Membrane systems in skeletal muscle of the lizard Anolis carolinensis

Abstract

The iliotibialis muscle of the iguanid lizard Anolis carolinensis is composed of fibers that display highly developed membrane systems. One of these systems is composed of the so-called “transverse” tubules, which may branch in both the horizontal and vertical planes and which usually are connected to the surface sarcolemma via intermediary passages composed of caveolar vesicles. The remaining membrane system comprises the various elements of the sarcoplasmic reticulum (SR). Three reasonably distinct yet confluent conformations of SR can be found: (1) highly developed “conventional” SR which surrounds each myofibril and is segmented into specialized regions according to the divisions of the sarcomeres, such as junctional SR at the A–I levels, “network” SR massed over the I/Z/I band complexes, and fenestrated collars at the M lines; (2) subsarcolemmal SR whose junctional SR cisternae form peripheral couplings with the inner surface of the external cell membrane; and (3) extensive masses of proliferated SR that form roughly cylindrical bodies oriented longitudinally within intermyofibrillar spaces. These last (“columnar SR”) are found at all depths of the fibers and may be collected into less discrete aggregates that lie just underneath the sarcolemma, including the soleplate regions of motor end plates. The degree of development of the Anolis skeletal muscle membrane systems may reflect the requirements for rapid movement of the fibers.