2 . M E T H O D S
25
is required. There is little doubt also that the presence of the bicarbonate ion is desirable for its own sake. Tissues tend to become inert
when cultured in vessels from which the C 0 2 is being constantly
removed, even though the p H is maintained within the required
limits. The ready penetration of C 0 2 into and out of cells is probably
important in maintaining intracellular ionic concentrations. The C 0 2
may also enter into certain important metabolic pathways (see p. 261)
and act as a source of carbon.
Once again, as with abnormal osmotic pressures, prolonged subjection of a culture to an abnormal pH eventually produces deleterious
effects. While cells may tolerate media with p H values between about
5-5 and 8-5 for a time, each tissue probably has its own optimum within
that range and any departure from that optimum has adverse effects,
in the end, on the tissue's activity (Fischer, 1921). Some tissues require
a p H between 6-8 and 7-6 and preferably between 7-2 and 7 4 (e.g.
bone), others do better at pH's between 7*6 and 8-0.
E. O X Y G E N
Stress has already been laid on the importance of the size of the
tissue explants. The absence of any circulation within the cultured
tissue leads to the accumulation of waste products and the lack of
oxygen and nutrients. As already mentioned, the lowered oxygen
tension within the explants at first encourages any anaerobic processes
of which the tissue may be capable, but often the beneficial effect is
offset by the deleterious effect of the metabolic products. It is thus
interesting that a close-packed tissue, like a colony of fibroblasts,
cannot for long tolerate being planted deep in a plasma clot, while
colonies of macrophages can be established at almost any depth
(Fischer, 1928), presumably because the latter cells scatter more
widely throughout the medium and do not crowd together, so exhausting the oxygen locally and producing a metabolic smog.
High concentrations of oxygen can be toxic to tissues, e.g. retina, so
that the problem of supplying tissues with adequate oxygen is by no
means simple. Tissues, organs, and cells have their own physiological
preferences in vivo, and adequate conditions in vitro may have to take
these preferences into account. Each cell, tissue or organ culture has
its own optimum conditions of oxygenation and these may have to be
experimentally established before successful culture can be achieved
(see pp. 71, 81).
F. S A L T C O N C E N T R A T I O N S
Sodium, potassium, calcium and magnesium chlorides have long
25
is required. There is little doubt also that the presence of the bicarbonate ion is desirable for its own sake. Tissues tend to become inert
when cultured in vessels from which the C 0 2 is being constantly
removed, even though the p H is maintained within the required
limits. The ready penetration of C 0 2 into and out of cells is probably
important in maintaining intracellular ionic concentrations. The C 0 2
may also enter into certain important metabolic pathways (see p. 261)
and act as a source of carbon.
Once again, as with abnormal osmotic pressures, prolonged subjection of a culture to an abnormal pH eventually produces deleterious
effects. While cells may tolerate media with p H values between about
5-5 and 8-5 for a time, each tissue probably has its own optimum within
that range and any departure from that optimum has adverse effects,
in the end, on the tissue's activity (Fischer, 1921). Some tissues require
a p H between 6-8 and 7-6 and preferably between 7-2 and 7 4 (e.g.
bone), others do better at pH's between 7*6 and 8-0.
E. O X Y G E N
Stress has already been laid on the importance of the size of the
tissue explants. The absence of any circulation within the cultured
tissue leads to the accumulation of waste products and the lack of
oxygen and nutrients. As already mentioned, the lowered oxygen
tension within the explants at first encourages any anaerobic processes
of which the tissue may be capable, but often the beneficial effect is
offset by the deleterious effect of the metabolic products. It is thus
interesting that a close-packed tissue, like a colony of fibroblasts,
cannot for long tolerate being planted deep in a plasma clot, while
colonies of macrophages can be established at almost any depth
(Fischer, 1928), presumably because the latter cells scatter more
widely throughout the medium and do not crowd together, so exhausting the oxygen locally and producing a metabolic smog.
High concentrations of oxygen can be toxic to tissues, e.g. retina, so
that the problem of supplying tissues with adequate oxygen is by no
means simple. Tissues, organs, and cells have their own physiological
preferences in vivo, and adequate conditions in vitro may have to take
these preferences into account. Each cell, tissue or organ culture has
its own optimum conditions of oxygenation and these may have to be
experimentally established before successful culture can be achieved
(see pp. 71, 81).
F. S A L T C O N C E N T R A T I O N S
Sodium, potassium, calcium and magnesium chlorides have long