100
BRIAN POOLE
of the equilibrium tissue mass. We shall return to a consideration of this
model after discussing the detailed time course of the hypertrophic response
in liver and kidney.
B. Other Theories
Teir (1951) has theorized that the cytolysis of cells of a particular kind
releases tissue-specific "wound hormones" which stimulate the growth of
homologous cells. When cells die in the tissues of an adult organism, they
must be replaced in order to maintain a constant tissue mass. If the rate
of tissue growth were related to the rate of cell death, it is possible that a
limited degree of stability might be achieved in an intact organism. However it is very hard to see how this theory could be used to explain hypertrophic growth when part of a tissue is cleanly removed. Teir explains
normal growth of the whole body by invoking nonspecific growth hormones.
The oldest theoretical explanation of hypertrophic growth is that the
elevated functional activity of the tissue remaining after surgical intervention itself causes the hypertrophy. This was probably suggested by the
well-known cellular hypertrophy of muscle which follows repeated exercise.
It has also been suggested that the increased blood flow through the reduced
tissue mass remaining is the stimulus to hypertrophic growth (Higgins
et al, 1932; and many others).
C. Experimental Analysis
One might suppose that it would be a relatively straightforward matter
to design and execute critical experiments to distinguish between the
various hypotheses. However, after more than 50 years of experimental
effort, it is still impossible to decide definitely the source of the stimulus
to hypertrophic growth. The best we can do is to examine the evidence and
draw some tentative conclusions.
III. Experimental Methods and Difficulties
A. Measurement of Growth Rate
An important obstacle to an experimental analysis of the mechanism of
control of normal and hypertrophic growth is the variability introduced
by the use of individual experimental animals. It has been essential to
study growth control in vivo, because growth in vitro seems to be uncontrolled or controlled by factors which are not ordinarily important in the
intact animal, for example, nutritional factors. The problem of individual
variation in response can be reduced, but not eliminated, by using inbred
strains as much as possible.
BRIAN POOLE
of the equilibrium tissue mass. We shall return to a consideration of this
model after discussing the detailed time course of the hypertrophic response
in liver and kidney.
B. Other Theories
Teir (1951) has theorized that the cytolysis of cells of a particular kind
releases tissue-specific "wound hormones" which stimulate the growth of
homologous cells. When cells die in the tissues of an adult organism, they
must be replaced in order to maintain a constant tissue mass. If the rate
of tissue growth were related to the rate of cell death, it is possible that a
limited degree of stability might be achieved in an intact organism. However it is very hard to see how this theory could be used to explain hypertrophic growth when part of a tissue is cleanly removed. Teir explains
normal growth of the whole body by invoking nonspecific growth hormones.
The oldest theoretical explanation of hypertrophic growth is that the
elevated functional activity of the tissue remaining after surgical intervention itself causes the hypertrophy. This was probably suggested by the
well-known cellular hypertrophy of muscle which follows repeated exercise.
It has also been suggested that the increased blood flow through the reduced
tissue mass remaining is the stimulus to hypertrophic growth (Higgins
et al, 1932; and many others).
C. Experimental Analysis
One might suppose that it would be a relatively straightforward matter
to design and execute critical experiments to distinguish between the
various hypotheses. However, after more than 50 years of experimental
effort, it is still impossible to decide definitely the source of the stimulus
to hypertrophic growth. The best we can do is to examine the evidence and
draw some tentative conclusions.
III. Experimental Methods and Difficulties
A. Measurement of Growth Rate
An important obstacle to an experimental analysis of the mechanism of
control of normal and hypertrophic growth is the variability introduced
by the use of individual experimental animals. It has been essential to
study growth control in vivo, because growth in vitro seems to be uncontrolled or controlled by factors which are not ordinarily important in the
intact animal, for example, nutritional factors. The problem of individual
variation in response can be reduced, but not eliminated, by using inbred
strains as much as possible.