Intermitochondrial junctions in the heart of the frog, Rana esculenta. A thin-section and freeze-fracture study.

In muscle fibers of the frog heart, junctions between outer membranes of adjacent mitochondrial profiles are occasionally found. In thin sections of embedded tissue and of mitochondrial pellets, the intermitochondrial junctional space is 5.4 +/- 0.15 nm; the external leaflets of the membranes are joined by periodic structures separated from each other by 16.3 +/- 0.29 nm. There are 65.3 +/- 2 periodic structures per micron of membrane measured on a section perpendicular to the junction. After cryofracture, the outer membrane is cleaved into two parts. Closely packed, parallel rows of large particles and furrows are found either on the P-, or on the E-faces. The rows of particles are 11 +/- 0.3 nm thick and are separated from each other by 16.5 +/- 0.46 nm, their density being 65 +/- 2.28 per micron of the membrane. In junctional areas, rows of particles on one membrane correspond with the furrows on the other membrane. Intermitochondrial junctions appear to be real structures and not artifacts due to preparation procedures. The conditions of their occurrence are discussed.

Ultrastructural studies of neuromuscular junctions in visceral and skeletal muscles of the chaetognath Sagitta setosa.

The ultrastructural characteristics of the neuromuscular junctions were studied in oesophageal (visceral) muscle and in four skeletal muscles of the head and trunk in Sagitta. Three types of neuromuscular junctions were encountered. The first is made up of nerve terminals which synapse with the surface of the muscle fiber, in a deep or in a slight depression. The second is characterized by muscle fiber protrusions that cross the connective tissue and form synapses with nerve endings; in this type, numerous post-junctional membrane folds are noted. In the third type, the synaptic cleft is very large (greater than 0.2 micron) and contains bundles of connective fibers. Nerve endings are partially ensheathed in glial cells; they contain mostly clear synaptic vesicles, though some dense-cored vesicles are noted. In many muscle fibers post-junctional membrane thickenings are also observed. All observed neuromuscular junctions resemble chemical synapses. Chaetognaths thus show a great variety of neuromuscular junction ultrastructure as do for instance Arthropods.

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.