Numerous Synaptic Vesicles Research Articles

Fine structure of the chemoreceptor sensillum in Limulus.

AbstractEach chemoreceptor sensillum of Limulus polyphemus consists of 6–15 bipolar neurosensory cells with distal processes confined within a single cuticular tubule as they extend to the outside environment. The cuticular tubule, which is enveloped by the cuticulo‐tubal cell, opens proximally into a fluid‐filled extracellular space through which the dendrite passes before entering the cuticular tubule. Between the neurosensory cells are one to three microvillar cells also exposed to the extracellular space. This space is enclosed by a sheath cell extending proximally from the inner opening of the cuticular tubule and enveloping the proximal portions of the dendrites, the distal portions of the microvillar cells, as well as the distal portion of some neurosensory cell bodies. Most of the remaining portions of the neurosensory cells and microvillar cells are enveloped by neuroglia. Tight junctions occur between the distal portions of the dendrites in or near the cuticular tubule. Each dendrite has a cilium‐like segment located where it traverses the extracellular space with a 9 + 0 pattern of fibers. Septuplelayered junctions occur among the proximal portions of some dendrites and some neurosensory cell bodies of the same sensillum. The subjacent processes of the sensillum frequently course proximally as isolated axons before joining nerve bundles. In the chilarial and gnathobasal chemoreceptors these nerve bundles course proximally to neuropile clumps of a peripheral nerve plexus. The presence of numerous synaptic vesicles in the neuropiles suggests that chemical transmission may occur among “en passant” synapses formed by the axons. Proximally the neuropiles are joined to the central nervous system by relatively long nerves.

Degeneration and regeneration of interneuronal synapses

1. Degeneration of synapses in the cat superior cervical ganglion after division of preganglionic fibers was expressed under the optical microscope by argyrophilia or argyrophobia, hypertrophy, and lysis. Under the electron microscope, two types of changes were observed during this process (“dark” and “light” degeneration of synapses). 2. The structure of the synaptic endings in sections impregnated with silver remains similar at all stages of regeneration, although their function undergoes considerable change during this period. Electron-microscopy showed that transmission of impulses through regenerating synapses is restored only after numerous synaptic vesicles have appeared in their presynaptic parts, and active zones of pre- and postsynaptic membranes have been formed. 3. Formation of the presynaptic part of the synapse precedes specific differentiation of the postsynaptic membrane. The symmetrical type of structure of the primary contact observed in early stages of regeneration of axonal endings is replaced by formations which are indistinguishable in structure and function from ordinary synapses.

Neuromuscular junctions in the body wall muscles of the earthworm, Lumbricus terrestris Linn.

ABSTRACT The fine structure of the neuromuscular junctions in the body wall muscles of the earthworm 13 described. The segmental nerves send branches into the muscle layers. Axons in the nerve branches contain numerous synaptic vesicles and contact is established between these axons and muscle fibres or muscle tails; the latter may extend for a considerable distance from the muscle fibre. The cleft between the axolemma and sarcolemma is 85–120 nm wide and contains basement membrane material. At intervals small aggregations of electron-dense material are attached to the axonal membrane and synaptic vesicles are associated with these. The sarcolemma bears rather larger masses of dense material and is also specialized extracellularly.

Changes in ultrastructure and electroretinogram of bullfrog retina during development

A study has been made with the purpose of attempting to correlate the ultrastructural and electroretinographic changes in the retina of the bullfrog during embryonic growth. On day 8 after fertilization, when we first examined the eyes for electrical responses to light, we obtained slow, cornea-negative waves. These persisted for the following 2 days and on day 11 a rapid negative deflection was added to the ERG. Cornea-positivity appeared on day 12 and remained thereafter. Electron microscopy revealed that during the period of negative responses the visual cells were well developed except for the synaptic apparatus, which reached maturity later than did the outer segments, the inner segments, and the visual cell nuclei. On day 12, when positivity was first recorded, the synaptic terminals of the receptors completed development, showing many synaptic ribbons and numerous synaptic vesicles within the synaptic body The meaning of the sequential development of the visual cells, in relation to the retinal origins of the a- and b-waves of the ERG is discussed.

Synaptic organization of the frog retina: an electron microscopic analysis comparing the retinas of frogs and primates.

The synaptic contacts in the inner and outer plexiform layers of the frog retina have been identified and studied by electron microscopy. In the inner plexiform layer, two types of synaptic contact were recognized. One type, believed to be the synaptic contact of the bipolar terminals, is characterized by a synaptic ribbon in the presynaptic cytoplasm. At such ribbon contacts, there are ordinarily two postsynaptic elements, both of which usually contain numerous synaptic vesicles and appear morphologically identical. The second type of synaptic contact in the inner plexiform layer has a more conventional morpho­logy and is observed very much more frequently than are the ribbon contacts. It is characterized by a dense aggregation of synaptic vesicles clustered close to the presynaptic membrane and is thought to be the synaptic contact of the amacrine processes. The conventional synapses are presynaptic to ribbon-containing processes, ganglion cell dendrites, and other amacrine cell processes. Reciprocal contacts between processes making ribbon synapses, and processes making conventional synapses are often observed. Serial synapses between morphologically identical processes, presumably amacrine processes, are frequently seen; and up to four synapses in series between five adjacent processes have been observed. These findings suggest that in the inner plexiform layer of the frog: (1) bipolar terminals synapse primarily with amacrine processes; (2) amacrine processes synapse extensively with the processes of other amacrine cells; and (3) ganglion cells are driven primarily by the amacrine cells. In the outer plexiform layer, processes penetrate into invaginations in the bases of the receptor terminals and lie in close proximity to the synaptic ribbons of the terminals, where the processes presumably receive synaptic input from the receptors. Elsewhere in the outer plexiform layer, knob-like processes, probably from horizontal cells, make conventional synaptic contacts with other horizontal cell processes and probably with bipolar dendrites.

Observations on the fine structure of photoreceptor cells in the planarian Dugesia tigrina

The bipolar photoreceptor cell of the planarian Dugesia tigrina was studied by electron microscopy. The photosensitive structure within the pigment-enclosed eyecup is attached to the free end of a cell process. This process, the dendritic fiber, contains large vesicles, mitochondria, and numerous neurotubules. It leads into an expanded cell body located lateral to and outside of the opening of the eyecup. The cell body contains a nucleus, nucleolus, mitochondria, several Golgi complexes, multivesicular bodies, vacuoles, free ribosomes, and ribosomes associated with an extensive endoplasmic reticulum longitudinally oriented in the cell. The axon, containing vesicles and few neurotubules, extends to the cerebral ganglion and ends in a terminal axonal enlargement which may contain mitochondria, numerous synaptic vesicles, and glycogen granules.

ELECTRON MICROSCOPY OF OPTIC NERVES AND OPTIC LOBES OF OCTOPUS AND ELEDONE.

Each optic nerve contains several bundles of axons. The axons have their surface membranes directly apposed and the bundles lie in troughs of the elongated Schwann cells. The axons have pronounced varicosities along their length. The axons enter the optic lobe and run between the granule cells to synapse in the plexiform zone. The granule cells are small neurons. Their cytoplasmic organelles include endoplasmic reticulum, ribosomes, agranular reticulum and of special interest, oval or spherical bodies with a lamellated cortex and granular medulla. The elongated varicose presynaptic bags of the optic axons contain mitochondria in the proximal region, numerous synaptic vesicles and, sometimes, neurofilaments. Below the mitochondrial zone, synaptic contacts are made with small spines invaginated into the bags. The spines probably originate from the trunks of the granule cells. Tunnel fibres that are probably trunks of the outer granule cells, run through channels in the synaptic bags.