Muscle as Food

461 Pages·1986·25.037 MB·English

1
Muscle Development and
Contractile Proteins
P. J. BECHTEL
Meat Science Laboratory
University of Illinois at Urbana - Champaign
Urbana, Illinois
I. Skeletal Muscle Structure 2
A. Gross Structure and Muscle Bundles 2
B. Muscle Cell 2
C. Other Cell Types in Muscle 9
D. Cardiac and Smooth Muscles 10
II. Muscle Development 10
A. Muscle Embryology 10
B. Myotube Formation H
C. Determinants of Muscle Fiber Number 12
D. Muscle Protein/DNA Ratios 12
E. Muscle Nuclei Number 13
F. Muscle Fiber Type 13
III. Factors Affecting Muscle Growth and Development 14
A. Growth Factors 14
B. Muscle Stretching 15
C. Muscle Regeneration 16
D. Nutritional Status 17
E. Environmental and Genetic Factors 17
F. Abnormalities of Skeletal Muscle 18
IV. Muscle Proteins 18
A. Thick Filament Proteins 18
B. Thin Filament Proteins 22
C. Ζ Line and α-Actinin 25
D. Interaction of Myosin and Actin 25
E. Contractile Protein Stoichiometry 26
F. Other Myofibrillar Proteins 27
G. Sarcoplasmic Proteins 28
H. Muscle Glycogen 29
I. Collagen 30
References 31
MUSCLE AS FOOD 1 Copyright © 1986 by Academic Press, Inc.
All rights of reproduction in any form reserved.
2 P. J. Bechtel
I. SKELETAL MUSCLE STRUCTURE
Muscle tissue is a major component of the body: it has been estimated that
the human body contains no fewer than 434 muscles, which constituet ~ 25%
of the body mass at birth and — 40% in the adult man (Adams, 1975).
A. Gross Structure and Muscle Bundles
Skeletal muscles are composed of bundles of muscel fibers embeddde in
connective tissue. The major cell component is called the muscel cell or muscel
fiber; howeve,r other types of cells found in muscel include those of vascula,r
connective, adipose, and nerve tissues. The gross organization of a muscel is
shown in Fig. 1. The muscel is encasde in a layer of connective tissue called the
epimysium, which is composed predominateyl of collagen. Individual muscel
cells are surrounded by connective tissue called the endomysium, which forms a
continuous connective tissue network from the muscel cell surface to the
epimysium. Muscle cells are grouped into bundles that are surrounded by a
layer of connective tissue called the preimysium. Small bundles are grouped
into larger bundles, which eventualyl form the muscle. As a generality, the
vascular system and nerves are found between muscel bundles, although capil-
laries and a nerve ending are found in each endomysial layer.
A single motor nerve axon innervates a group of muscel cells and sends a
single axonal filament to each cell. Large muscles such as the gluteus maximus
may contani up to 200 muscel cells per group; the small eye muscles, only five
muscle cells per group (Lockart, 1973).
Most muscles attach to bones. The muscel connective tissue (type III colla-
gen) becomes continuous with the tendon connective tissue. Tendons contani
type I collagen and are lubricated to facilitate the gliding action of the tendon
(Ramachandrna and Redd,i 1976).
B. Muscle Cell
The muscel cell is the cellular contractiel unit of skeletal muscle. The
number of total muscel cells in large meat animasl is unknown but is probabyl in
the range of hundreds of millions. Muscle cells are large multinucleated cells
that vary in both width and length. The diameter of muscel cells ranges from 10
to 100 ìçé and their length can vary from several millimeters to more than
30 cm (Lockhart, 1972). The diameter of individual muscel cells varies over
their length and often is smaller at the ends. Muscle cell diametesr vary with
exercise, animal age, animal sex, nutritional state, breed, specie,s and type of
muscle. Individual muscel cells can span the length of small muscles but
1. Muscle Development and Contractile Proteins 3
generalyl span only a fraction of the muscel length in large animals. Fibers can
run parallel to the length of the muscel but often are arranged at an angle.
1. SARCOLEMMA AND T-TUBULE SYSTEM
The sarcolemma is the plasma membrane of the muscel cell and is similar in
structure to other plasma membrane.s The membrane has a thickness of
~ 7 nm and has the same trilaminate structure as other cell membranse (Adams,
4 P.J. Bechtel
Myofibril
Muscle fiber nuclei
Sarcolemma
Transverse tubules
Satellite cell
Basal lamina
Endomysial reticular fibers
Capillary endothelial cells
Endomysial collagen
and elastin fibers
Fibroblast
Fig. 2 Organization of muscle cell. Courtesy of J. Novakofski.
1975). The structure is formed from a bimolecular leaflet of phospholipids in
which the nonpolar portions of the molecuel are oriented inward and the polar
moieties are on the surface. Carbohydraet moieties and proteins are integral
componenst of the membran.e Some proteins are located only on the outer or
inner membrane surface while others span the membran.e Many of the pro-
teins on the outer membrane surface are glycoproteins.
The muscel cell membrane is surrounded by a basemetn membrane structure
which contains reticular fibers and has a thickness > 20 nm. The basal lamina
is enclosed by the endomysial collagen sheaht (Fig. 2). The endomysial collagen
sheaht is attached to the basemetn membran,e which in turn is attached to the
sarcolemm.a
The sarcolemma has a number of functions, the first of which is to compar-t
mentalize the contractiel units. Another major function is regulation of uptake
and release of some molecules by the muscel cell. Probabyl the major distin-
guishing feature of the sarcolemma is its ability to depolarize in respones to a
nerve impulse. The site at which the nerve axon is attached to the muscel
membrane is the motor endplate. The muscel membrane under the motor
1. Muscle Development and Contractile Proteins 5
endplaet region contains an increased number of acetylcholine receptor,s mito-
chondria, and other component.s Once the nerve is stimulated and releases
acetylcholine, receptosr on the sarcolemma bind the acetylcholine and go
through a series of evenst ultimately resulting in the depolarization of the
sarcolemm.a
The transveres tubule (T-tubule) system is a series of small (20-nm diamete)r
invaginations of the sarcolemm.a There are two T-tubule systems per sarco-
mere, which surround the myofibrils and are located at the site at which the A
and I bands meet (A-I junction) in mammalian skeletal muscel (Fig. 3). The
function of the T-tubule system is to bring the membrane depolarization event
from the sarcolemma to the inner regions of the muscel cell. T-Tubule mem-
brane is thought to have electrical excitability properties similar to the sarco-
lemma. There is nine times as much T-tubule membrane as sarcolemma mem-
brane.
The structure of the sarcoplasmci reticulum is shown in Fig. 3. This mem-
brane structure has a biochemical composition different from both the sarco-
lemma and T-tubule system. Its main function is to release and sequestre
calcium in the muscel cell. Sarcoplasmci reticulum forms a matrix around the
myofibril between the T-tubules. The sarcoplasmci reticulum is not continu-
ous with the T-tubule system; howeve,r a series of small structures called feet
bridge the gap between the T-tubule membrane and the sarcoplasmci reticu-
lum. When the muscel cell is stimulated, the sarcoplasmci reticulum releases
calcium; howeve,r the mechanims of transmitting the signal from the T-tubule
system to the sarcoplasmci reticulum is not fully understoo.d The sarcopla-s
mic reticulum has the ability to sequestre calcium against a concentration
gradien,t a process that requires energy. On a relative scale, there is one part
sarcolemma membrane to nine parts T-tubule membrane to one hundred and
forty-four parts sarcoplasmci reticulum membran.e
There is a correlation between the amount of sarcoplasmci reticulum and the
speed of contraction (Peachy and Porter, 1959). Muscles with faster contrac-
tion speeds have a more developed sarcoplasmci reticular system. A more
developed sarcoplasmci reticulum may allow for a faster, more uniform release
and uptake of calcium.
2. MYOFIBRIL
Myofibrils are the long thin contractiel elemenst inside the muscel cell that
give the characteristci striated pattern (Fig. 4). Vertebraet myofibrils are 0.5-
1.0 μτη in width and have a fairly uniform diameter throughout their length
(Adams, 1975). The sarcomeer is the unit of muscel structure between the two
Æ lines (Bloom and Fawcet,t 1975). Other bands that can be observed with the
light microscope include the A band, I band, and Æ line (Fig. 4). Areas that
appear darkest are the Æ line and the regions of the A band where thick and thin
Fig. 3 Organization of the sarcoplasmic reticulum and transverse tubule systems.
filaments overlap. Sarcomeer length changes depending on the contractiel
state of the muscel and a resting mammalian psoas muscel will have a sarcomeer
length of approximateyl 2.6 μτη (Huxley, 1973).
The sarcomer,e depicted in Fig. 4, is constructed of thick filaments ,thin
filaments ,and Æ lines. Mammalian thick filaments have diametesr of 10 to
12 nm and lengths of 1.5 μτη and thin filaments have diametesr of 5 to 7 nm and
lengths of 0.375 μπ\ (Huxley, 1973).
The sliding filament model for contraction has the thick and thin filaments
1. Muscle Development and Contractile Proteins 7
Myofibril
H Ζ A I
Band Disc Band Band
é—ι I I II
Resting State
A Thin Thick
Band Band Filament Filament Ζ line
Z line i * I
—1
Contracted State
Fig. 4 Myofibril organization.
sliding past one anothe,r resulting in an increase or decreaes in sarcomeer
length. The thick and thin filaments do not change length, but the degree of
overlap between thick and thin filaments change.s A cross-sectionla examina-
tion of the myofibril reveasl a hexagonla array of six thin filaments around a
thick filament.
The Æ line maintains the spacing between the thin filaments and is ~ 50 -
65 nm in diameter depending on the muscel type. Structural analyssi of the Æ
line has not been completed, but the protein á-actinin is presen.t When the
Z-line is examined at high magnification, it appeasr as a zigzag structure that
anchors the thin filaments .Thin filaments do not span the Æ line (Knappesi and
Carlsen, 1962).
Two other myofibril structures are the M line and N band. The M line
2
appeasr as a line running parallel to the Æ line that connecst the centesr of the
thick filaments togethe.r The M line is located in the center of the H zone.
Knappesi and Carlsen (1968) described a complex model for the M line struc-
ture that consisted of both M filaments and M bridges in conjunction with the
thick filaments . The N line is a structure located in the I band that runs
2
parallel to the Æ lines. This structure may be composed of a high molecular
weight protein (Wang and Williamson, 1980).
8 P. J. Bechtel
Poorly understood myofibrillar structures are the 2-nm filaments observed in
stretched myofibrils (Locker and Leet, 1975). Maruyama et al. (1977b) and
Wang et al. (1979) described the purification of new proteins that could form
the 2-nm filaments .Much work remains to be done on the identification and
characterizatino of the thin 2-nm filaments and associatde proteins.
Other types of filaments that can be found in skeletal muscel include small
numbers of microtubules, especialyl in developing muscle, and the 10-nm inter-
mediate filaments . The intermediaet filaments in skeletal muscel are thought
to connect adjacent myofibrils together at the level of the Æ line (Robson et al,
1981).
3. OTHER CELL ORGANELLES
The mature skeletal muscel cell is multinucleated and can contani several
thousand nuclei per cell. In mammalian muscle, the nuclei are located directly
beneaht the sarcolemma and have a width of 1 to 3 μτη and a length of 5 to
12 μτη (Adams, 1975). These small nuclei are surrounded by a nuclear envelope
and the nuclei inside mature muscel cells do not divide. The primary function
of skeletal muscel nuclei is to provide the DNA templaet for RNA synthesi.s
Skeletal muscel contains lysosome,s which are small spherical particles 0.2-
2 μτη in diameter surrounded by a membrane and located in the interior of the
muscle cell (Bird et al, 1980). Lysosomes are thought to be involved in intra-
cellular digestion and turnover of biochemical component.s Lysosomes con-
tain a variety of enzymes including protease,s nuclease,s glycosidase,s lipases,
phospholipase, sand phosphatase. s As a generality, muscel contains far less
lysosomal enzyme activity than most other tissues such as liver and spleen.
There is more lysosomal enzyme activity in slow-twitch than fast-twitch mus-
cles. Bird et al. (1980) have shown that the lysosomal enzymes cathepsins  and
D are found inside the muscel cell.
Muscle mitochondrai are found in the sarcoplasma and are aligned with the
contractile fibers of skeletal muscle. These mitochondrai are usualyl cigar-
shaped and consist of two separaet membrane sacs. The inner membrane has
infoldings called cristae, which are studded with small protruding structures
called inner membrane particles. Mitochondria contani enzymes involved in
the citric acid cycle, electron transpotr assemblag,e fatty acid oxidation, other
lipid-metabolizing enzyme,s some nitrogen-metabolizign enzyme,s and some of
the enzymes involved in urea synthessi (White et al, 1978). Slow-twitch oxida-
tive muscles contani more mitochondrai than fast-twitch glycolytic muscle.
There does not appear to be anything unique about muscel mitochondrai
except that they are located in close proximity to the myofibrils.
A unique feature of muscel is the large number of glycogen particles found in
the sarcoplasm. The glycogen particle is not enclosed in a membrane and
1. Muscle Development and Contractile Proteins 9
isolated particles have variable diameter,s often in the range 20-30 nm (Preiss
and Walsh, 1981). The glycogen particle is associatde with a number of the
enzymes involved in glycogen metabolism. The particle size will change as the
glycogen is being synthesizde or degraded to suppyl glucose units for energy
production.
Muscle cells have a number of small Golgi bodies located near one pole of
each muscel nucleus (Bloom and Fawcet,t 1975). The Golgi organelel plays an
important role in the secretion of proteins from many types of cells. Skeletal
muscle does not secreet as many proteins as many other cell types and the Golgi
apparatus of muscel cells is not a major cellular componen.t
C. Other Cell Types in Muscle
There are a number of different cell types in muscel including nerve cells, cells
involved in the synthessi of collagen, fat cells, and the different types of cells
that constituet the vascular system. There is a large variation in the number of
mature fat cells in muscle. Differences in fat cell number can be attributed to
species, breed, animal maturity, nutrition state, and a large number of other
variables (Allen et ai, 1976). Fat cells are derived from mesenchym;e when they
start to accumulaet lipid, they are called adipoblast.s The mature lipid-filled fat
cells are called adipocyte.s The mature adipocyet is a large spherical cell that
can have a diameter >100//m; the small precursor cells have diametesr of
~ 5 μπι. The potential of changing the diameter of adipose tissue cells 50-fold
through dietary manipulation is an indication of the plasticity of this tissue.
In skeletal muscle, fat cells are usualyl located next to small vessesl and often
associatde with the perimysial connective tissue. The lipid inside the fat cell is
in the form of a large droplet; other cellular component,s such as the nucleus,
are located next to the cell membran.e There are two major types of adipose
tissue, commonyl called white and brown fat. White fat is found in skeletal
muscle and constitutes the bulk of the adipose tissue. Brown fat is found in
some fat pads and has been associatde with thermal regulation.
Connective tissue is a major component of skeletal muscel and is composed
of extracellular fibers of collagen, elastin, and reticulin, ground substanec mu-
copolysaccharide, sand fibroblast cells. The protein and ground substanec are
synthesizde by fibroblasts and form the fibrous connective tissue matrix that
surrounds the muscle. During embryonci developmen, tit is difficult to distin-
guish between cell types; howeve,r the connective tissue in muscel begins to
acquire a recognizabel form at about the time muscel cells are formed. At this
stage, the connective tissue will divide the muscel into primary and secondayr
groups of muscel cells. Endomysial connective tissue will extend between
individual muscel fibers late in fetal developmetn (Adams, 1975). The amount
10 P. J. Bechtel
of connective tissue between the muscel fibers increases with age. There is a
higher content of connective tissue in muscles used for locomotion such as
those in the leg.
D. Cardiac and Smooth Muscles
Muscles can be classified as either striated or nonstriated. Striated muscles
exhibit regularyl spaced transveres bands along the length of the cell and can be
further subdivided into skeletal and cardiac muscle. Nonstriated or smooht
muscle is composed of cellular units that are not subject to voluntary control.
Smooth muscel is found in the walls of the digestive tract, ducts of glands,
respiratoyr passage, swalls of blood vessel,s and many other places. Smooht
muscle cells have one nucleus per cell and are long spindle-shapde cells, having
lengths up to 0.5 mm (Bloom and Fawcet,t 1975). The cells are often found in
bundles or sheet.s Contraction of smooht muscel can be initiated by nerve
impulses, hormonal stimulation, or changes in the muscel cell such as stretch-
ing. The major contractiel proteins in smooht muscel are myosin, actin, tropo-
myosin, á-actinin, and filamin; howeve,r troponin is not presen.t As the name
implies, nonstriated muscel lacks the filamentous organization found in cardiac
and skeletal muscle.
Cardiac muscel cells are 50-120 μτη in length and have an intercalated disc
that connecst adjacent cardiac cells. Cardiac muscel cells contani only one or
two nuclei, which are located in the cell interior. Cardiac muscel contains
more cytoplasm and mitochondrai than skeletal muscle. Differences between
atrial and ventrical cardiac muscles include smaller cell size, fewer Ô tubules in
atrial muscle, and different isoforms of some contractiel proteins such as myo-
sin. The intercalated discs of cardiac muscel are located on the opposing ends
of cardiac muscel cells and have complex interdigitating structures that main-
tain cell - cell cohesion. The thin filament of the adjacent I bands terminaet in
the matrix of the intercalated disc, resulting in continuity from cell to cell. In
the mature meat anima,l the heart constitutes < 1% of the live animals' weight.
II. MUSCLE DEVELOPMENT
A. Muscle Embryology
Cells forming limb muscel are derived from the mesoderm. Striated volun-
tary muscel is derived from paired somites (limb muscles) and from mesenchyme
or the branchial arches (head and neck muscles.) The portion of a somite left
after emigration of the schlerotome mass to form a vertebrae is the myotome or