Skeletal Muscle

Staci Bryson, MD

Larissa V. Furtado, MD

Key Facts

Embryology

Skeletal muscle derives from mesoderm
- Pharyngeal arches give rise to muscles of head
- Paraxial mesoderm gives rise to muscles of trunk
- Lateral plate mesoderm gives rise to visceral and limb muscles
Paraxial and lateral plate mesoderm are formed by cells migrating laterally from the primitive streak in week 3 of gestation
Paraxial mesoderm develops into somitomeres and then somites during week 3
Somites form from cranial to caudal at rate of 3-4 per day up until day 30 of gestation
Somites differentiate into myotomes (and sclerotomes and dermatomes) during week 4 of gestation
- Myotomes further subdivide into dorsal and ventral parts, which are separately innervated by spinal nerves (dorsal and ventral primary rami)
Neck and trunk muscles contract spontaneously by week 7
Embryo can respond to skin stimulation, and some postural reflexes are present by week 12
Muscle development progresses to the point where fetal movements are felt by mother by weeks 16-20

Macroscopic Anatomy

Muscle comprises 25% of body weight at term
Individual muscles vary greatly in size and shape
Individual muscles are composed of varying numbers of muscle fibers, up to 1 million in gastrocnemius
Muscles are connected at both ends by tendons or epimysium
Blood supply to individual skeletal muscles is not well delineated, and arterial supply can vary from person to person
- Most skeletal muscles receive blood from multiple arteries, making skeletal muscle relatively resistant to ischemia
Gross color of muscle generally progresses from pink to red over course of fetal life due to accumulation of myoglobin and increasing vascular supply

Microscopic Anatomy

Earliest cells with distinct muscle differentiation are myoblasts, containing desmin
Myoblasts are spindle-shaped cells with finely granular cytoplasm, oval nuclei, and prominent nucleoli
Myoblasts begin to form primary myotubes, precursors to muscle fibers, by week 7
Primary myotubes are characterized by longitudinally arranged nuclei
- Cytoplasm is eosinophilic due to presence of mitochondria, glycogen, and myofibrils; cross-striations are not yet present
- Myofibrils accumulate in myoblasts even as myotube formation continues
- Myofibrils are composed of thin actin and thick myosin filaments
Secondary myotubes form during early fetal period by fusion of myoblasts, progress toward multinucleated myocytes
Secondary myotubes mature into myofibers, recognizable at midpoint of 2nd trimester
- Myofibers are marked by development of cross-striations and progression to peripheral nuclei
Early muscle fibers are undifferentiated as far as type 1 vs. type 2
- Differentiation to type 1 fibers occurs from 16-20 weeks gestation
- Differentiation of type 2 fibers occurs later with type 2b fibers appearing at ˜ 25 weeks and type 2a at ˜ 35 weeks
- Most muscle fibers have differentiated by term