MORPHOGENESIS OF CROWN
GALL
71
number of multistranded chromosomes, these bundles would fall apart in
the first mitosis so that reduction, in another division, would again require
somatic pairing.
The differentiation of tumor cells has been discussed extensively in
the anatomical parts of the present review. On the cytological level the
lignification often starts with a few or even a single cell. Kupila
(1956) has presented the hypothesis that the crown gall cells are
able to divide only a certain number of times and thereafter are forced to
differentiate. This idea could be tested by feeding radioactive thymidine
to the original crown gall cells and finding out whether all diploid lignify -
ing cells display the same amount of labeling.
The lignification in the crown gall cells agrees with the same process
in normal cells (cf. Therman, 1956). When a cell is about to differentiate,
its cytoplasm becomes denser; thereafter it forms a pattern on the walls
along which lignification proceeds. At the same time changes occur in
the nuclei which finally become pycnotic. The cytoplasmic and later on
the lignification patterns in adjacent cells correspond exactly. In the
broad bean gall the first cells to lignify display scalariform or reticulate
thickenings. In older tumors, other types of lignified cells appear, fibers
with practically no pits and small-pitted large cells. Each cell type differentiates without previous growth or other visible changes. In the old
galls, all but the very largest giant cells finally lignify.
VI. Origin of Crown Gall
Since the classic studies by Erwin F. Smith and his associates in the
beginning of the century, the similarities of crown gall and animal
cancer have been stressed repeatedly. It is, therefore, not surprising that
the origin of crown gall has been considered the most important problem
in the field. It is clear that a normal cell is changed into a crown gall
cell through a mutation, in the widest sense of the term—a change takes
place in a cell, and all its descendants show the new characteristics. The
site and mode of this change are, however, far from clear, and a number
of hypotheses have been put forward to explain its nature.
These hypotheses (cf. Braun and Wood, 1961) can be divided into
three groups. According to the first, the crown gall change would be
nuclear, either due to a gene mutation or to a change in chromosome
structure or number. Into the second group fall all the ideas postulating
an extranuclear effect. These would include crown gall induction by the
bacteria in some modified form, by viruses or episomes, or by some
GALL
71
number of multistranded chromosomes, these bundles would fall apart in
the first mitosis so that reduction, in another division, would again require
somatic pairing.
The differentiation of tumor cells has been discussed extensively in
the anatomical parts of the present review. On the cytological level the
lignification often starts with a few or even a single cell. Kupila
(1956) has presented the hypothesis that the crown gall cells are
able to divide only a certain number of times and thereafter are forced to
differentiate. This idea could be tested by feeding radioactive thymidine
to the original crown gall cells and finding out whether all diploid lignify -
ing cells display the same amount of labeling.
The lignification in the crown gall cells agrees with the same process
in normal cells (cf. Therman, 1956). When a cell is about to differentiate,
its cytoplasm becomes denser; thereafter it forms a pattern on the walls
along which lignification proceeds. At the same time changes occur in
the nuclei which finally become pycnotic. The cytoplasmic and later on
the lignification patterns in adjacent cells correspond exactly. In the
broad bean gall the first cells to lignify display scalariform or reticulate
thickenings. In older tumors, other types of lignified cells appear, fibers
with practically no pits and small-pitted large cells. Each cell type differentiates without previous growth or other visible changes. In the old
galls, all but the very largest giant cells finally lignify.
VI. Origin of Crown Gall
Since the classic studies by Erwin F. Smith and his associates in the
beginning of the century, the similarities of crown gall and animal
cancer have been stressed repeatedly. It is, therefore, not surprising that
the origin of crown gall has been considered the most important problem
in the field. It is clear that a normal cell is changed into a crown gall
cell through a mutation, in the widest sense of the term—a change takes
place in a cell, and all its descendants show the new characteristics. The
site and mode of this change are, however, far from clear, and a number
of hypotheses have been put forward to explain its nature.
These hypotheses (cf. Braun and Wood, 1961) can be divided into
three groups. According to the first, the crown gall change would be
nuclear, either due to a gene mutation or to a change in chromosome
structure or number. Into the second group fall all the ideas postulating
an extranuclear effect. These would include crown gall induction by the
bacteria in some modified form, by viruses or episomes, or by some