Douglas E. Kelly

The fine structure of skeletal muscle triad junctions.

Triad junctions in skeletal muscle of young newts and adult frogs have been examined after a wide variety of fixation procedures. When collidine-osmium is employed, the T-tubules of younger larval newt muscle appear as dilated channels which retain normal junctional relations with adjacent terminal cisterns. In older muscle, or after other fixation techniques, the T-tubule is narrow. This latter condition is less useful for analysis of junctional morphology, particularly when sections parallel to and including the junction are utilized. The images (including stereo electron micrographs) suggest that a given triad junction contains a gap distance of about 150 A bridged by an array of cementing materials. “Dimples” in the terminal cistern membrane bring it into tight or close junctional proximity to the T-tubule membrane. The overall appearance resembles an intermediate junction within which small foci of tight or close junctional contact exist. Evidence of direct pore-like membrane continuities between T-tubules and terminal cisterns was not observed.

AN ULTRASTRUCTURAL ANALYSIS OF THE PARAPHYSIS CEREBRI IN NEWTS.

SummaryParaphyses from the brains of adult Triturus pyrrhogaster and adult and larval Taricha torosa have been examined by light and electron microscopy.Adjacent epithelial cells of the CSF-filled paraphyseal saccules are partially separated by extensive intercellular compartments containing variable amounts of granular, electron dense material. Each compartment is open basally against the basement membrane but narrows apically to a tight junction. Finger-like projections of neighboring cells interdigitate across the compartments. Toward the apical surface of the epithelium, the interdigitations are attached to each other by numerous desmosomes. The interdigitations and basal infoldings of the plasma membrane impart a paraphyseal morphology resembling cells known to be involved in fluid transport. Luminal surfaces often bulge and are microvillous near tight junctions. A single cilium is common. Interior cytoplasmic components include scattered mitochondria, smooth and rough endoplasmic reticulum, Golgi complexes, basal lipid accumulations, and variable amounts of glycogen granules. External to the epithelial cells, unmyelinated nerve bundles course through the connective tissue separating the epithelium from the underlying venous sinusoidal network. Presumed mast cells are present in the connective tissue in addition to fibroblasts.Thorotrast particles (colloidal thorium dioxide) introduced into the venous sinusoids rapidly traverse the endothelium and accumulate at the basement membrane of the paraphyseal epithelium. After two hours, the smallest particles of the tracer suspension have penetrated the basement membrane and are found scattered through the length of the intercellular compartments. Thorotrast neither invades cytoplasmic components of the epithelium nor crosses tight junctions to enter the CSF, even when the particles circulate for as long as nine hours before fixation.These findings are discussed in relation to previous light microscopical examinations and theories on possible paraphysis function.

Septate junctions in the gastrodermal epithelium of Phialidium: A fine structural study utilizing ruthenium red.

The septate junctions of the gastrodermis of the hydromedusa, Phialidium gregarium, are composed of septa (80 A thick) which bridge the gap (130 A) between the outer leaflets of the plasma membranes of adjacent cells. The septa are parallel walls, presumably continuous around the cells, and en face show a periodicity of 110 A. Examination of material fixed in a ruthenium red-containing mixture shows that this dye penetrates the interseptal compartments and illucidates the finer structure of the septa. A model of an interpretation of the three-dimensional structure of the junction is presented and relevance of the results to current theories of cell communication is discussed.