DIFFERENTIATION OF KIDNEY MESENCHYME
265
ated albumin-coated kieselguhr (MAK) chromatography (Miettinen et
al., 1966). At 30 hours of cultivation, a difference was observed in the
relative distribution of labeled precursor in various RNA fractions obtained from induced and noninduced mesenchymes (Fig. 10). After 48
hours' contact with the inductor tissue, the relative distribution of
3 2
P
in the various RNA classes had undergone a marked change from the
pattern found at 30 hours. Correspondingly, the relative rate of total
RNA synthesis was significantly lower. Among the various RNA fractions, ribosomal ribonucleic acid (rRNA) was losing its dominance in
comparison with the situation in the control mesenchyme.
At 62 hours of cultivation the differences observed earlier were hardly
detectable. The MAK fractionation method did not disclose any major
differences in the distribution of radioactivity among the deoxyribonucleic acid (DNA)-like fractions between differentiating and non-differentiating tissues. Nor did the relative rates of
3 2
P uptake by these
fractions, namely Q x and Q 2 plus the tenaciously bound ribonucleic acid
(tRNA) (Ellem and Sheridan, 1964), suggest any definite differences in
the kinetics of their synthesis. The same held true of the base ratio
analyses of these RNA fractions; between induced and noninduced tissues no differences could be detected (Miettinen et al., 1966).
The pronounced emphasis on the synthesis of rRNA at 30 hours
(above) coincides with the time at which uridine incorporation shows
maximal sensitivity to actinomycin (Vainio et al., 1965). This effect of
actinomycin is chiefly due to inhibition of rRNA synthesis, because most
of the precursor taken up is used for this synthesis and because of the
syntheses of various RNA classes, that of rRNA is the one most sensitive
to the drug (Clark and Ellem, 1966).
In attempting to evaluate the possible significance of this enhancement
of rRNA synthesis as an indication of differentiation at the macromolecular level and, thus, a sign of preparation for subsequent overt differentiation, at least the following two alternatives should be taken into
consideration as explanations.
1. The enhancement is an expression of recovery from the metabolic
arrest caused by transference of the tissue to culture conditions.
As stated earlier in Section I,B, both induced and noninduced mesenchymal tissues pass through a metabolic standstill period, from which
recovery only begins after some 15 to 20 hours of cultivation (Fig. 2).
Thus it may be argued that the observed increase in the rRNA synthesis
of differentiating mesenchyme only reflects recovery to the normal in
vivo level. The protein-synthesizing machinery thus rebuilt might have
265
ated albumin-coated kieselguhr (MAK) chromatography (Miettinen et
al., 1966). At 30 hours of cultivation, a difference was observed in the
relative distribution of labeled precursor in various RNA fractions obtained from induced and noninduced mesenchymes (Fig. 10). After 48
hours' contact with the inductor tissue, the relative distribution of
3 2
P
in the various RNA classes had undergone a marked change from the
pattern found at 30 hours. Correspondingly, the relative rate of total
RNA synthesis was significantly lower. Among the various RNA fractions, ribosomal ribonucleic acid (rRNA) was losing its dominance in
comparison with the situation in the control mesenchyme.
At 62 hours of cultivation the differences observed earlier were hardly
detectable. The MAK fractionation method did not disclose any major
differences in the distribution of radioactivity among the deoxyribonucleic acid (DNA)-like fractions between differentiating and non-differentiating tissues. Nor did the relative rates of
3 2
P uptake by these
fractions, namely Q x and Q 2 plus the tenaciously bound ribonucleic acid
(tRNA) (Ellem and Sheridan, 1964), suggest any definite differences in
the kinetics of their synthesis. The same held true of the base ratio
analyses of these RNA fractions; between induced and noninduced tissues no differences could be detected (Miettinen et al., 1966).
The pronounced emphasis on the synthesis of rRNA at 30 hours
(above) coincides with the time at which uridine incorporation shows
maximal sensitivity to actinomycin (Vainio et al., 1965). This effect of
actinomycin is chiefly due to inhibition of rRNA synthesis, because most
of the precursor taken up is used for this synthesis and because of the
syntheses of various RNA classes, that of rRNA is the one most sensitive
to the drug (Clark and Ellem, 1966).
In attempting to evaluate the possible significance of this enhancement
of rRNA synthesis as an indication of differentiation at the macromolecular level and, thus, a sign of preparation for subsequent overt differentiation, at least the following two alternatives should be taken into
consideration as explanations.
1. The enhancement is an expression of recovery from the metabolic
arrest caused by transference of the tissue to culture conditions.
As stated earlier in Section I,B, both induced and noninduced mesenchymal tissues pass through a metabolic standstill period, from which
recovery only begins after some 15 to 20 hours of cultivation (Fig. 2).
Thus it may be argued that the observed increase in the rRNA synthesis
of differentiating mesenchyme only reflects recovery to the normal in
vivo level. The protein-synthesizing machinery thus rebuilt might have