3. C O N S T R U C T I O N A N D USE OF S Y N T H E T I C M E D I A
107
placed by a mixture of the component amino acids (Rose, 1938).
When this was seen to be possible for the whole animal, the way was
opened to the application of the same principle to isolated cells. The
fact that amino acids are nutritionally useful for cells was slow to gain
credence, especially among those who accepted the views of Burrows
and Neymann (1917), that amino acids are toxic to cells. In a sense
this is true, for the particular crude amino acid mixtures tested, in the
very high concentrations used, certainly were toxic. This half-truth
obscured the subsequent thinking of a generation of students of cell
nutrition and barred the way to the disclosure that the right amino acids,
in the right proportions, are used by cells when supplied in the right
concentrations.
One of the commonest practices for maintaining cultures is to subject
them to periodical renewal of the culture medium. The cells are thus
exposed to fluctuating concentrations of the nutrients and, at the end
of the cycle before the next renewal, to products of the cells' metabolism. It has quite commonly been supposed that accumulated
products are toxic. Hanks (1955) has called this "the great myth that
cells in a used medium are damaged by their own end-products rather
than by exhaustion of primary nutriments", and claimed that "maximal
longevity occurs in media of highest nutrient reserve and greatest yield
of end-products". These general statements were evidently designed
to shock his colleagues and successors out of an entrenched prejudice.
It is now (and was even then) recognized that excessively high concentrations of almost any nutrient can be deleterious, though optimal
concentrations may well be above those thought of as "physiological",
and in practice substances of low stability are generally supplied at
levels above the probable actual needs.
Securing a suitable balance of a whole range of components, the
need for any one of which seems highly probable in the light of general
nutritional and metabolic information, is really the crux of the problem
in designing defined media for cells. The utility of no single component
can be considered in isolation. Any one nutrient substance may have
multiple functions, and the degree to which it exercises any of these
may depend on the rest of the cellular environment as well as on the
equipment of the cells to deal with it in one or more ways. Thus, amino
acids may be used for incorporation into cell proteins; for conversion
into other amino acids, if these are not supplied or are present in
suboptimal concentrations; as energy sources, if others, e.g. carbohydrates, fall below a critical level; as buffers; as chelators of ions;
and as the starting materials for building other structures than proteins,
e.g. nucleic acids. The possibilities are very numerous and, realizing
this, we must recognize the futility in most instances of saying that a
107
placed by a mixture of the component amino acids (Rose, 1938).
When this was seen to be possible for the whole animal, the way was
opened to the application of the same principle to isolated cells. The
fact that amino acids are nutritionally useful for cells was slow to gain
credence, especially among those who accepted the views of Burrows
and Neymann (1917), that amino acids are toxic to cells. In a sense
this is true, for the particular crude amino acid mixtures tested, in the
very high concentrations used, certainly were toxic. This half-truth
obscured the subsequent thinking of a generation of students of cell
nutrition and barred the way to the disclosure that the right amino acids,
in the right proportions, are used by cells when supplied in the right
concentrations.
One of the commonest practices for maintaining cultures is to subject
them to periodical renewal of the culture medium. The cells are thus
exposed to fluctuating concentrations of the nutrients and, at the end
of the cycle before the next renewal, to products of the cells' metabolism. It has quite commonly been supposed that accumulated
products are toxic. Hanks (1955) has called this "the great myth that
cells in a used medium are damaged by their own end-products rather
than by exhaustion of primary nutriments", and claimed that "maximal
longevity occurs in media of highest nutrient reserve and greatest yield
of end-products". These general statements were evidently designed
to shock his colleagues and successors out of an entrenched prejudice.
It is now (and was even then) recognized that excessively high concentrations of almost any nutrient can be deleterious, though optimal
concentrations may well be above those thought of as "physiological",
and in practice substances of low stability are generally supplied at
levels above the probable actual needs.
Securing a suitable balance of a whole range of components, the
need for any one of which seems highly probable in the light of general
nutritional and metabolic information, is really the crux of the problem
in designing defined media for cells. The utility of no single component
can be considered in isolation. Any one nutrient substance may have
multiple functions, and the degree to which it exercises any of these
may depend on the rest of the cellular environment as well as on the
equipment of the cells to deal with it in one or more ways. Thus, amino
acids may be used for incorporation into cell proteins; for conversion
into other amino acids, if these are not supplied or are present in
suboptimal concentrations; as energy sources, if others, e.g. carbohydrates, fall below a critical level; as buffers; as chelators of ions;
and as the starting materials for building other structures than proteins,
e.g. nucleic acids. The possibilities are very numerous and, realizing
this, we must recognize the futility in most instances of saying that a