I. PROBLEM OF THE ORGANIZER
35
In a recent publication, Niu (1958) concluded that calf thymus RNA
causes specific differentiation in ectoderm cultured in a hanging drop.
The ectoderm of young gastrulae of Ambystoma tigrinum was used,
which when cultured in the salt solution showed neural differentiation
in up to 15% of cases. An addition of the saline extract of the tissue
containing 8% of RNA to the culture was followed by 56% neural,
and 17% non-neural differentiation. According to his description the
neural differentiation does not show any sign of specificity. Nonneural differentiation appears to be specific to the extent that upon
implantation into larvae of the same species it gives rise to a group of
cells similar to the thymus of the host larva. The ribonuclease-treated
sample of the saline extract in which RNA was reduced from 31 ¿¿g/ml
to 18 ¿¿g/ml gave rise to 28% non-neural and 18% neural inductions,
indicating an increase in specific effects and a decrease in non-specific
effects. This gives one the impression that RNA is related to the nonspecific induction, while other components may be related to the
specific induction. This conclusion was not, however, drawn by Niu.
He adjusted the RNA level of the ribonuclease-treated saline extract
to 40 ju,g/ml and obtained a very high frequency of non-neural differentiation (64%), the highest recorded in the paper. Niu interpreted the
results by assuming that the undigested core of RNA was qualitatively
different from the digestible fraction of RNA in possessing specific
effects which were counteracted by the digestible fraction. This interesting interpretation would be convincing, if the experiment had been
conducted on a pure RNA sample. However, the saline extract actually
used in the experiments contained probably many proteins, so that it is
possible and even probable that one or more proteins have the nonneural effects, as in bone marrow and other tissues (cf. Section IV), and
in the last series, in which the amount of protein was increased, a high
frequency of non-neural differentiation resulted.
Next, he studied the effect of the RNA sample isolated from calf
thymus by a modification of Kirby's technique. The sample used at a
concentration of 40 /¿g/ml induced 6% non-neural, and 28% neural
differentiations. In an attempt to increase the effect, trypsin inhibitor
(100-300 /¿g/ml) was added to the RNA as a non-inducing protein.
In this case 18% non-neural, and 40% neural differentiations were
registered. The results were interpreted by Niu as indicating that
specific inducing activity of RNA not revealed in pure solution was
expressed in the presence of protein which plays an indifferent role.
It should be pointed out that a crucial test for the tissue-specific
effect of RNA was made by implanting the non-neural part of the
culture into the flank of the young larva. However, the culture used for
the test was from the saline extract series, and not from the RNA-series
35
In a recent publication, Niu (1958) concluded that calf thymus RNA
causes specific differentiation in ectoderm cultured in a hanging drop.
The ectoderm of young gastrulae of Ambystoma tigrinum was used,
which when cultured in the salt solution showed neural differentiation
in up to 15% of cases. An addition of the saline extract of the tissue
containing 8% of RNA to the culture was followed by 56% neural,
and 17% non-neural differentiation. According to his description the
neural differentiation does not show any sign of specificity. Nonneural differentiation appears to be specific to the extent that upon
implantation into larvae of the same species it gives rise to a group of
cells similar to the thymus of the host larva. The ribonuclease-treated
sample of the saline extract in which RNA was reduced from 31 ¿¿g/ml
to 18 ¿¿g/ml gave rise to 28% non-neural and 18% neural inductions,
indicating an increase in specific effects and a decrease in non-specific
effects. This gives one the impression that RNA is related to the nonspecific induction, while other components may be related to the
specific induction. This conclusion was not, however, drawn by Niu.
He adjusted the RNA level of the ribonuclease-treated saline extract
to 40 ju,g/ml and obtained a very high frequency of non-neural differentiation (64%), the highest recorded in the paper. Niu interpreted the
results by assuming that the undigested core of RNA was qualitatively
different from the digestible fraction of RNA in possessing specific
effects which were counteracted by the digestible fraction. This interesting interpretation would be convincing, if the experiment had been
conducted on a pure RNA sample. However, the saline extract actually
used in the experiments contained probably many proteins, so that it is
possible and even probable that one or more proteins have the nonneural effects, as in bone marrow and other tissues (cf. Section IV), and
in the last series, in which the amount of protein was increased, a high
frequency of non-neural differentiation resulted.
Next, he studied the effect of the RNA sample isolated from calf
thymus by a modification of Kirby's technique. The sample used at a
concentration of 40 /¿g/ml induced 6% non-neural, and 28% neural
differentiations. In an attempt to increase the effect, trypsin inhibitor
(100-300 /¿g/ml) was added to the RNA as a non-inducing protein.
In this case 18% non-neural, and 40% neural differentiations were
registered. The results were interpreted by Niu as indicating that
specific inducing activity of RNA not revealed in pure solution was
expressed in the presence of protein which plays an indifferent role.
It should be pointed out that a crucial test for the tissue-specific
effect of RNA was made by implanting the non-neural part of the
culture into the flank of the young larva. However, the culture used for
the test was from the saline extract series, and not from the RNA-series