The manner in which skeletal muscle of the pig adapts to meet the changing
mechanical and metabolic demands of an increase in body weight during postnatal
growth was studied using two different methods.
(1) Histochemical profiles of individual muscle fibres were established using
the myosin ATPase, succinate dehydrogenase (SDHase) and glycogen phosphorylase
(GPase) reactions. Samples of longissimus and diaphragm muscles from a series
of 18 Large White pigs between birth and 60 kg liveweight, and from a series
of 16 Large White pigs of mean liveweight of 93 kg, were used.
In both muscles, the number of fibres low in myosin ATPase activity
increases with growth, and these fibres are grouped into bundles. In
m, longissimus, the estimates of the total fibre population and the number of
myosin ATPase low bundles in a complete transverse section of the muscle remain
constant, while the mean number of myosin ATPase low fibres per bundle increases
from one at birth to 3'2 at 93 kg liveweight. Whereas the complete transverse
sectional area of the muscle increases in proportion to the 2/3 power of the
body weight, the area occupied by myosin ATPase low fibres increases in direct
proportion to the body weight. This observation suggests the mechanism by
which larger animals are supported without a relative increase in their muscle
mass. Some histcchemical evidence was obtained that this is achieved by a
transformation of the physiological properties of certain fibres.
The diaphragm of smaller pigs contains a greater proportion of a myosin
ATPase high, SDHase high and GPase low fibre type than that of more mature pigs.
After initial neonatal differentiation, the muscles studied do not change their
proportion of SDHase high fibres during growth,
In both longissimus and diaphragm, the mean transverse sectional areaof
myosin ATPase high fibres is greatest when the SDHase activity is low. Also,
the mean transverse sectional area of SDHase high fibres is greatest when the
myosin ATPase activity is low, but this difference is significant only for the
diaphragm,
(2) Eighteen female pigs of both the Pietrain and Large White breeds, from birth
to 72 kg liveweight, were dissected, and the major tissues were weighed. The
growth of fat, muscle and bone, relative to carcass growth, were compared for
both breeds. Fat is the fastest developing tissue; fat and muscle grow at
a rate higher, and bone at a rate lrwer, than overall carcass growth. The
musclesbone ratio increases during the growth interval studied.
Growth changes in the distribution of muscle and bone were studied by
weighing these tissues after separation into 93 muscle units and 10 b^ne units.
The pattern of growth of both these tissues can be described as an increasing
craniocaudal gradient axially and an increasing distcproximal gradient in the
limbs. It is proposed that these gradients are an adaption to enhance the
animal's propulsive force. The cardiac muscle:skeletal muscle ratio decreases
with growth.
Of the major tissues, only the growth of bone is significantly different
between the two breeds; this tissue develops faster relative to carcass growth
in the Large White. When the breeds are compared at the same body weight, the
weight of muscle is greater in the Pietrain over the entire growth range studied.
Musclejbone ratios, compared at the same values of total muscle plus bone, are
higher for the Pietrain. There are no significant differences between the
muscle:bone ratios of the Large Whites used in the present study and the Large
Whites dissected by McMeekan ever 30 years ago.
A comparison of the difference in tissue distribution between the two breeds
shows that growth gradients for muscle are accentuated in the Pietrain. Although
the weight of muscle in all regions is higher in the Pietrain at birth, only the
weights of abdominal and femoral muscles and m. longissimus are significantly
higher in the Pietrain when these weights are compared in pigs of 60 kg body
weight. No difference in bone distribution is apparent between the breeds.
Consequently, although the ratio of brachial muscle weight to humerus weight is
higher for the Pietrain, in pigs of 60 kg body weight the difference between the
breeds is very much greater for the ratio of femoral muscle weight to femur weight.
At 60 kg body weight, the heart of the Large White is heavier; the cardiac
muscle{skeletal muscle ratio of the Large White is higher over the entire growth
range studied.
Since the proportions and distribution of the skeletal muscle, cardiac muscle
and bone of the Pietrain are an exaggeration of the changes observed in the Large
White during growth, it is concluded that the Pietrain is more mature at the same
body weight than the Large White, and that a genetic control of muscle
distribution is possible.
A study of samples of m. longissimus, removed from the muscle without
restraint to contraction, suggests that the greater development of this muscle
in the Pietrain is due to relative hypertrophy of a similar number of component
fibres. No difference between the breeds is observed in the proportion of fibre
types, as determined by the myosin ATPase reaction.
