Electron-dense material within sarcoplasmic reticulum apposed to transverse tubules and to the sarcolemma in dog papillary muscle fibers

Focal laminate segments in cytoplasmic processes of mouse myocardial fibroblasts

In mouse ventricular myocardium, we have found unusual fibroblasts whose cellular processes in some regions are particularly flattened and which contain linearly-arranged, electron-opaque structures ('central laminae"). The morphology of these focal laminate segments of fibroblast processes suggests that the intracellular laminae are adhesive entities which hold the plasmalemmata above and below them in close parallel apposition for short distances.

Sarcoplasmic reticulum in the conducting fibers of the dog heart

The ultrastructure or sarcoplasmic reticulum (SR) was studied in the conducting fibers of the dog heart. A dense network of sarcoplasmic tubules occurred in well preserved cytoplasmic areas. Some of the tubules extended into large sacs filled with finely filamentous material. The sacs often appeared in subsarcolemmal apposition connected to the sarcolemma by electron dense projections. The flattened cisternae of SR occurred beneath the sarcolemma in the myofibrillar region of the cell. Long flattened cisternae only partially apposed to the sarcolemma were quite common in one specimen. Sarcoplasmic reticulum was continuous throughout the interfibrillar spaces surrounding each myofibril. At the level of the Z-line the SR formed a quasi tubular flattened structure surrounding the Z-line and closely adhering to it.

Junctional subsurface organs in frog sympathetic ganglion cells

Subsurface cisternae in frog sympathetic ganglion cells were studied and shown to have similar features to those of the C.N.S. A number of special features were, however, revealed by high resolution microscopy. Highly flattened subsurface cisternae occurred in close proximity to the ganglion cell membrane and formed structures comparable to gap junctions. These subsurface cisternae appeared to be elongated plates (about 0.3 X 2.5 mum) specifically restricted to the area of the ganglion cell membrane adjacent to nerve endings, although often with the intervention of a thin satellite sheath. Thus they have been termed here 'junctional subsurface organs', although the nerve terminals opposing them did not show any synaptic specialization. The junctional subsurface organ was often accompanied by closely arrayed endoplasmic reticulum and/or mitochondria. Where the junctional subsurface organ intervened between plasma membrane and endoplasmic reticulum or mitochondria, faint particles appeared to traverse both sides and bridge the narrow spaces to the opposing plasma membrane and endoplasmic reticulum or mitochondria. The possible functional significance of the junctional subsurface organs is discussed.

Spheroidal bodies in the junctional sarcoplasmic reticulum of lizard myocardial cells

The sarcoplasmic reticulum (SR) of lizard (Anolis carolinensis) myocardial cells has been examined, with particular attention being paid to the structural details of the peripheral couplings (junctional SR). Spheroidal bodies are present within the opaque core of junctional SR; these can be seen both in sections made en face and in sections cut to show the apposition of the junctional SR with the sarcolemma. Opaque junctional processes extend between the sarcolemma and the peripheral junctional SR. The myocardial cells in addition contain some SR cisternae deep within the cells which also possess opaque cores composed of spheroids. Although the significance of the junctional SR spheroidal bodies is unknown, it is thought that they could act as a matrix on which enzymes such as calcium-specific ATPase may be located.