8. P R O T E I N M E T A B O L I S M IN C E L L L I N E S
285
of the order of 100,000,000 cells/ml. The additional burden placed on
the dispersed culture cell, in terms of retaining metabolic intermediates
and products essential for survival and growth, is self-evident.
Despite the relatively narrow limits of cellular population density
within which cells can be cultured in vitro, their nutritional requirements
have proved to be profoundly modified by the exact population
density (Eagle and Piez, 1962a). A number of compounds known
to be synthesized by the cell have nevertheless proved, essential for
their survival and growth, and in every one of the situations so far
examined that requirement has been found to disappear at a sufficiently
high population density (Table I I ) . The reason for this population
dependence appears to be the fact that for each of these compounds
there is equilibration between the cellular pool and the environment,
in consequence of which the compound itself, or intermediates required
for its synthesis, are lost to the medium in amounts which exceed the
biosynthetic capacity of the cell. At a sufficiently high population
density the medium can be " conditioned", i.e. the concentration in the
medium and the intracellular pool can be brought to a level consistent
with cellular survival and growth, before the cells die of the specific
deficiency.
Some of these effects of population density on the nutritional requirements of cultured cells are observed only at the maximum cell
population possible to achieve in culture. It is a reasonable speculation
that at the several hundred-fold higher population densities of organized tissues, the nutritional requirements and metabolic activities may
differ in additional respects from those observed in culture. The possibility suggests itself that a wide variety of metabolic intermediates,
cofactors and perhaps even macromolecules may be lost to the environment in dispersed cell cultures, which are retained in organized
tissues for the very reason that the cells then constitute approximately
50% of the organ volume. The phenomenon of "dedifferentiation"
in cultured cells, i.e. the loss of their specialized functional capacities,
may therefore be referable, at least in part, to the prohibitive loss to the
medium of essential elements in the biosynthetic apparatus. In cells
that are only several generations removed from the parent host tissue,
that loss could conceivably be reversible, capable of being restored by
appropriate concentration of the dispersed cells into an organized
cellular mass.
The same considerations may apply also in the process of embryonic
differentiation, and in the ability of the differentiated cell to exercise
its newly developed biosynthetic capacities. The possibility of a critical
mass for cellular function appears a fruitful area for further exploration.
285
of the order of 100,000,000 cells/ml. The additional burden placed on
the dispersed culture cell, in terms of retaining metabolic intermediates
and products essential for survival and growth, is self-evident.
Despite the relatively narrow limits of cellular population density
within which cells can be cultured in vitro, their nutritional requirements
have proved to be profoundly modified by the exact population
density (Eagle and Piez, 1962a). A number of compounds known
to be synthesized by the cell have nevertheless proved, essential for
their survival and growth, and in every one of the situations so far
examined that requirement has been found to disappear at a sufficiently
high population density (Table I I ) . The reason for this population
dependence appears to be the fact that for each of these compounds
there is equilibration between the cellular pool and the environment,
in consequence of which the compound itself, or intermediates required
for its synthesis, are lost to the medium in amounts which exceed the
biosynthetic capacity of the cell. At a sufficiently high population
density the medium can be " conditioned", i.e. the concentration in the
medium and the intracellular pool can be brought to a level consistent
with cellular survival and growth, before the cells die of the specific
deficiency.
Some of these effects of population density on the nutritional requirements of cultured cells are observed only at the maximum cell
population possible to achieve in culture. It is a reasonable speculation
that at the several hundred-fold higher population densities of organized tissues, the nutritional requirements and metabolic activities may
differ in additional respects from those observed in culture. The possibility suggests itself that a wide variety of metabolic intermediates,
cofactors and perhaps even macromolecules may be lost to the environment in dispersed cell cultures, which are retained in organized
tissues for the very reason that the cells then constitute approximately
50% of the organ volume. The phenomenon of "dedifferentiation"
in cultured cells, i.e. the loss of their specialized functional capacities,
may therefore be referable, at least in part, to the prohibitive loss to the
medium of essential elements in the biosynthetic apparatus. In cells
that are only several generations removed from the parent host tissue,
that loss could conceivably be reversible, capable of being restored by
appropriate concentration of the dispersed cells into an organized
cellular mass.
The same considerations may apply also in the process of embryonic
differentiation, and in the ability of the differentiated cell to exercise
its newly developed biosynthetic capacities. The possibility of a critical
mass for cellular function appears a fruitful area for further exploration.