Go
to the main content page
Vertebrate
muscle systems. Basic pattern of development
The
gastrula is the stage of embryonic development at which
the embryo appears as three distinct layers of cells (the
germ layers): the exterior ectoderm, the middle mesoderm,
and the interior endoderm. The mesoderm differentiates to
form most of the remaining tissues, structures, and organs
of the body. As the embryo lengthens, the mesoderm lying
along the midline differentiates to form the chordamesoderm,
which gives rise to the notochord, a hollow, cartilaginous
nerve tube and a major distinguishing feature of the vertebrates
and their closest relatives, the protochordates. In the
adult the notochord contributes only to the structure of
the vertebrae. The mesoderm lateral to this midline chordamesoderm
then divides into three parts: the epimere, nearest the
chordamesoderm; the hypomere, farthest away from it; and
a small mesomere between these two.
Along
the length of the embryo, the epimere divides into segmental
blocks called somites, which again divide into three cell
blocks: closest to the notochord appears the sclerotome,
which will form the greater part of the vertebrae; in the
middle lies the myotome, which gives rise to somatic muscle;
and to the side lies the dermatome, which contributes to
the skin. Appendicular muscles form either directly as outgrowths
of the myotomes or from wandering cells (mesenchyme) that
bud off from the myotomes. The mesomere gives rise only
to structures of the urogenital system. The hypomere remains
unsegmented, and, except in the region of the neck where
it remains a solid mass, it splits longitudinally, giving
rise to a central space. This space, together with its counterpart
on the other side of the developing body, will eventually
form the major cavity of the body, the coelom. The exterior
layer gives rise to the lining of the body wall. The interior
layer, together with the adjacent epithelial lining of the
yolk sac, develops into the muscles of the heart and other
viscera. The solid mass of the hypomere in the neck region
differentiates to form the various branchiomeric visceral
muscles, which areassociated with the visceral skeleton,
the gill bars, or their derivatives. Posterior to (dorsal
to) this region is the developing ear capsule, where the
myotomes are rudimentary or absent. Farther forward, however,
three more myotomes appear, which give rise to a distinct
group of axial muscles, namely the external muscles of the
eyes. The first few myotomes behind the ear region typically
bend forward at their anterior (ventral) ends to produce
slips of muscle that migrate to a position ventral to the
gill pouches and that form the hypobranchial musculature
(somatic muscles that run between the pectoral skeleton
and the jaw).
