Dense particles within synaptic vesicles fixed with acid-aldehyde

A sulphide-osmium procedure for detection of metal-containing synaptic boutons in the lizard cerebral cortex

Two procedures of the Timm sulphide-silver method for the ultrastructural location of heavy metals and a sulphide-osmium procedure proposed here were applied to studying the Timm-reacting axonal boutons of the lizard cerebral cortex. It was shown that metal-containing boutons which would be labelled when using the Timm method, could be labelled by the sulphide-osmium procedure as well, i.e. perfusion and immersion in a glutaraldehyde fixative saturated with H2S vapour and then osmium postfixation. When the sulphide-osmium procedure was used, Timm-reacting boutons displayed an electron-dense loading in some of their synaptic vesicles. The histochemical significance of this labelling is suggested; metal contents (possibly zinc) in the synaptic vesicles are first transformed into metal sulphides that afterwards interact with OSO4 and render an electron-dense labelling. Moreover, chain-like electron-dense aggregates, which could not be detected in the Timm method, have been shown in endoplasmic reticulum cisternae using the sulphide-osmium procedure. The importance of both the Timm method and the sulphide-osmium procedure in ultrastructural studies in the lizard cerebral cortex could be paramount, as they define specifically one of the projection systems in the reptilian cortex. The sulphide-osmium procedure could be more useful than the Timm method since it is a simple procedure that avoids physical development, allows a better ultrastructural quality and a more precise location of the labelling.

Smooth endoplasmic reticulum and other agranular reticulum in frog retinal photoreceptors

Frog retinal photoreceptors are favourable material for studying a number of unresolved issues concerning the interconnections, three-dimensional organization and functions of intracellular membrane systems in neurons. At least two distinct regions of smooth endoplasmic reticulum (SER) are present in these cells. One region, the subellipsoid SER, is located in rod cells at the base of the mitochondria-rich ellipsoid region, and is comprised of arrays of stacked tubules which exhibit frequent continuities with the rough endoplasmic reticulum (RER). The subellipsoid SER is also present throughout the ellipsoid region and at the apex of the inner segment. The second region of SER, the axonal SER, is comprised of agranular sacs and tubules present in the axons of rod cells, the perinuclear and Golgi regions of rod and cone cells and the synaptic terminals of rod and cone cells. There sacs and tubules exhibit continuities with cisternae of RER and with the nuclear envelope. Serial section analyses indicate that this SER can extend as a continuous networking along the entire length of the rod axons and throughout synaptic terminals. The axonal SER is distinct from the subellipsoid SER not only in location and morphology but also in its ability to bind divalent lead ions, a property it shares with synaptic vesicles, with agranular sacs at one face to the Golgi apparatus and with sacs extending from the Golgi apparatus toward the axons hillock. These latter sacs may serve in transport from the Golgi region to the axon. The axons SER in the axon, terminals, and the perinuculear and Golgi regions appear to be a source of synaptic vesicles as evidenced by this lead binding capacity and by the observation of vesicles, with the size (50-75 nm) and appearance of synaptic vesicles, budding from SER in direct continuity, with RER. The endoplasmic reticulum (ER) in synaptic terminals of frog photoreceptors is not continuous with endocytic structures found in the same region, such as blunt-ended tubules or anastomosing networks of tubules. Nor does the ER acquire exogenous horseradish peroxidase. These observations suggest that the ER does not play a direct role in membrane recycling in photoreceptors.

Localization of calcium in presynaptic nerve terminals. An ultrastructural and electron microprobe analysis

Ultrastructural techniques and electron probe microanalysis were used to determine whether or not the smooth endoplasmic reticulum (SER) within presynaptic nerve terminals is a Ca-sequestering site. The three-dimensional structure of the SER was determined from serial sections of synaptosomes. The SER consists of flattened cisterns that may branch and are frequently juxtaposed to mitochondria. To investigate intraterminal Ca sequestration, synaptosomes were treated with saponin to disrupt the plasmalemmal permeability barrier. When these synaptosomes were incubated in solutions containing Ca, ATP, and oxalate, electrondense Ca oxalate deposits were found in intraterminal mitochondria, SER cisterns, and large vesicular profiles. Saponin-treated synaptosomes that were incubated in the presence of mitochondrial poisons contained electron-dense deposits within SER cisterns and large vesicular profiles, but very rarely in mitochondria. Similar deposits were observed within saponin-treated synaptosomes that were not post-fixed with OSO4, and within saponin-treated synaptosomes that were prepared for analysis by freeze-substitution. Electron-probe microanalyses of these deposits confirmed the presence of large concentrations of Ca. When oxalate was omitted from the incubation solutions, no electron-dense deposits were present in saponin-treated synaptosomes. In other control experiments, either the Ca ionophore A23187 or the Ca chelator EGTA was added to the incubation media; electron-dense deposits were very rarely observed within the intraterminal organelles of these saponin-treated synaptosomes. The data indicate that presynaptic nerve terminal SER is indeed a Ca-sequestering organelle.

Visualization of proteinaceous granules in the clear synaptic vesicles

A method for demonstration of electron-dense particles within clear synaptic vesicles from various areas of the CNS as well as from neuromuscular junctions of rat is described. Electron-dense granules of 70-250 A were visible in the center of the synaptic vesicles, or in some cases excentrically situated and bound to the vesicular membrane. Digestion with proteolytic enzymes lead to a negative reaction, whereas treatment with hyaluronidase and neuraminidase, as well as the lipid extraction had no effect. Based on the obtained data, it may be assumed that this method manifests the proteinaceous structures.