Download Muscle Structure, Myogenesis, and Growth

Muscle Structure
Animal Growth and Development
Types of Muscle
► Skeletal
► Cardiac
► Smooth
► Classification




Striated
Non-striated
Voluntary
Involuntary
Skeletal structure
► Based
on size, shape and location
► Origin- where muscles originate on one side of the
joint
► Insertion- where muscles terminate on the other
side of the joint
► Tendon – point of attachment at either insertion or
origin
► Fascia- a thin sheet of connective tissue at
attachment
Organization
► Epimysium
► Perimysium
► Endomysium
Sarcolemma
Muscles are multinucleated
Muscle Cytoskeleton
► Microfilaments
► Intermediate
filaments
► Microtubules
► Myofibrils
are microfilamentous organelles
of muscle fibers
► Sarcomere – smallest contractile unit of
muscle
Myofibrils
► Made
up of thick and thin filaments called
myosin and actin
► Z-line – outer demarcation of the sarcomere
 Made up predominantly of alpha-actinin
► A-
band
► H – Zone
► I - band
Myosin
► Occupies
80-87% of the total volume of the
muscle fiber
► Contains both heavy and light chain molecules
(heavy and light meromyosin)
► Positioned in the center of the sarcomere
► Rachet effect – acts in contraction to make the
sarcomere shorten because it pulls on actin
filaments, thus shortens one Z-line to another
Actin
► Second
most abundant in muscle fibers and
makes up almost 20% of the total protein
► Filamentous actin (F-actin) is predominant
► G-actin or globular is smaller yet is found to
aid in the development of F-actin
► Numerous G-actins develop one F-actin
Regulatory proteins
► Tropomyosin
– another filamentous protein
of the thin filament
► Troponin – also is a filamentous protein that
surrounds actin and holds the actin strands
together
► Troponin is involved in calcium binding upon
release of or increased calcium levels in the
muscle to initiate contraction
Additional Components of Muscle
Fibers
► Sarcoplasmic
reticulum – membranous
system of tubules that forms a network
around each myofibril
► T – tubules – (transverse tubules) surround
myofibrils at the A-I band jct.
► Figure 2.34
► In fact, the system between two T-tubules
is called the Sarcoplasmic reticulum
Additional Components of Muscle
Fibers
► Sarcoplasmic
reticulum consists of two structures
 Terminal cisternae
 Fenestrated collar
► These
► The
act as Ca storage reservoirs
structure formed from the two terminal
cisternae and on each side of the T-tubule is called
a triad. Note: there is no physical contact
between the T-tubule and the terminal cisternae
► Communication occurs via electrical messages
Additional Components of Muscle
Fibers
► The
electrical system
 Dihydropyridine and ryanodine recptors
 When an electrical stimuli occurs at these receptors, the
SR channel changes conformation and allows Ca into
the cytoplasm
 Calsequestrin is the part of the fiber/cell (protein) that
binds calcium in the SR
► Nucleus
– muscle fibers are multinucleated
► Mitochondria- most of the energy is made
► Sarcoplasm – cellular cytoplasm of the muscle cell
Cardiac Muscle
► Cardiocytes
–cardiac muscle fibers
► Striated and involuntary (uniform
contractions)
► Communication of these type contractions
occurs at connections called intercalated
discs
► Cardiac myofibrils are attached directly to
the intercalated discs and allow the heart to
act in syncytium or as a group
Cardiac muscle
► Cardiac
thin filaments are more variable in length
as compared to skeletal muscle fibers
► Cardiac muscle must contract even under extreme
conditions
► Because of the increased variability, this allows
more overlap for contraction during stressed times
for more uniform, consistent contraction
► Cardiac muscle fibers are more branched and have
more mitochondria than skeletal muscle
Smooth muscle
► Seen
as sheet-like muscle masses
► Is very elastic and pliable
►One
example is around the uterus where it has to
stretch
►It thickens during pregnancy and becomes more thin
while the female is open
► Smooth
muscle is more triangular and only
has one nucleus per cell
► Thick and thin filaments are not
symmetrically arranged like striated muscle
Smooth muscle
► As
compared to skeletal muscle, smooth
muscle has dense bodies that are analogous
to Z-lines
 Actin molecules are attached to dense bodies
 Thick filaments are likewise positioned
throughout the cells that overlap the thin
filaments
 As a result, when contraction occurs, forces
react in varying directions
Myogenesis
► Origin
of muscle which begins at fertilization
► Develops from mesodermal somites
► DNA responsibility
► Muscle is continually modified throughout growth
and development
► Mitosis – increase in cell numbers or cell division
► During mitosis, the entire genome or DNA has to
be replicated
► Figure 4.4
Muscle cell cycle
► Four
distinct phases
 Gap 1 (G1) all cells enter the cell cycle from this
phase
►Most
variable phase in length
►Respond to cues from the environment
 Synthetic or S-phase is the time where the cell
dedicates to DNA replications or synthesis
 Third phase is called G2, and follows the Sphase
Muscle cell cycle
► G2
phase represents when the intracellular
architecture remodels itself to accommodate
physical division of the cell mass in mitosis
or M-phase
► M-phase is the shortest phase
► After this, it re-enters the G1 phase
► Growth factors are often small proteins that
influence the environment that initiate cell
growth
Muscle cell cycle
► These
small proteins bind to receptors
► Also, steroids can bind and create an environment
for growth
 Ex. IGF-1, testosterone, etc.
► Another
alternative for cells in the G1 phase is to
exist in a protracted G1 phase or G0 phase.
► Cells are capable of remaining dormant, which can
re-enter the proliferative cycle without dividing
► These cells in muscle are called satellite cells
Muscle cell determination
► Determination
 Cells must migrate from the somites (matured
mesodermal cells (Fig. 3.1 and 3.2)
 Cells that are determined to develop into
muscle are termed as myoblasts (precursors for
muscle fibers) Fig. 4.4
 Muscle regulatory factor gene (MRFs)
►These
trigger the expression of one or more genes
for myogenesis
Muscle cell determination
► Transcription
factors include MRFs and are
responsible for turning on transcription of
other genes located in the nucleus and
contain helix-loop-helix motif
► When two of these proteins combine they
bind to the regulatory region and cause the
gene to be expressed or repressed
► Thus, MRFs are control point for myogenesis
Muscle cell determination
► Helix-loop-helix




proteins include:
Myogenin
MRF-4
Myo-D
Myf-5
► Fig.
4.5 MRFs action
Basic Genetics and Cytogenetics
Two Basic Concepts
DNA- Polymer of Nucleotide Bases
Complementary Base Pairs
I.
1 Gene – 1 Protein (Gene determining the Phenotype)
DNA is the Genetic Code for Protein Synthesis
II.
Gene is the Unit of inheritance (Inherited from both
Parents) Complementary Base Pares – DNA Duplication
and Gamete production.
Cell Division – Mitosis - Meiosis
DNA- Contain the Genes – The unit of
Inheritance
DNA- Complementary Bases
Chromosomes- Chromatin
and DNA
DNA – Genetic Code for Protein
Synthesis
DNA – Polymer of Nucleotide Bases
Nucleotide Sequence –Amino acid
Sequence of the Protein (Shape and
function)
I - DNA is the Genetic Code for Protein
Synthesis
Proteins can be Enzymes, Hormones,
Transcription Factors, Structural, etc.
Nucleotide Sequence –Amino acid Sequence of the Protein
Shape Determine the function/effect)
Genetic
Code for
Protein
Synthesis
DNA Replication
Complementary
Bases
Differentiation and Fusion
► Replication
competent – capable of
progressing the cell cycle and giving rise to
additional myoblasts even though
determined muscle cells & myoblasts have
express muscle regulatory factor genes
► Myoblasts do not contract because they do
not contain contractile proteins yet
► Before this, cells must receive signals that
induce them to differentiate
The “Differentiating” process
► Cells
stop dividing
► Cells begin to align with one another
► Cell membranes begin to fuse together to
form an immature muscle fiber (myotube)
► Muscle specific genes are regulated to
initiate
 Genes for muscle creatine kinase and
acetylcholine receptor subunit, myosin and actin
are triggered
Fusion
► The
exact process is still cloudy
► Yet, we know that small tubules or attachments
are apparent and dense structures beneath
membranes are generated to form tight junctions
between myoblasts
► Finally, two bilipid membranes become one and
dissolve in the cytoplasm of the newly formed
multinucleated cell. Note: Ca ions play a
significant role in this process
Maturation
► Once
myoblasts differentiate and fuse to
form myotubes, resulting cells do not
continue to express a given set of tenes
► The new muscle fibers change following
fusion
► This process is called maturation
Morphological Aspects of Myogenesis
► Once
the somite has reached the two
layered, dermomyotome stage cells begin to
accumulate ventrally to form the myotome.
► Myotome – a compartment of the somite
► This is believed to involve migration of cells
from the dorsomedial ridge and
ventrolateral part of the dermomyotome to
below it creating the myotome
Morphological Aspects of Myogenesis
► Recall
that skeletal muscle originates from
somites which is developed from the
mesoderm
► Myogenesis appears to involve migration
from the dorsomedial ridge and
ventrolateral spects of the dermomyotome
to immediately below the dermomyotome,
thus creating a myotome Fig 4.8
Morphological Aspects of Myogenesis
► Muscle
does not just appear during prenatal
development
► It results from biphasic processes
► Occurs from two populations of muscle fiber
myoblasts
► During myoblast differentiation, myoblasts
cluster align in a vast network of connective
tissue and fuse to form an immature muscle
fiber referred to as a myotube
Morphological Aspects of Myogenesis
► Primary
myotubes are the first to develop
► These primary myotubes are the structure
for which others attach and complete the
final fiber structure - Figure 4.11
► The myotube formation can be referred to
as the second phase of myogenesis (fetal
myoblasts) whereas the first phase of
primary myotubes are referred to as
embryonic myoblasts
Morphological Aspects of Myogenesis
► Upon
receiving signals to differentiate, the
secondary myotube use the primary myotubes as
a template for alignment and organization
► This alignment of fetal with embryonic myotubes
align closely and facilitates the fusion process
► After contraction the primary fibers remain intact
and constant whereas the secondary muscle fibers
or myotubes are splintered away
► This process of producing primary myotubes is
self-governing in nature
Morphological Aspects of Myogenesis
► Depending
on the species, muscle
development occurs during the first 2/3 of
prenatal development
► The total # of myofibers is considered
established by 90 days post-conception with
pigs Fig. 4.12 - K State gestation study
► As muscle growth increases, muscle fiber
diameter increases
Myofibrillogenesis
► The
primary f(x)n of muscle is to contract
► This is done by the addition of myuofibrils
to each cell during development
► Again, skeletal muscle is highly organized
and contains many proteins
► Stress fibers are those that are bundles of
myofilaments which have contractile
possibilities
Myofibrillogenesis
► The
stress cells are located at the periphery and
interact with membrane bound proteins called
extracellular and cell-adhesion molecules
► Integrins (extracellular adhesion molecules) are
proteins that interact with extracellular proteins of
connective tissues
► Two families that are responsible for holding cells
close in contact are: adherins and NCAM’s
► Development of stress fibers to the cell membrane
represents a template for the development of
nascent myofibrils (those initially responsible for
developing muscle cells)
Muscle Growth
► Hypertrophy
vs Hyperplasia Fig. 5.1
► Muscle fibers are oriented in an oblique
fashion rather than parallel to the long axis
► Muscle fiber # is best assessed when
muscles contain more fibers that are
arranged in the long axis orientation
► Individual muscle fibers do not necessarily
extend the entire length of the muscle
Muscle Growth
► Mean
packing density = # of fibers per unit of a
cross-sectional area
► Again, minimal increases in #’s of fibers occur
during postnatal development
► The occurrence when reported that #’s increased
was due to a muscle fiber lengthening process
► This may occur during muscle regeneration
following an injury or some type of death to the
myofiber
Muscle Growth
► Satellite
cells of the muscle fiber that are
located between the sarcolemma and basal
lamina are activated to to proliferate and
ultimately fuse with other satellite cells to
form new fibers
Factors affecting muscle fiber
numbers
► Animal
variation
 Varies from animal to animal and from muscle
to muscle
► Muscle
 The primary diff. in the size of muscle is the
number of muscle fibers contained within each
 This may range from a few thousand to billions
depending on the muscle
►What
would be one of the largest muscles????
Factors affecting muscle fiber
numbers
► Species
 Excess muscle growth both #’s and size (both
length and diameter) contribute to the size of
muscles in various species
 Also, size of muscle fibers and thus muscles
contribute to the variation between species
this would include mature cattle versus sheep, etc.
►Porcine semitendinosus has roughly 1/3 of the # of
fibers as does the beef semitendinosus muscle
►
Factors affecting muscle fiber
numbers
► Nutrition
 Nutrition has a greater impact on muscle at
specific stages of growth thus altering muscle
development at various stages of growth is
imperative
 Younger animals require more protein for
muscle growth than older animals
 Also, in young animals, 600 lb. calves require
more protein than a 900 lb. calf and those
require more protein than those at 1000 lbs.
Factors affecting muscle fiber
numbers
► Swine
– litter bearing species has to
prioritize across the embryos and fetuses;
thus, variations of muscle development
among littermates occur often
 Genetic differences may occur among siblings
 Ex. Runts – those that weigh less than 2/3 of
the mean wt. of a given litter
Factors affecting muscle fiber
numbers
 Runts ex.
►If
they survive, they will usually enter the fattening
phase earlier (earlier maturity pattern) and not be as
heavy muscled.
►This may be due to undernourishment or simply
lesser development prenatally
 Yet, if adequate nutrition was available during
fetal muscle development, then numbers of
muscle fibers are not affecting during nutrition
depravation during a postnatal state.
Factors affecting muscle fiber
numbers
► Age
– the # of muscle fibers that an animal
develops is fixed at birth because
hyperplasia occurs “in ovo” or “in utero”
► Muscle fibers continue to die and regenerate
with age
► Yet, with age muscle fiber number does not
change but when tissue mass is no longer
maintained then fibers are lost
Factors affecting muscle fiber
numbers
► Breed
and genetic selection
 Faster growing breeds have more muscle fibers
than slower growing breeds
 This would also be true for lean vs fat type hogs
 Tables 5.1 and 5.2
 Selection for protein accretion is a result of
selection for heavier muscled faster gaining
animals
 Therefore, domesticated animals will have more
muscle fibers than wild animals (ie. Pigs)
Factors affecting muscle fiber
numbers
► Sex
– males tend to have greater numbers of
muscle fibers than females at birth
 Species dependent – pigs are not really different yet
bulls have more than heifers
 This may be a result of greater androgenic activity
across the membrane for cattle greater than for pigs.
Ex. Freemartin in cattle vs litters of pigs within the
uterus
 When there are more myofibers developed prenatally,
this gives rise to more hypertrophy postnatally
Factors affecting muscle fiber
numbers
► Genetic
“conditions”
 Double muscle (culard/doppellender) in cattle
►Do
not have double the # of muscles rather have
muscles that have twice the number of fibers
►Muscle to bone ratios are closer to 6:1 instead of 5:1
in normal beef cattle
 Callipyge condition in sheep
 Association with adverse effects in reproduction,
stress resistance, locomotion, etc.
Muscle Fiber Size
► Both
longitudinal and radial growth effects
the growth of muscle fibers
► Mechanisms for increasing size
 Structural changes
►Splitting
of myofibrils accounts for the majority of
increased numbers during major stages of muscle
fiber hypertrophy
►Work or exercise stimulates increasing the radial
growth of muscle fibers
Muscle Fiber Size
► Structural
changes cont.
 Length is related to stretch by imposing bone
lengthening during skeletal growth
 Length is added during growth by adding
sarcomeres
 Sarcomere #’s can multiply 3x from birth to
maturity
 Lengthening of sarcomeres also increases the
length of muscle fibers
Muscle Fiber Size
► Protein
Synthesis and Degradation
 The ability of the cell to synthesize or manufacture
proteins affects the growth of the cell – DNA
 DNA synthesis
► Transcribing
DNA in to mRNA
► Movement of mRNA out of the nucleus into the cytoplasm
► Translation of mRNA into protein
► Post-translational processing of the protein
► Positioning of the protein to specific locations
Muscle Fiber Size
► Protein
degradation or proteolysis
 Use of enzymes within the cell
 Net protein accretion or accumulation
determines muscle fiber hypertrophy
 Three protein proteolysis systems in muscle
►Lysosomal
►Calpain
►Ubiquitin-proteosome
proteolytic pathway
Muscle Fiber Size
► Lysosomal
 Cellular organelles packed with enzymes to
degrade proteins
 One family of enzymes are called Cathepsins
►Non-specific
proteases that work best at low pH
►Cystatin is responsible for deleterious degradation
► Calpain
System
 Consists of at least four major components
Muscle Fiber Size
► Calpains
 Three of the four are specific intracellular
proteases: u-calpain (1), m-calpain (2), skeletal
muscle calpain (3 or p94)
 Calpastatin – the fourth component- inhibits
calpains
 The calpain system requires calcium
 The calpain system is responsible for protein
turnover; ie. Myogenesis and myofibril assembly
as well as proteolysis
Muscle Fiber Size
► Ubiquitin-proteosome
proteolytic pathway
contains multiple enzymes, ATP, and
polypeptide co-factors
 Ubiquitin is covalently attached to proteins
intended to be broken down
►Enzymes
are: E1, E2, E3
Muscle Fiber Size
► Protein
Accretion Rates
 Building vs degradation
►When
young, building will exceed degradations and
eventually becomes zero
►In an senescent state or with diseases, degradation
may exceed building or accretion and muscle atrophy
will occur
►Factors such as nutrition, hormonal control, energy
availability, etc. frequently make an impact on
accretion
Muscle Fiber Size
► Satellite
cell recruitment
 Increases in myofiber size is continually achieved
through accretion of proteins
 Are similar to myoblasts in that most are determined
and restricted to normal muscle accretion through
protein accretion
 Once activated cells proliferate and is capable of
differentiation and fusing with adjacent muscle fibers
 Satellite cells provide growing muscle fibers with added
DNA that increases the capacity of fibers to synthesize
greater amounts of protein during cellular hypertrophy
Muscle Fiber Size
► Satellite
cells
 The number of satellite cells are highest at birth
 The capacity of muscle to synthesize protein is
evaluated by using the RNA:DNA ratio
 Fig 5.26 participation of adult satellite cells in
regenerating muscle fibers
Muscle Fiber Size
► Factors
affecting muscle fiber size
 Sex – Table 5.5
►Regardless
of breed, males have ~ 20% greater area
of muscle fibers of bulls than castrates
►This is a consequence of androgens such as
testosterone that enhances protein accretion
Muscle Fiber Size
► Nutrition
 Tables 5.6, 5.7, & 5.8 (Feed intake, protein, and
paylean effects)
 Pigs with ad libitum feeding have larger fiber
sizes
 Pigs with a higher plane of nutrition will have
larger fibers
 Overall, the effect of nutrient restriction on
postnatal muscle mass is mediated through
changes in muscle fiber size
Muscle Fiber Size
► Age
–
 Muscle fiber size increases with age until
maturity
 This does not occur or refer to normally
matured slaughter animals at the end of a
finishing period
 This refers to older more mature animals such
as older cows, sows, ewes, etc.
Muscle Fiber Size
► Growth
promotants
 Again, Muscle growth is a result of increasing
muscle fiber size
 Many growth promotants act on the
endocrine/hormonal system to promote muscle
fiber size increase or protein accretion
► Genetic
anomalies
 Double muscling, callipyge examples