Muscle fiber development during
gestation determines the muscle structure at birth and
establishes the conditions for muscle development in
growing cattle. Differences in muscle structure among
beef cattle breeds and between beef and dairy cattle are
obvious already shortly after birth. The objective of the
study was to investigate the development of muscle fibers
and muscle fiber bundle structure in semitendinosus
muscle of divergent cattle breeds from 3 mo of gestation
until birth. Fetuses of German Angus (GA), Galloway
(GW), Belgian Blue (BB), and Holstein Friesian (HF)
were harvested at 3, 4.5, 6, or 9 mo of gestation. Muscle
sections were analyzed for fiber size and types as well as
for bundle structure. The results confirmed that primary
muscle fiber development occurs mainly during the first
trimester of gestation. All fibers were initially positive
for fetal fast myosin. Slow myosin as a marker for
fiber maturation was detected in primary fibers at 3 mo
of gestation showing a weak immunostaining. During
the second trimester, the intensity of immunostaining
strongly increased indicating increased slow myosin
protein expression. Concurrently, the shape of primary
fibers changed from myotubes to myofibers whereas the
size stayed nearly constant. The main increase in muscle
mass during the second trimester was caused by secondary
fiber development. As an example, the ratio between
secondary and primary fibers increased in Holstein
Friesian fetuses from 5.9 at 4.5 mo of gestation to 21.6 at
6 mo of gestation. Primary and secondary fibers continued
to growth during the third trimester. Regional differences
in the density of slow muscle fibers were detected leading
to greater variation within the muscle than among breeds.
Structural organization of muscle fibers in muscle fiber
bundles developed early in fetal life. At first, large main
bundles were visible. Smaller structural units defined as
primary bundles were measurable at 6 mo of gestation
when most fibers were developed. The size of primary
bundles nearly doubled from 6 mo of gestation to birth
in all breeds. In summary, differences among breeds in
the early fetal muscle fiber development were detected
in contractile differentiation and partly in muscle fiber
bundle structure. A prolonged secondary fiber generation
and altered contractile differentiation may be involved in
breed differences of postnatal muscle development.