38
TUNEO
YAMADA
Johnen (1957) and Vahs (1957) studied the effect of ribonuclease treatment on the inducing abilities of a large number of mammalian, and
amphibian adult tissues. The total induction frequency remained in
most cases almost unchanged, except for a few where a slight reduction
was indicated. In several cases changes in the regional specificities were
noticed. The inability of ribonuclease treatment to diminish inducing
activity was further demonstrated on tobacco mosaic virus and tissue
homogenates by Kuusi (1951, 1953) and on tissue extracts by Hayashi
(1955). It has already been mentioned that although ribonucleoprotein
of guinea-pig liver has very strong archencephalic and deuterencephalic
effects on isolated ectoderm, a treatment of the sample with ribonuclease
which removes the major part of RNA, does not interfere with its
inducing ability (Hayashi, 1959).
On the other hand, the claim that ribonuclease suppresses the
inducing ability of an inductor was made by Brächet (1943), concerning
the induction caused by tobacco mosaic virus and liver granules.
However, Brächet himself (Brächet, Kuusi and Gothié, 1952) admitted
later that the sample of ribonuclease used in his experiment might have
had proteolytic activity which could be responsible for the observed
reduction of induction.
Thus the weight of evidence speaks against the active role of RNA in
the mediation mechanism of specific induction. On the other hand, it
should not be overlooked that not all RNA species, and not all techniques of RNA isolation have been studied so far. Hence the conclusion
inevitably remains provisional.
VII. Transmission of Mediating Factors
In the foregoing sections suggestions have been obtained as to the
agent which mediates the embryonic induction of various types. In the
present section, attempts will be made to discuss available information
concerning the possible mechanism whereby the agent may be transmitted from the inductor to the reacting system.
First let us discuss the most simple case in which the protein is
dissolved or suspended in the medium, and the cells cultured in the
medium are induced to differentiate specific structures (Section IV).
As the mode of differentiation evoked in these cases is quite different
from the mode of differentiation observed after subcytolytic stimulation,
it is improbable that the mechanism suggested in Section III is operative
in these cases. It has not been ascertained yet whether the protein really
enters the cell, or attaches itself to the cell membrane and thence exerts
its influence on cell behaviour. That the latter alternative is theoretically
possible, is illustrated by the scheme of Weiss (1950), in which a foreign
TUNEO
YAMADA
Johnen (1957) and Vahs (1957) studied the effect of ribonuclease treatment on the inducing abilities of a large number of mammalian, and
amphibian adult tissues. The total induction frequency remained in
most cases almost unchanged, except for a few where a slight reduction
was indicated. In several cases changes in the regional specificities were
noticed. The inability of ribonuclease treatment to diminish inducing
activity was further demonstrated on tobacco mosaic virus and tissue
homogenates by Kuusi (1951, 1953) and on tissue extracts by Hayashi
(1955). It has already been mentioned that although ribonucleoprotein
of guinea-pig liver has very strong archencephalic and deuterencephalic
effects on isolated ectoderm, a treatment of the sample with ribonuclease
which removes the major part of RNA, does not interfere with its
inducing ability (Hayashi, 1959).
On the other hand, the claim that ribonuclease suppresses the
inducing ability of an inductor was made by Brächet (1943), concerning
the induction caused by tobacco mosaic virus and liver granules.
However, Brächet himself (Brächet, Kuusi and Gothié, 1952) admitted
later that the sample of ribonuclease used in his experiment might have
had proteolytic activity which could be responsible for the observed
reduction of induction.
Thus the weight of evidence speaks against the active role of RNA in
the mediation mechanism of specific induction. On the other hand, it
should not be overlooked that not all RNA species, and not all techniques of RNA isolation have been studied so far. Hence the conclusion
inevitably remains provisional.
VII. Transmission of Mediating Factors
In the foregoing sections suggestions have been obtained as to the
agent which mediates the embryonic induction of various types. In the
present section, attempts will be made to discuss available information
concerning the possible mechanism whereby the agent may be transmitted from the inductor to the reacting system.
First let us discuss the most simple case in which the protein is
dissolved or suspended in the medium, and the cells cultured in the
medium are induced to differentiate specific structures (Section IV).
As the mode of differentiation evoked in these cases is quite different
from the mode of differentiation observed after subcytolytic stimulation,
it is improbable that the mechanism suggested in Section III is operative
in these cases. It has not been ascertained yet whether the protein really
enters the cell, or attaches itself to the cell membrane and thence exerts
its influence on cell behaviour. That the latter alternative is theoretically
possible, is illustrated by the scheme of Weiss (1950), in which a foreign