Fine structure and organization of the sarcoplasmic reticulum in the sino-auricular fibres of frog heart.

The ultrastructure of the sarcoplasmic reticulum (SR) of the sino-auricular fibres of frog heart has been investigated especially in order to evaluate its possible relationship with several cellular organelles. The SR appears as a complex unit which can be divided into three regions:--peripheral SR, localized beneath the sarcolemma. It is composed of junctional SR, which forms specialized complexes (couplings) with the sarcolemma, and peripheral SR s. str. which frequently extends its anastomotic tubules around the caveolae.--juxtafibrillar SR, in relation with myofibrils, at the level of Z lines and I bands.--intermediary SR which is the most abundant. It contains dense granules, able to bind strontium (and calcium) and, from place to place, is formed into flattened saccules bearing fenestrations where microfilaments frequently pass through. It allows peripheral SR and juxtafibrillar SR to communicate, giving unity to the reticular system and, on the other hand, shows close topographical relations with mitochondria. The great extension of the SR, its heterogeneity and its relationships with several cellular organelles, especially mitochondria, are interpreted from the functional point of view.