Orgin of skeletal muscle
A large proportion of the skeletal musculature is
formed from the myotomal cells of the somites. Note that the most
cranial somites - the 4 occipital somites - provide myotomal cells
to the developing tongue. The remaining myotomes more caudally
become organised into a dorsal column called the epimere, from which
the deep extensor muscles of the back develop, while the more ventro-laterally
placed hypomere contributes muscles to the body wall.
Note how the spinal nerves reflect this subdividion
by having a posterior primary ramus (to the epimere), and an
anterior primary ramus (to the hypomere).
Some body wall muscles develop in situ from local
mesenchyme. The same is true of the limb musculature - this develops
from the mesenchyme of the limb buds. The embryonic pharyngeal
arches give rise to numerous muscles in the head and neck - the
muscles of facial expression, mastication, swallowing, speaking,
Selected muscle-forming zones in the embryo - myotomes in the
somites, limb bud mesoderm, septum transversum, pharyngeal arch
musculature, and orbital muscles.
Transverse section through the embryo -
myotomal derivatives separated into epimere (alongside neural tube)
and hypomere (forming body wall - three-layered anterolaterally).
groups in the early fetus
Origin of cardiac
This develops from splanchnic mesoderm around the
endothelial heart tube. Recall that the heart soon begins to beat -
this demonstrates that some cardiac muscle fibres have
differentiated rapidly in week 3.
Origin of smooth muscle
Smooth muscle fibres develop from mesodermal cells
throughout the body, but in terms of bulk, the greatest proportion
is derived from the splanchnic mesoderm around the gut tube and its
derivatives, for example the respiratory system.
Development of the diaphragm
Although generally speaking you will not need to know
how particular skeletal muscles develop, there is one that is of
practical interest - the diaphragm. Occasionally the diaphragm
develops with a deficiency large enough to allow abdominal contents
to herniate into the thorax - usually when the baby tries to
establish breathing at birth. This condition is known as
diaphragmatic hernia. Notice that development of the diaphragm
begins in the cervical region of the embryo. You will recall that
the motor supply of the diaphragm comes from the phrenic nerves
which arise from cervical levels of the spinal cord: "C3, 4, 5,
keeps the diaphragm alive".
A note on the limb buds
The limb buds first appear at the end of week 4. The
upper limb buds are slightly in advance of the lower limb buds. The
arm buds develop alongside the lower cervical and upper thoracic
segments; the leg buds develop alongside the lower lumber and upper
Each limb buds consists of a mesodermal core covered
by ectoderm. The ectoderm at the tip of the limb bud is thickened
and known as the apical ridge. The apical ridge is thought to
control initial development of the limb bud. In the 6th week, the
distal ends of the limb buds flatten to form hand and foot plates.
The digits are defined by radially-organised zones of cell-death in
the hand and foot plates. Failure of this process results in fusion
or webbing of the digits - syndactyly. Extra fingers or toes may
develop, a condition known as polydactyly.
Mesoderm within the limb buds condenses to form
cartilagenous models of future bones, and also develops into the
muscles. The developing muscles are grouped into compartments.
Branches of the spinal nerves enter the limb buds early in
development and play a crucial role in organising further
differentiation. The muscles depend on a motor nerve supply for
normal development, and sensory fibres are distributed to the skin
to establish the pattern of dermatomes detectable in the adult.
(Note that the word dermatome has two uses: one in the embryological
context and another in the context of adult anatomy.)
The developing limbs rotate. At first, the arm buds
and the leg buds extend laterally, approximately at right angles to
the long axis of the trunk. The preaxial border in both cases lies
cranially and the postaxial border caudally. Towards the end of the
embryonic period, the upper limb becomes adducted at the shoulder,
so that the preaxial border of the arm lies laterally. (The fetus
usually flexes and pronates its forearm, so this will reposition the
preaxial border of the more distal portions of the limb.) The lower
limb rotates medially so that the original dorsal surface of the
limb is brought to face ventrally, particularly in the lower
segments. These rotations can be identified in the patterns of the
dermatomes, muscle groups, and nerve plexuses.
Endochondral ossification of the limb bones continues
throughout the fetal period and after birth, until fusion of the
epiphyses occurs in the late teens/early twenties.
Major abnormalities of the limbs such as amelia or
meromelia tend to be uncommon. However, the drug Thalidomide caused
a high incidence of these abnormalities in the early 1960's.
(Thalidomide was prescribed to mothers who were suffering from
severe `morning sickness' in the early stages of pregnancy.)
Abnormalities of the muscular system
Often, there is a close relationship between skeletal
and muscular abnormalities. Revise the abnormalities you encountered
in the previous section on skeletal development, but this time note
the associated muscular defects. Another abnormality to look up is
Questions on the muscular system
1 Which germ layer gives origin to almost all the
2 Compare and contrast the general organisation of
the muscles of the thoracic wall with those of the abdominal wall.
3 What tissues are derived from:
a) occipital somites
4 Which part of the diaphragm is formed from the
septum transversum? Which other embryonic structures contribute to
the definitive diaphragm? How do you explain the nerve supply of the
diaphragm in embryological terms?
5 What might be the consequences of a defect in the
6 In a baby with spina bifida, the erector spinae
muscles become displaced antero-laterally in the region of the
lesion. What effect might this have on the already damaged spine?