Ultrastructural study of the organization of axonal agranular reticulum in Octopus nerve

Distribution and calcium-sequestering ability of smooth endoplasmic reticulum in olfactory axon terminals of frog brain

In the present study, the structural and functional role of smooth endoplasmic reticulum was investigated in bullfrog olfactory axon terminals. Structural evidence obtained from this study indicated that this vesiculotubular organelle becomes a more elaborate network of anastomosing tubules near the nerve terminal, located in the olfactory lobe of frog brain. Further structural evidence suggested that membranes of the smooth endoplasmic reticulum pinch off to give rise to some electron-lucent vesicles of approximately 50 nm diameter (microvesicles). Ultrastructural cytochemistry was employed in the present study to demonstrate that olfactory axon terminal smooth endoplasmic reticulum actively sequesters Ca2+. However, a variable amount of electron-dense product (calcium oxalate) was associated with microvesicles located at a distance from the synapse, in contrast to those clustered near the synapse which usually did not contain this reaction product. Results from Ca2+-Mg2+-adenosine-5'-triphosphatase (ATPase) cytochemistry showed a similar pattern of distribution, with smooth endoplasmic reticulum being densely labeled with ATPase reaction product (lead phosphate), but aggregated microvesicles in the nerve terminal generally lacking this electron-dense product. Therefore, it is concluded that olfactory axonal smooth endoplasmic reticulum plays a role in the regulation of intraneuronal Ca2+ levels, and that the Ca2+-sequestering activity of this membranous organelle is dependent upon enzymatic hydrolysis of ATP. Conversely, the microvesicles, particularly those accumulated near the synapse, lack this Ca2+-pumping capacity. Thus, if some of the microvesicles originate from smooth endoplasmic reticulum membranes which are capable of pumping Ca2+, but these vesicles themselves lack this capacity, one can postulate that the Ca2+ pumps are either removed from the newly formed microvesicle membranes or are somehow incapacitated in situ in the membrane.

Movement of newly synthesized membrane by fast transport along the axon of an identified Aplysia neuron

In order to ascertain the form in which newly synthesized membrane is moved by fast axonal transport, we examined the distribution of label along an axon of the identified giant serotoninergic neuron (GCN) in the cerebral ganglion of Aplysia californica. Membrane glycoproteins were labeled by intrasomatic injection of 3H-fucose, and segments of GCN's axon in the posterior lip nerve containing the transported organelles were examined at 1, 5, 15, and 24 hours by quantitative electron-microscopic autoradiography. To show that membrane which is rapidly transported is contained only in discrete organelles rather than in continuous sheets of axoplasmic reticulum, we systematically varied conditions of fixation. We found that we could distinguish vesicles from axoplasmic reticulum most reliably in axons fixed with 2% formaldehyde and 2% glutaraldehyde in cacodylate buffer. At short times after intrasomatic injection of 3H-fucose, dense-cored vesicles and multivesicular tubules were the only axonal organelles labeled.

The axoplasmic reticulum within myelinated axons is not transported rapidly

The axoplasmic reticulum in myelinated axons is an extensive system of branched smooth membranous tubules which is found throughout the length of large axons. To investigate its motility and possible role in fast axonal transport, a focal chilling method was used to arrest transport at two sites separated by a 3 mm wide warm region along the saphenous nerve of mice. The experiments ran for 3-4 h since axoplasmic material travelling faster than 25 mm/day would clear from the central warm region. The nerve was subsequently fixed and processed by a technique that enhances the electron density of the axoplasmic reticulum. Thin and thick sections from several regions along the nerve were then systematically studied using conventional and high voltage electron microscopy. In these studies we found that: 1. the axoplasmic reticulum does not accumulate against the proximal sides of the cold blocks; 2. although often closely associated, there is no evidence of continuity between the axoplasmic reticulum and the discrete membranous compartments that do accumulate proximal to the chilled regions; 3. the axoplasmic reticulum remains in the central 3 mm wide warm region; 4. the axoplasmic reticulum does not accumulate against the distal sides of the cold blocks; 5. retrogradely moving elements that do accumulate distal to the cold blocks do not fuse with the axoplasmic reticulum and are not contained in it; and 6. both retrograde and anterograde vector types are often closely associated with elements of axoplasmic reticulum. These results were supported by quantitative morphometric analysis. We conclude that the axoplasmic reticulum represents a discrete membrane system, separate from either anterogradely or retrogradely moving rapid transport vectors, and that this interconnected cisternal system itself is not rapidly transported.

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.