BRIAN POOLE
and tissues more or less constant during the process of growth in higher
animals. The problem is most evident during the period of increase in body
size, but we know from the work of Leblond and Walker (1956) and others
that there is a continuous process of cell division and cell death in all the
tissues of the body with the exception of the central nervous system. Even
in the mature animal, control mechanisms are necessary to maintain constancy of tissue and organ mass. In some tissues, notably liver and kidney,
this stability of tissue mass is so great that the whole mass will be restored
after surgical removal of a large fraction of the tissue provided that enough
functional tissue is left to tide the animal over the immediate postoperative
period. Partial hepatectomies can be performed repeatedly on the same
animal with complete restoration (Ingle, 1954; G. E. C. Simpson and
Finckh, 1963). The difficulty of partial kidney removal has prevented the
experimental reduction of kidney mass more than twice in the same animal
(Pearce, 1908).
It is probably a mistake to consider the capacity for compensatory
growth in these two tissues as evidence for a special mechanism of growth
control restricted to these tissues. Only in a tissue with a high potential
growth rate and a functional capacity greater than that normally required
by the organism (so that part of the tissue can be safely removed) would
the capacity for compensatory growth ever become evident. Other tissues
may very well be under the same type of organismic control. There is some
evidence of compensatory hypertrophy in the lungs of tadpoles (Liozner
et al. f 1964), in rat submandibular gland (Alho, 1961), and in other tissues
(Liozner, 1963). Perhaps the only tissue which has been shown not to
exhibit compensatory hypertrophy is the outer orbital gland of the rat
(Teir and Sundell, 1953).
The growth of many endocrine glands is partly controlled by the secretion of trophic hormones, whose secretion in turn is partly controlled by
levels of the hormone secreted by the target gland. In particular, the
adrenals, thyroid, and gonads show compensatory hypertrophy, presumably
as a result of increased secretion of trophic hormones. This is a special
mechanism of growth control restricted to a small number of tissues. We
shall confine our discussion of hypertrophic growth control to the liver
and kidney, but our conclusions may well be valid for a wide variety of
tissues.
B. Use of Terms
Three words are frequently used in connection with these restorative
processes—hypertrophy, hyperplasia, and regeneration. We shall use the
term "hypertrophy" to refer to an increase in protoplasmic mass of a tissue
or organ. Hypertrophy is sometimes restricted in meaning to an increase
94
and tissues more or less constant during the process of growth in higher
animals. The problem is most evident during the period of increase in body
size, but we know from the work of Leblond and Walker (1956) and others
that there is a continuous process of cell division and cell death in all the
tissues of the body with the exception of the central nervous system. Even
in the mature animal, control mechanisms are necessary to maintain constancy of tissue and organ mass. In some tissues, notably liver and kidney,
this stability of tissue mass is so great that the whole mass will be restored
after surgical removal of a large fraction of the tissue provided that enough
functional tissue is left to tide the animal over the immediate postoperative
period. Partial hepatectomies can be performed repeatedly on the same
animal with complete restoration (Ingle, 1954; G. E. C. Simpson and
Finckh, 1963). The difficulty of partial kidney removal has prevented the
experimental reduction of kidney mass more than twice in the same animal
(Pearce, 1908).
It is probably a mistake to consider the capacity for compensatory
growth in these two tissues as evidence for a special mechanism of growth
control restricted to these tissues. Only in a tissue with a high potential
growth rate and a functional capacity greater than that normally required
by the organism (so that part of the tissue can be safely removed) would
the capacity for compensatory growth ever become evident. Other tissues
may very well be under the same type of organismic control. There is some
evidence of compensatory hypertrophy in the lungs of tadpoles (Liozner
et al. f 1964), in rat submandibular gland (Alho, 1961), and in other tissues
(Liozner, 1963). Perhaps the only tissue which has been shown not to
exhibit compensatory hypertrophy is the outer orbital gland of the rat
(Teir and Sundell, 1953).
The growth of many endocrine glands is partly controlled by the secretion of trophic hormones, whose secretion in turn is partly controlled by
levels of the hormone secreted by the target gland. In particular, the
adrenals, thyroid, and gonads show compensatory hypertrophy, presumably
as a result of increased secretion of trophic hormones. This is a special
mechanism of growth control restricted to a small number of tissues. We
shall confine our discussion of hypertrophic growth control to the liver
and kidney, but our conclusions may well be valid for a wide variety of
tissues.
B. Use of Terms
Three words are frequently used in connection with these restorative
processes—hypertrophy, hyperplasia, and regeneration. We shall use the
term "hypertrophy" to refer to an increase in protoplasmic mass of a tissue
or organ. Hypertrophy is sometimes restricted in meaning to an increase
94