Craniofacial Muscles

3 Extraocular Muscle Structure and Function
37
Fig. 3.6 The microscopic
anatomy of the EOM is quite
complex. Cross-sections
through the EOM show that
the muscles are not
homogeneous, but rather are
composed of two distinct
layers, the orbital (ORB) and
global (GLOB) layers. These
vary by muscle fi ber size,
which is much smaller in the
orbital layers, and by patterns
of expression of various
molecules. ( a ), ( c ), and ( e )
are serial cross-sections from
the middle of the medial
rectus muscle of a macaque
monkey. ( b ), ( d ), and ( f ) are
serial cross-sections from a
region midway between the
midregion of the muscle and
the tendon end. ( a ) and ( b )
are immunostained for fast
myosin heavy chain (MyHC)
isoform, ( c ) and ( d ) are
immunostained for
embryonic MyHC isoform,
and ( e ) and ( f ) are
immunostained for neonatal
MyHC isoform. Note the
reduction in numbers of
fi bers positive for both the
fast and neonatal MyHC
isoforms as sections move
anteriorly and away from the
muscle midregion. Note the
small increase in the numbers
of fi bers in the orbital layer
positive for embryonic
MyHC as sections move
anteriorly and away from the
midregion