Growth & Development of Skeletal Muscle

Skeletal, Striated, Voluntary Muscle

6 weeks from conception

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.

Dorsal View of an Embryo at about 22 days (8 somite stage) Somite Development Dorsal View of an Embryo at about 22 days (8 somite stage) 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.

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 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 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: increase in # of sarcomeres (major) increase in length of sarcomeres length increase primarily at musculotendinous juncton in respone to functional length

Muscle Composition 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: 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

Muscle Mass Estimated from Creatinine Clearance Creatinine levels sensitive to diet and exercise

K40 estimate of Fat Free Mass 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 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)

Strength Differences No gender difference in strength if expressed per unit of cross-sectional 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) 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 Type I Type II (IIa & IIb) Red (Slow Twitch Fibres) Type II (IIa & IIb) 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 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.