Muscular System
Time period:
day 22 to week 9
Introduction
Most muscle
cells originate from the paraxial mesoderm (Figure 25.1), and
specifically the myotome portion of the somites. The three types of muscle
described here are skeletal, smooth and cardiac muscle.
Skeletal
muscle
Within each
somite the myotome splits into two muscle‐forming parts: a ventrolateral
edge and a dorsomedial edge (Figures 25.2 and 25.3). The ventrolateral edge
cells will form the hypaxial musculature (i.e. that of the ventral body
wall and, in the limb regions, musculature of the limbs) (Figures 25.4 and
25.5). The dorsomedial edge will form the epaxial musculature (the back
muscles).
During
formation of skeletal muscle multiple myoblasts (muscle precursor cells)
fuse to form myotubes at first, and then long multi-nucleated muscle fibres
(Figure 25.6). By the end of month 3, microfibrils have formed and the
striations of actin and myosin patterning associated with skeletal muscle are
visible. Important genes involved in myogenesis include MyoD and Myf5,
which cause mesodermal cells to begin to differentiate into myoblasts, and then
MRF4 and Myogenin later in the process.
A fourth
part of the somite, the syndetome, has been recently shown to contain
precursor cells of tendons (Figures 25.2 and 25.3). The cells of the syndetome
lie at the ventral and dorsal edges of the somites between the cells of the
myotome and sclerotome; blocks of cells whose tissues they will connect. They
also migrate, but develop independently of muscles and connect later in development. However, tendon cells will also arise from lateral plate mesoderm to
populate the limbs, so the full story of tendon development is not limited to
the somite.
The upper
limb bud is visible from day 26 around the levels of cervical somite 5 to
thoracic somite 3. The lower limb starts at the level of lumbar somite 2 and
finishes between lumbar 5 and sacral 2 (see Figure 26.1). The migrating muscle
precursors migrate into the limbs, coalesce and form specific muscle masses
which then split to form the definitive muscles of the limbs (see Chapter 26).
It is known that, as in skeletal development, cell death is important in the
development of these muscle masses. Joints within the limbs develop
independently from the musculature (see Chapter 23) but foetal musculature and
the motions that occur are required to retain the joint cavities.
Neurons of
spinal nerves that follow migrating myoblasts are specific to their original
segmental somites. By roughly 9 weeks most muscle groups have formed in their
specific locations. The migration of whole myotomes and fusion between them
accounts for the grouping of muscular innervation seen in adult limb anatomy.
Movements of
the limbs can be detected using ultrasound at 7 weeks and isolated limb
movements from around 10–11 weeks.
In the head
area the somitomeres undergo similar changes but never fully develop the three
compartments of the somite, and this process remains less well understood.
Myogenesis
in the head differs from trunk and limb myogenesis as these muscles have
different phenotypic properties, although myoblasts still develop from the
paraxial mesoderm of the somitomeres and migrate into the pharyngeal arches and
their terminal locations.
The
surrounding connective tissues coordinate migration and differentiation of
muscle as elsewhere, but the nerves to these muscles are present before their
formation, as they are cranial nerves. Musculature formed from pharyngeal
arches and their innervation is described in Chapters 40–43.
Extraocular muscles probably arise from
mesenchyme near the prechordal plate (a thickening of endoderm in the embryonic
head). Muscles of the iris are derived from neuroectoderm, whereas ciliary
muscle is formed by lateral plate mesoderm. Muscles of the tongue form
from occipital somites, as does the musculature of the pharynx. Movement of the
mouth and tongue and the ability to swallow amniotic fluid begins around week
12.
Most smooth
muscle of the viscera and gastrointestinal tract (Figure 25.7) is derived from splanchnic
mesoderm that is located where the organs are developing (Figure 25.8).
Developing blood vessels surround local mesenchyme that forms smooth muscle.
Larger blood vessels (aorta and pulmonary vessels) receive contributions from neural
crest cells.
Exceptions
to the splanchnic mesoderm rule include muscles of the pupil, erector pili
muscles of hair, salivary glands, lacrimal glands, sweat glands and mammary
gland smooth muscle, all of which are derived from ectoderm.
Cardiac
muscle cells are also derived from splanchnic mesoderm surrounding the early
heart tube. The cardiac
myoblasts differ from skeletal myoblasts in that they do not fuse to form
multinucleated fibres, and they remain individual but connected via
intercalated discs (Figure 25.9).
At
approximately 22 days a cardiac tube has formed that can contract (see Chapter
27).
Clinical relevance
Muscular dystrophy is a group of over 20
muscular diseases that have genetic causes and all produce progressive weakness
and wasting of muscular tissue.
Duchenne muscular dystrophy affects boys (in
extremely rare cases symptoms show in female carriers) and affects the gene
coding for the protein dystrophin. Patients develop problems with walking between
1 and 3 years of age, wheelchairs are necessary between 8 and 10 years, and
life expectancy is limited to late teens to early adulthood as cardiac muscle
is affected in the later stages of the disease. There is no cure but research
into using stem cells in forms treatment is ongoing.
An absence
or partial absence of a skeletal muscle can occur (e.g. Poland anomaly which
exhibits a unilateral lack of pectoralis major). Other commonly affected
muscles include quadriceps femoris, serratus anterior, l and palmaris longus, and
are relatively common.
