Transcript The Growth and Development of Skeletal Muscle

Growth & Development
of Skeletal Muscle
Skeletal, Striated, Voluntary
Muscle
6 weeks from conception
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At the end of week 3, the intra-embryonic
mesenchyme differentiates into three loose
aggregate pairs of mesenchyme on each side of
the neural tube (Paraxial, Intermediate, Lateral)
Paraxial mesoderm differentiates into the future
dermatome (dorsal surface), myotome (middle
layer), and sclerotome (ventral layer), forming
dermis, muscle, and connective tissue respectively.
Intermediate mesoderm, will form the future
urogenital system.
Lateral mesoderm will develop into future body
cavities and parts of the body wall.
Somite Development
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The paraxial mesoderm will develop into
paired cuboidal bodies, or somites. These
will eventually develop into the bones
(sclerotome), muscles (myotome), and
dermis (dermatome) of and surrounding
the axial skeleton.
Somites appear as bumps on the dorsal
surface of the embryo.
At the end of week 3, 4-12 somites are
present By the end of week 5, 42-44 can
be counted. However, most appear
between days 20-30, giving this period the
title of the somite period of development.
Somites appear cranially to caudally,
beginning at the occipital end. They can be
counted and are used to roughly estimate
the age of the embryo.
5 - 7 coccygeal pairs disappear leaving 37
pairs of somites.
Dorsal View of an Embryo
at about 22 days (8 somite stage)
Law of Original Innervation
The myoblasts (future muscle cells) form concurrently with the
spinal nerves and they migrate out from the notochord together.
This results in the formation of 31 spinal nerves with associated
skin, muscle, and connective tissue.
Dermatome: an area of skin receiving mesenchyme from
a specific somite that is supplied by a single spinal nerve and
its ganglion
6 weeks
8 weeks
Mesenchyme
Myoblasts
Myotubules
Single Muscle Fibre
Peripheral Nuclei
Myogenesis
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Mesoderm – pluripotent connective tissue cell
Presumptive myoblast – undergoing mitosis,
mononucleated cell incapable of fusion or contractile
protein synthesis
Myoblast – mononucleated cell not undergoing mitosis,
cell capable of fusion and of synthesizing myofibrillar
proteins
Myotubule – multinucleated cell from fusion of
myoblasts, may contain sarcomeres depending on stage.
Nuclei at centre in early stages migrating to periphery as
matures to muscle fibre
Muscle fibre – mature multinucleated muscle cell with
myofibrils
Growth of Muscle
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Muscle fibre number increases prenatally and
for a short time postnatally
Fiber number doubles between 32 weeks
gestation and 4 months of age
Girth and length increases continue into
postnatal period
Postnatal increase in muscle girth due almost
entirely to hypertrophy not hyperplasia
Fibres increase length by:
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increase in # of sarcomeres (major)
increase in length of sarcomeres
length increase primarily at musculotendinous
juncton in respone to functional length
Muscle Composition
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FETUS - fibres small in number and widely
separated by extracellular material
TERM - still small, greater number, more
closely packed
ADULT - Larger diameter with little space
between them
Therefore:
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decrease in sodium and chloride (extracellular)
increase in potassium (intracellular)
decrease in % water content
Changes in Muscle Composition
in Water, Sodium (Na), Chloride (Cl) and Potassium (K)
as a percentage of adult levels
from 13 weeks of gestation to Adulthood
400
350
300
Water
Na
Cl
K
250
200
150
100
50
0
13-14 Weeks
20-22 Weeks
Full Term
4-7 Months
Adult
Muscle Mass Estimated from Creatinine Clearance
Creatinine levels sensitive to diet and exercise
K40 estimate of Fat Free Mass
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Assumed constant
proportion of potassium
in Fat Free Mass
Average values
acceptable, whereas
indivudual estimates
have considerable error
due to variability in
proportion of potassium
in Fat Free Mass
Skinfold-adjusted
Girths
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Indicators of
“muscularity”
Cross-sectional
data from the
Coquitlam
Growth Study
(1979)
Estimated Muscle Mass
as a Percentage of Body Weight
(data collected from various sources)
55
% Muscle Mass
Male
Female
50
45
40
35
5
7
9
11
13
13.5
15
Age (yrs)
15.5
17
17.5 20-29
Strength Differences
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No gender difference in strength
if expressed per unit of crosssectional area of muscle
Disproportionate strength
increase in male adolescence
more in upper extremities than
trunk or lower extremities
No significant difference (7-17
years) in lower extremity
strength after adjusting for
height, between boys and girls
Peak Strength Velocity
occurs after Peak Height Velocity
Muscle Width
(distance and velocity curves)
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Peak velocities occur later than peak height velocity in boys and
girls
Strength “Maturity” not reached until late twenties
Resistance
Training
Children can weight train with individual monitoring
Effort/Benefit ratio is very high prior to puberty
Training prior to puberty has lasting effect
Fibre Typing
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Type I
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Type II (IIa & IIb)
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Red (Slow Twitch Fibres)
White (Fast Twitch Fibres)
High proportion Type I and undifferentiated Type
IIc fibres during early and mid-childhood in
comparison to adulthood
Little known about sex associated differences in
fibre type distribution
Dahlström, M. (Karolinska Institute)
The dancer: Physical effort, muscle fibre types, and
energy intake and expenditure
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Muscle fibre type composition in young dancers, even at
the very beginning of their professional dance training,
differs from that in the average individual and is
characterised by a high % of type I fibres, similar to that
found in 20 years old dancers.
This, together with the fact that detraining did not change
the muscle fibre type composition, supports the idea that
the high percentage of type I fibres in dancers is due to a
selection of individuals with suitable muscle fibre
composition to the dance profession rather than being
an effect of training.