292
H A R R Y E A G L E A N D L E O N L E V I N T O W
factor is apparently an a-globulin, but its specific identification as
fetuin has been questioned. In addition, the serum protein provides
unidentified factor(s) necessary for the sustained growth of animal
cells, and which are not provided by the a-globulins which promote their
attachment. Serum protein is therefore just as necessary in suspension
cultures as it is in monolayers. The molecular species of protein which
embodies the growth-promoting factor(s) has not been identified,
although a recent report (Holmes, 1962) indicates that it may be
concentrated in a small subfraction of the a-2 globulin. There is some
evidence that growth promotion is due, not to the protein as such, but
to relatively small molecular weight components which are either
liberated from or produced from protein in the course of its proteolytic
degradation (Eagle, 1960).
The problem of the protein factor is, however, complicated by three
distinct observations. One is the fact that at least one cell strain, a
variant of the L-strain of mouse fibroblasts, can be grown in a minimum
medium containing only the twenty-seven demonstrably essential
growth factors (Table I) and without supplementation by serum protein
(Merchant and Hellman, 1962). This variant can thus dispense with
both the attachment and nutritional factors deriving from protein
and required by most cells. Several additional cell lines have been
grown in a protein-free medium provided that they are placed on an
appropriately treated glass surface (Rappaport, Poole and Rappaport,
1960). These cells can apparently dispense with the nutritional factor(s),
while the provision of a properly charged glass surface eliminates the
need for an attachment factor. Finally, a somewhat larger number of
cell strains have now been propagated in monolayer culture in proteinfree media (cf. Levintow and Eagle, 1961) containing a number of components over and above the twenty-seven minimum essential compounds
which suffice for growth in the presence of serum protein. The specific
compounds which have replaced the protein for these cell strains have
not yet been identified, nor is it clear whether the cells which grow are
indeed those of the parent culture, mutants which grow out selectively
in the protein-free medium, or cells which have adapted to growth in a
protein-free environment.
In keeping with the evidence that protein per se is not necessarily
required by cultured cells, serum protein provided in the form of the
usual supplement to the minimal medium is utilized by the cell only
to a very limited degree for protein synthesis (Eagle and Piez, 1960).
The magnitude of the utilization is such as to leave serious doubt as
to whether the cell in fact possesses the enzymatic mechanisms for
degrading exogenous protein to amino acids, prerequisite for its incorporation into cellular protein.
H A R R Y E A G L E A N D L E O N L E V I N T O W
factor is apparently an a-globulin, but its specific identification as
fetuin has been questioned. In addition, the serum protein provides
unidentified factor(s) necessary for the sustained growth of animal
cells, and which are not provided by the a-globulins which promote their
attachment. Serum protein is therefore just as necessary in suspension
cultures as it is in monolayers. The molecular species of protein which
embodies the growth-promoting factor(s) has not been identified,
although a recent report (Holmes, 1962) indicates that it may be
concentrated in a small subfraction of the a-2 globulin. There is some
evidence that growth promotion is due, not to the protein as such, but
to relatively small molecular weight components which are either
liberated from or produced from protein in the course of its proteolytic
degradation (Eagle, 1960).
The problem of the protein factor is, however, complicated by three
distinct observations. One is the fact that at least one cell strain, a
variant of the L-strain of mouse fibroblasts, can be grown in a minimum
medium containing only the twenty-seven demonstrably essential
growth factors (Table I) and without supplementation by serum protein
(Merchant and Hellman, 1962). This variant can thus dispense with
both the attachment and nutritional factors deriving from protein
and required by most cells. Several additional cell lines have been
grown in a protein-free medium provided that they are placed on an
appropriately treated glass surface (Rappaport, Poole and Rappaport,
1960). These cells can apparently dispense with the nutritional factor(s),
while the provision of a properly charged glass surface eliminates the
need for an attachment factor. Finally, a somewhat larger number of
cell strains have now been propagated in monolayer culture in proteinfree media (cf. Levintow and Eagle, 1961) containing a number of components over and above the twenty-seven minimum essential compounds
which suffice for growth in the presence of serum protein. The specific
compounds which have replaced the protein for these cell strains have
not yet been identified, nor is it clear whether the cells which grow are
indeed those of the parent culture, mutants which grow out selectively
in the protein-free medium, or cells which have adapted to growth in a
protein-free environment.
In keeping with the evidence that protein per se is not necessarily
required by cultured cells, serum protein provided in the form of the
usual supplement to the minimal medium is utilized by the cell only
to a very limited degree for protein synthesis (Eagle and Piez, 1960).
The magnitude of the utilization is such as to leave serious doubt as
to whether the cell in fact possesses the enzymatic mechanisms for
degrading exogenous protein to amino acids, prerequisite for its incorporation into cellular protein.