Muscle
Hyperplasia
Tetsuro Tamaki
Department of
Physiology
Tokai University School of Medicine
Kanagawa,
Japan
It has been generally accepted that the fiber content of
mammalian skeletal muscle remains constant after birth and that
muscle hypertrophy occurs exclusively through enlargement of the
existing individual fibers. Recently, however, considerable evidence
has indicated that growth and enlargement of the muscle may be the
result of both hypertrophy of existing fibers and an increase in
fiber numbers (muscle fiber hyperplasia). Technical difficulties have
always existed in the evaluation of muscle hyperplasia because it is
impossible to count all of the fibers in the muscle from histological
cross-sections. All fibers do not always appear in the histological
cross-sections because of the existence of pinnately fibered muscle
(many mammalian skeletal muscles show this structure). Furthermore,
whole muscle is always required for correct evaluation of muscle
hyperplasia, and an animal model is essential. To overcome such
problems, a muscle fiber counting method using a nitric acid
digestion technique which teases free all of the fibers, and animal
models in which muscle hypertrophy occurs following muscle
hyperplasia have been developed by several researchers over the past
20 years. It now appears that muscle hyperplasia may occurs in
mammalian skeletal muscle.
Mechanisms Of Muscle
Hyperplasia
There have been two mechanisms proposed concerning
the appearance of muscle hyperplasia. One is fiber division induced
by longitudinal fiber splitting, and the other is new fiber formation
depending on muscle fiber precursor cells.
1) Longitudinal
fiber splitting
Originally,
increases in fiber numbers occurring as a result of longitudinal
fiber splitting was the main concept of muscle hyperplasia. This
suggestion is based in part on the observation in histological
sections that fibers with points of branching or with central
cleavages exist in skeletal muscle. The presence of such fibers in
skeletal muscle was reported in the second half of the 19th century,
and has been frequently observed in various muscle disorders such as
muscular dystrophy and muscle trauma. However, the relationship among
hypertrophy, hyperplasia and splitting of fibers begin to be noticed
in the late 1960s. Such fibers have been called "splitting
fibers", "bifurcated fibers", or "branched
fibers". Concerning the mechanisms of onset of fiber splitting,
one possibility that has been suggested is an increased metabolic
gradient (gradient of oxygen and energy supply) across the fiber
occurring due to hypertrophic effects, and resulting less efficient
metabolism; hypertrophied muscle fibers divide or split to maintain
an optimal fiber cross-sectional area. In fact, it seems that
branched fibers tend to increase following resistance training.
However, at present, this concept is not widely accepted because it
has been suggested that these branched or splitting fibers may be
derived from muscle fiber precursor cells, termed satellite cells.
Satellite cells are responsible for muscle regeneration following
injury or damage, and it appears that when muscle fibers are damaged
by resistance exercise, satellite cells are stimulated by such fiber
damage. Daughter branching of branched fibers may also be a feature
of the development of these cells. Recently, it has been confirmed
that a few of these branched or splitting fibers are present in
normal non-exercised muscle, and the features of these fibers can be
clearly observed three-dimensionally by scanning electron microscopy
(Fig. 1).
2) New fiber formation
Another mechanism of
muscle hyperplasia is new fiber formation by muscle precursor cells
(satellite cells). This is the main concept of muscle hyperplasia at
present.
Satellite
cells are found in all mammalian skeletal muscle. Every skeletal
muscle fiber has a great many nuclei (multinucleated cells). About
100 of these nuclei are present per millimeter length of fiber. At
least 95% of them are muscle nuclei under the cell membrane (plasma
membrane of the muscle fiber). The remaining 5% are satellite cell
nuclei which lie outside the cell membrane but within the basement
membrane. Satellite cells are a mononucleated cells which have a
small amount of cytoplasm and no myofilaments, but which are capable
of undergoing mitotic division to increase the number of available
muscle nuclei. They are considered to be the only cells in the muscle
which can divide. Ordinary muscle nuclei do not, or can not, divide.
Thus, it is thought that satellite cells are reserve cells used to
supply an extra nuclear population if the muscle is damaged. When
muscle is damaged, satellite cells belonging to the damaged fiber are
activated to become activated satellite cells or myoblasts and enter
the cell cycle (mitotic cycle of the cell) in the damaged portion of
the parent fiber. After several mitoses to produce a population of
cells, these cells fuse within the damaged portion, and form a
multinucleated syncytial tube of cells called a "myotube".
They are then re-innervated, develop into a myofiber (muscle fiber)
and repair the damaged portion. This is the regeneration process of
muscle fiber, and this process is an important not only to in
understanding new fiber formation but also the formation of branched
fibers (mentioned above). It appears that branched fibers are the
result of an incompletely regenerated muscle fiber.
Returning
to the main theme of this section, the formation of new muscle fibers
might be supported by satellite cells which function as reserve
muscle cells. If certain stimulations which can activate satellite
cells are applied to the muscle by resistance training, and if
mitosis and development of the satellite cells take place out-side
the parent fiber (out-side the plasma membrane of parent fiber)
without causing any damage to the parent fiber, it is possible that
new muscle fiber will appear along the parent fiber. Schematic
drawing of the formation of new fiber and branched fiber is shown in
Fig. 2.
Muscle Hyperplasia Following Growth
Activation
of the satellite cells is also observed in growing muscles after
birth. The concept of postnatal growth of the muscle occurring by a
combination of longitudinal and circumferential hypertrophy of the
existing fibers has been recognized for a long time. Recently,
however, several reports have indicated that increases in the fiber
number are observed in postnatal growing muscles of the rat. First,
the rate of total satellite cells to the total muscle nuclei in adult
rat muscle is about 5%, but in the newborn they account for some
30-35% of all muscle nuclei. At 4 weeks of age (comparable to human
childhood), the rate dropped to 10%, and reached 5% by 10 weeks of
age (comparable to young adults). Second, many proliferating cells
within the muscle (it has been suggested that almost all of them are
activated satellite cells) are observed in the newborn, but the
numbers decrease with age, and they are hardly seen at about 10 weeks
of age. Third, activated satellite cells can be frequently observed
in growing muscle by electron microscopy, but in adult muscles, they
are hardly observed. Fourth, it has been confirmed that the total
number of muscle fibers gradually increased with age until 10 weeks
of age by direct counting using the nitric acid method. These
findings actually suggested that postnatal growth of the muscle
depends on longitudinal and circumferential hypertrophy, and
increases in new fibers from satellite cells (muscle hyperplasia).
Stimulation for Inducing Muscle Hypertrophy Following Muscle
Hyperplasia
There
are some methods for inducing muscle hypertrophy (refer to the
section on "Muscle Hypertrophy"), but they seem to be
limited to methods which induce hypertrophy following hyperplasia.
The basic method consists of prolonged heavy resistance training with
relatively long exercise duration. Many researchers have studied
muscle hyperplasia using various hypertrophied muscles (refer to the
section on "Muscle Hypertrophy") in humans and laboratory
animals.
Interestingly, however, muscle hypertrophy following hyperplasia seems to be observed only as exercise-induced muscle hypertrophy by prolonged weight-lifting exercise. In another types of muscle hypertrophy, activation of satellite cells is certainly observed in the early stages of the hypertrophy, but no increases in fiber numbers appear finally.
Cases
showing hypertrophy and hyperplasia are as follows:
1)
Prolonged weight-lifting exercise of cats.
2) Prolonged
weight-lifting exercise of rats.
3) Bodybuilders with
extraordinarily enlarged muscles.
Evidence
of hyperplasia in cats and rats is obtained by direct counts of total
muscle fibers using nitric acid treatment. Evidence in human
bodybuilders is obtained by estimation from small biopsy samples or
electrophysiological analysis. However, prolonged weight-lifting
training is common to all of these cases. Weight-lifting training is
performed by various athletes, especially by power athletes, but the
appearance of hyperplasia is reported only in bodybuilders.
Bodybuilders generally use different training systems from other
power athletes, and tend to display different adaptations of the
muscles from others (full details are given in the section "Muscle
Hypertrophy"). They use a moderately high load and relatively
high number of repetitions, and certain muscle groups are exercised
separately. This exercise is usually followed by or combined with two
or more additional exercises which activate the same muscle group,
interspersed with short resting times. Furthermore, as many as 16-20
consecutive sets
stressing a certain muscle might be executed
within 30-40 minutes
to achieve the state termed "muscle pumping up". This
system is exhaustive for the muscle. In addition, the training
regimen used in the experiment on rats is also composed exhaustively,
comparable to the human bodybuilder's system (the training regimen of
the cat(1) appears to be slightly different from those for the rat(2)
and humans(3)). Moreover, another possibility of hyperplasia has been
reported for swimmers and kayakers who display hypertrophied
deltoideus muscle, despite surprisingly small fiber diameters in
muscle biopsy analysis. Both sports also apply intensive and
exhaustive stimulations to the muscles. Prolonged training or
exercise consisting of relatively high-intensity and endurance
(wholly exhaustive) regimens induce muscle hypertrophy following
hyperplasia.
Why Is Muscle Hyperplasia Induced Only By
Prolonged And Repeated Bouts Of Exhaustive Heavy Resistance Training
?
The
common point in the above evidence is that relatively intense and
long duration (exhaustive) exercise induced hyperplasia. Such
exercise is actually associated with fiber damages, because it has
been found that fibers displaying abnormal features such as central
nuclei (proof that the fibers have been damaged) and necrosis were
observed in biopsy samples from elite bodybuilders. There have also
been reports that degenerative changes were observed in some fibers
following weight-lifting training on cats and rats. Such damage would
certainly activate the satellite cells and lead to fiber
regeneration, but the process by which the satellite cells cause
hyperplasia is still unknown. Perhaps, the severe stress applied to
the muscle by such exercise directly stimulates and activates the
satellite cells without regard to muscle damage. This might be the
reason why activated satellite cells in fibers with no alteration are
detected by electron microscopy in cat muscle subjected to prolonged
weigh-lifting.
In
short, both direct stimulation by physical training and stimulation
from damaged fibers should activate the satellite cells. Some of
these satellite cells will be engaged in repair of damaged fiber, and
others will be engaged in new fiber formation.
There
are several other investigative methods that induced dramatic muscle
hypertrophy, such as a tenotomy, ablation and stretch-induced
hypertrophy (refer to the section on "Muscle Hypertrophy").
Activation of satellite cells is also observed in these muscles but
the hyperplasia is not induced in the end stage. It is hypothesized
that these methods are too stimulating for the muscle; passive
stretch and compensatory overloads chronically stimulate the muscle
for several days or weeks. Furthermore, the damaged portion of the
muscle is wide-spread. This state can hardly be considered as a
normal physiological condition. In contrast, training regimens
inducing hyperplasia consist of repeated bouts and provide sufficient
recovery time under normal physiological conditions. The damaged
portion of the muscle is small and randomly distributed (unpublished
data). It is also speculated that these recovery phases may
contribute to the hyperplasia. A resting state under normal
physiological conditions is required for better adaptation of the
muscle. In fact, if the training schedule is too frequent and too
exhaustive, overtraining will bring about inevitable results.
Furthermore, abnormal fibers (complex branching fibers) are
frequently observed in compensatory hypertrophied muscles.
It
is recognized that high-intensity and exhaustive exercise is the most
severe state for the muscle under normal physiological conditions.
Perhaps this muscle crisis (lack of blood and oxygen supply, severe
fatigue, and demand for maximum energy metabolism) is required for
the appearance of hyperplasia, but this crisis should be followed by
a sufficient recovery phase under normal physiological conditions.
In
conclusion, muscle hyperplasia is one of the adaptation mechanisms of
the muscles in the same way as muscle hypertrophy. It can be
speculated that muscle hyperplasia might be the final adaptation
mechanism for the muscle crisis. Generally, skeletal muscle attempts
to adapt to the applied overload which results in individual fiber
hypertrophy. However,
if crisis stress under normal physiological conditions is applied to
the muscle, muscle hypertrophy following hyperplasia should occur to
adapt to such a load. If the crisis occurs under abnormal
physiological conditions such as compensatory and stretch-induced
hypertrophy or other muscle disorders, many complex branched fibers
would appear.
Training
regimens which cause muscle hypertrophy and hyperplasia at the same
time are effective in creating extraordinarily enlarged muscle
(bodybuilder's muscle), but the functional significance of these
muscles is doubtful. The cause or mechanism of muscle hyperplasia has
yet to be sufficiently clarified, and largely depends upon future
multilateral studies. However, muscle hyperplasia undoubtedly occurs.
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Figure legend
Fig. 1
Branched fibers observed in normal rat skeletal muscle. A) Y type
branched fiber, it is the most frequently observed.
B) Subtype of
the Y type, Y type fused with one more fiber.
C) X type, two
fibers fused with middle portion of their length. Bar:
100#61549;m
Fig.2 Schematic drawing of the formation of branched
and new muscle fiber (speculation).