalipoprotein", Nutritional Biochemicals C o . ; 0-5 mg/ml) recohesion
of cells takes place. Fetuin, which promotes attachment of cells to
glass (Fisher, Puck and Sato, 1958) is less effective in supporting
cell aggregation than a-lipoprotein and requires higher concentrations.
Since these serum fractions are of heterogeneous composition (Lieberman and Ove, 1958) the precise identity of the effective constituents is
not known. It is however of interest that a material comprising lipid
and globulin, both of which are thought to contribute to the composition of cell membranes (Danielli, 1958; Willmer, 1961), aids in maintaining the functional integrity of dissociated embryonic cells.
Detailed comparative studies on requirements for aggregation of cells
dissociated by different procedures are not available; however, our
experience indicates that, in this, as in other respects, the responses of
differently treated cells can be expected to vary considerably. It is not
always possible to decide whether a given component of the medium is
prerequisite specifically for cell reattachment, or for the function of
more general processes essential to normal cell maintenance. Yet such
distinctions are necessary if more is to be learned about the nature of
cell-binding mechanisms.
Another way of approaching this problem is by studying various
aggregation-inhibiting factors (Moscona, 1962a, b ) ; interpretation of
results is qualified by the complexities involved, but under suitable conditions highly useful and indicative information can be obtained. The
inhibitory effect of sodium-metaperiodate (10~
4 M) is of interest due to
the selective—though not exclusive—reaction of this agent with carbohydrates; the reversible inhibition by periodate of aggregation of