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LAURI SAXEN ET AL.
been programmed by the original genomic setting so that no new gene
activations need be postulated. If this were the case, the inductive role
of the spinal cord would merely be to supply the mesenchyme with factors necessary for further development, such as nutrients. These are
needed during the time before the new level of differentiation is stabilized (Wessells, 1964d).
2. Alternatively, it is an expression of preparation for synthesis of
"differentiative" proteins, specific to a certain stage of tubule cell differentiation.
According to the latter explanation, the increase in the rate of synthesis
of rRNA is regarded as an early expression of chemodifferentiation and
as a specific response to the inductive stimulus. Occurring at about 30
hours of cultivation, it may thus represent the very first detectable
change in the differentiating cells, in which hardly any other biochemical
or histochemical differentiative markers were visible at that time. In this
context it is appropriate to cite the data of Sundelin et al. (unpublished),
concerning the distribution of total RNA between individual cells in
differentiating mesenchymal tissue. These microspectrophotometric analyses reveal that at the stage at which the very first aggregate appear, the
mesenchyme cells contain RNA in quantities exceeding the level detected
in the noninduced control mesenchymes. This rise in RNA is seen both in
the aggregated cells and also, to a lesser extent, in the cells of the loose
mesenchyme between the aggregates. The observations that even the
nonaggregated cells had an increased RNA content may indicate that all
the cells in the induced mesenchyme have by that time reached the
"threshold level" needed for further differentiation.
In any event, the increase in rRNA synthesis might be a reflection
either of new genome-regulated protein synthesis or of activation at the
translation level.
If the rapid synthesis of rRNA and the increase of polysomal aggregates, as revealed by electron microscopy (Wartiovaara, 1966b; Sundelin
et al., unpublished), play some essential role in morphological and biochemical differentiation, we need to look for a synthesis of proteins which
could be linked with the antecedent changes in RNA synthesis.
B. Response at the Level of Protein Synthesis
In view of the observation of increased RNA synthesis in differentiating mesenchyme, we studied whether a qualitative or quantitative change
in protein synthesis could be observed in these explants. Any new protein
molecules can legitimately be regarded as related to the increased
LAURI SAXEN ET AL.
been programmed by the original genomic setting so that no new gene
activations need be postulated. If this were the case, the inductive role
of the spinal cord would merely be to supply the mesenchyme with factors necessary for further development, such as nutrients. These are
needed during the time before the new level of differentiation is stabilized (Wessells, 1964d).
2. Alternatively, it is an expression of preparation for synthesis of
"differentiative" proteins, specific to a certain stage of tubule cell differentiation.
According to the latter explanation, the increase in the rate of synthesis
of rRNA is regarded as an early expression of chemodifferentiation and
as a specific response to the inductive stimulus. Occurring at about 30
hours of cultivation, it may thus represent the very first detectable
change in the differentiating cells, in which hardly any other biochemical
or histochemical differentiative markers were visible at that time. In this
context it is appropriate to cite the data of Sundelin et al. (unpublished),
concerning the distribution of total RNA between individual cells in
differentiating mesenchymal tissue. These microspectrophotometric analyses reveal that at the stage at which the very first aggregate appear, the
mesenchyme cells contain RNA in quantities exceeding the level detected
in the noninduced control mesenchymes. This rise in RNA is seen both in
the aggregated cells and also, to a lesser extent, in the cells of the loose
mesenchyme between the aggregates. The observations that even the
nonaggregated cells had an increased RNA content may indicate that all
the cells in the induced mesenchyme have by that time reached the
"threshold level" needed for further differentiation.
In any event, the increase in rRNA synthesis might be a reflection
either of new genome-regulated protein synthesis or of activation at the
translation level.
If the rapid synthesis of rRNA and the increase of polysomal aggregates, as revealed by electron microscopy (Wartiovaara, 1966b; Sundelin
et al., unpublished), play some essential role in morphological and biochemical differentiation, we need to look for a synthesis of proteins which
could be linked with the antecedent changes in RNA synthesis.
B. Response at the Level of Protein Synthesis
In view of the observation of increased RNA synthesis in differentiating mesenchyme, we studied whether a qualitative or quantitative change
in protein synthesis could be observed in these explants. Any new protein
molecules can legitimately be regarded as related to the increased