THE STIMULUS TO HYPERTROPHIC GROWTH
95
in cell size, but we shall refer to this process as cellular hypertrophy.
'"Hyperplasia" will be used to refer to an increase in the number of cells in
a tissue, whatever changes may occur in the tissue mass. The term "regeneration" suggests a process of organized growth to replace a lost structure which does not occur in the tissues we are considering. After the
removal of one or more lobes of a liver, these are not restored. The residual
lobes merely hypertrophy until the tissue mass is restored (Fishback,
1929). New lobules are formed within the hypertrophying lobe (G. E. C.
Simpson and Finckh, 1963), but this is not in any sense a regeneration of
the lobes removed. Similarly, in the kidney remaining after unilateral
nephrectomy, no new nephrons or glomeruli are formed; there is merely
an hypertrophy of the structures already present (Arataki, 1926; Rollason,
1949).
C. The Process Controlled
Somehow the organism as a whole exerts a close control over the growth
of a particular tissue so that the mass of that tissue is in proportion to the
size of the organism as a whole. However, the process of tissue growth
involves two processes—the synthesis of new cytoplasm and the synthesis
of new nuclear material with cell division. The organismic control of tissue
growth could be exerted on either of these two processes: (1) The rate of
deoxyribonucleic acid (DNA) synthesis or of mitosis is controlled by the
organism and the cytoplasm is synthesized in an amount appropriate to the
number of nuclei in the tissue; or (2) the rate of cytoplasmic synthesis is
controlled by the organism and DNA synthesis and cell division keep pace
with the increase in tissue mass.
From a teleorganic point of view, the functional capacity of a tissue is
the variable that must be controlled, and this property depends much more
on the cytoplasmic mass than it does on the number of nuclei present. However, there is no apparent reason why control of cytoplasmic mass could
not be exerted indirectly through control of the rate of cell division. Indeed,
the assumption that growth control is exerted this way has been implicit
in most experimental studies and two excellent reviews (Abercrombie,
1957; Bullough, 1962).
It may well be that in epidermis, growth control is mitotic control (Iversen, 1961; Bullough and Laurence, 1964), but in liver and kidney the
experimental evidence strongly suggests that growth control in these tissues
is the control of cytoplasmic synthesis. Lieberman et al. (1963a,b) have
studied the metabolism of freshly cultured rabbit kidney cortex cells in
which there seems to be no control of the growth rate. They found that
there was always a period of intensive ribonucleic acid (RNA) and protein
synthesis lasting for almost 2 days before significant synthesis of the DNA
95
in cell size, but we shall refer to this process as cellular hypertrophy.
'"Hyperplasia" will be used to refer to an increase in the number of cells in
a tissue, whatever changes may occur in the tissue mass. The term "regeneration" suggests a process of organized growth to replace a lost structure which does not occur in the tissues we are considering. After the
removal of one or more lobes of a liver, these are not restored. The residual
lobes merely hypertrophy until the tissue mass is restored (Fishback,
1929). New lobules are formed within the hypertrophying lobe (G. E. C.
Simpson and Finckh, 1963), but this is not in any sense a regeneration of
the lobes removed. Similarly, in the kidney remaining after unilateral
nephrectomy, no new nephrons or glomeruli are formed; there is merely
an hypertrophy of the structures already present (Arataki, 1926; Rollason,
1949).
C. The Process Controlled
Somehow the organism as a whole exerts a close control over the growth
of a particular tissue so that the mass of that tissue is in proportion to the
size of the organism as a whole. However, the process of tissue growth
involves two processes—the synthesis of new cytoplasm and the synthesis
of new nuclear material with cell division. The organismic control of tissue
growth could be exerted on either of these two processes: (1) The rate of
deoxyribonucleic acid (DNA) synthesis or of mitosis is controlled by the
organism and the cytoplasm is synthesized in an amount appropriate to the
number of nuclei in the tissue; or (2) the rate of cytoplasmic synthesis is
controlled by the organism and DNA synthesis and cell division keep pace
with the increase in tissue mass.
From a teleorganic point of view, the functional capacity of a tissue is
the variable that must be controlled, and this property depends much more
on the cytoplasmic mass than it does on the number of nuclei present. However, there is no apparent reason why control of cytoplasmic mass could
not be exerted indirectly through control of the rate of cell division. Indeed,
the assumption that growth control is exerted this way has been implicit
in most experimental studies and two excellent reviews (Abercrombie,
1957; Bullough, 1962).
It may well be that in epidermis, growth control is mitotic control (Iversen, 1961; Bullough and Laurence, 1964), but in liver and kidney the
experimental evidence strongly suggests that growth control in these tissues
is the control of cytoplasmic synthesis. Lieberman et al. (1963a,b) have
studied the metabolism of freshly cultured rabbit kidney cortex cells in
which there seems to be no control of the growth rate. They found that
there was always a period of intensive ribonucleic acid (RNA) and protein
synthesis lasting for almost 2 days before significant synthesis of the DNA