Macromolecular Arrangement
within Muscle

Councilman Morgan, George Rozsa,' Albert Szent-Gytirgyi'
and Ralph W. G. Wyckofi

Laboratory of Physical Biology,
Exfierimental Biology C6 Medicise Imtitzrte,
National Institutes of Health, Bethesda, Maryland

Electron microscopy has already given much informa-
tion about the macromolecular components of striated
muscle (1, 4).  Thus it is shown that myofibrils of teased
muscle are ribbons composed of parallel arrays of fila-
ments associated with an amount of seemingly amorphous
material that is great,est in the anisotropic regions.

For the present work, strips of psoas muscle separated
from a rabbit at death were tied in situ to strips of wood
at their resting length, then cut out and immediately
fixed in formalin. Pieces of this fixed muscle were do-
hydrated by passage through alcohols, embedded in meth-
acrylate, and thinly sectioned for electron microscopy
by the procedures recently outlined by Neumann, Borysko
and Swerdlow (2) and finally shadowed with gold-
Manganin.

In favorable instances an astonishingly regular macro-
molecular arrangement can be seen in these sections.  dt
a moderate magnification a section of a fiber cut nearly
at right angles to the long axis will appear as in Fig. 1.
The macromolecular filaments constituting the myofibrillar
blocks are seen almost end-on as either dots or short
rods.  Their diameters correspond to those of the fila-

Though the study of such teased preparations has much
to say about the macromolecular structure of muscle, it
does not and cannot bc expected to tell much about the
relation between these thin strips or ribbons and the way
they are built up in three dimensions in the intact myo-
librils.  This can I)e dooe only by the investigation of
transverse and longitudinal sectious.  We are here report-
ing certain preliminary results of such an investigation.2

ments seen in electron micrographs of teased prepara-
tions.  The order that is obvious in the arrangement of
these macromolecules is c.lear at the higher maguificatiou
of Fig. 2.  The section here is almost. exactly normal to
the fiber axis, the molecular net is approximately hex-
agonal, and therefore the filaments must be fairly closc-
packed in the fiber itself.

Longitudinal sections also have shown regularity in
particle arrangement (Fig. 3).  The macromolecular fila~
merits, which run nearly vertical in this figure, have au
obvious beaded structure and the beads of adjaceut fila-
ments are regularly aligned to give a net which might
he rectangular if the section were cut exactly parallel to
the fiber axis.  The cxisteuce of such a uct in longitudlin;ll
st&iou, together with the hexagonal net seen in tr:Insvers(!


2