Cellular organization of rhesus extraocular muscle.

Rhesus extraocular muscle appeared to be divided into three zones: an outer small-fiber layer, an intermediate area containing a mixture of small and large cells, and a large-cell area in the center of the muscle and adjacent to the globe. The outer and intermediate layer had the histochemical and electron microscopic characteristics of red muscle, tohile the large-cell area appeared to be white muscle. The small cells xoere of two types with varying mitochondrial configuration and histochemical characteristics. The sarcotubular apparatus of both cells was limited almost entirely to the I band. The M line was absent from the center of the A band. The large cells of the intermediate zone contained abundant columns of mitochondria with the sarcoplasmic reticulum outlining the entire length of the myofibrils, and the M line was absent. The large cells were found in the white portion of the muscle with the addition of two other cellular types. A Felderstruktur type fiber contained a smaller number of transverse tubules and triads. A reduced number of mitochondria were paired on either side of the Z line and partially encircled the I band. The M line was either absent or poorly formed, and all enzymatic determinations were reduced. The majority of the ichite portion of the muscle consisted of typical twitch fibers with a well-developed M line, sarcoplasmic reticulum and transverse tubular system, fewer mitochondria, and the histochemical characteristics of white fibers. It is proposed that the outer red portion of the muscle is utilized for slower eye movements and the inner xohite portion for faster movements. The variation of sarcoplasmic reticulum and enzymatic content of the cells without an M line may also represent a modification of slow fibers loith different degrees of slowness.