A. R. Lieberman

Neurons and their synaptic organization in the visual cortex of the rat

SummaryCells in the visual cortex (area 17) of adult rats were impregnated by the rapid Golgi method and characterized by light microscopy. Selected cells were then sectioned for electron microscopy and their cytological characteristics and the pattern of synapses on their cell bodies and dendrites were studied Twelve classical pyramidal cells from layers II–VI, two pyramid-like cells from layer VI, two inverted pyramidal cells from layers V and VI, ten spine-free non-pyramidal cells from layers II–VI and two spinous non-pyramidal cells from layer IV were examined.The cytoplasmic features of the identified cells, where these could be discerned, corresponded to those previously reported for the different cell types in conventionally prepared tissue. Pyramidal Cells received exclusively type 2 synaptic contacts on their cell bodies, type 1 contacts on their dendritic spines and a mixture of synaptic types (type II predominating) on their shafts, where synaptic density was relatively low. This pattern of synaptic contacts was consistent for all portions of the dendritic tree; inverted pyramidal cells and pyramid-like cells showed the same synaptic organization as classical pyramids. The axon collaterals of pyramidal cells established type I contacts with dendritic spines (or, rarely, shafts) of unknown origin. Non-Pyramidal Cells received both type 1 and type 2 contacts (the former predominating) on their cell bodies and dendrites. The spinous variety also received type I contacts on their dendritic spines. Axon terminal of spine-free non-pyramidal cells established type II synaptic contacts with dendritic shafts of unknown origin. The similarity in synaptic organization between the spine-free and spinous non-pyramidal cells examined in this study suggest that the latter correspond to the sparsely spinous stellate cells rather than to the spinous stellate cells of cat and monkey visual cortex.

Microtubule-associated smooth endoplasmic reticulum in the frog's brain

SummaryElectron microscopic studies of neural processes in the cerebellum, optic tectum, and cerebral hemisphere of the frog reveal a distinctive system of SER cisternae lying at intervals (commonly 1–2 μm apart) perpendicular to the long axis of axons and dendrites, interconnected by tubular, longitudinally orientated SER elements, and in direct continuity with the outer membrane of mitochondria. The transverse cisternae are fenestrated, with a single mierotubule (or rarely, two) passing through the centre of each 50–75 nm fenestration. Extensions of the SER-microtubule complex may be located parasynaptically in axon terminals and dendrites. The SER of dendritic spines also appears to be continuous with the fenestrated cisternae.Possible roles for the specialized SER (particularly of the parasynaptic extensions), such as calcium ion sequestration and ATP or monoamine oxidase transport, are discussed.

The connective tissue elements of the mammalian nodose ganglion

SummaryThe results of an electron microscope study of ganglionic connective tissue are reported (rabbit, rat and guinea pig nodose ganglia). The perineurium is about 30 μ thick, with more than a dozen cellular laminae enclosed within thick basement membranes. Hemidesmosome-like specializations are found at both surfaces of the cells, caveolae and micropinocytotic vesicles are prominent, and tight junctions are found along intercellular contact zones. Irregular collagen bundles, occasional fibroblasts and blood vessels lie between the laminae and collagen is sometimes found between the basement membrane and plasma membrane of the cells. Large diameter epineurial collagen (800–1,100 Å) abuts directly onto the outermost perineurial lamina. The endoneurium is extensively compartmentalized by thin perineurial sheets which relate to the functional organization of the ganglion, separating nerve fibre bundles from cell bodies and also groups of neurons with different peripheral connections. Perineurial sleeves around endoneurial blood vessels are sometimes found. Microfibrils (100–125 Å), with a tubular appearance in cross section are found in the epineurium, perineurium and endoneurium: elastic fibres only in the capsule. Microfibrils and collagen are particularly prominent in guinea pig endoneurium.

Scanning electron microscopy of isolated peripheral nerve fibres. Normal surface structure and alterations proximal to neuromas.

SummaryThe surface morphology of normal myelinated nerve fibres prepared in different ways for scanning electron microscopy has been studied and compared with the surface features of similar fibres undergoing retrograde changes. Nodes of Ranvier, paranodal specializations, artefactual fractures of the myelin, and the endoneurial collagen sheaths are described. A regular pattern of elevations, usually with a pitted or depressed surface seen on normal myelinated fibres after certain preparative procedures are thought to be artefacts produced during preparation and to be related to the neurokeratin network.Alterations in the surface structure of fibres central to long-standing nerve transections include irregular protuberances, serial surface corrugations and large swellings, all associated with demyelination. Fibres that have undergone retrograde degeneration consist of endoneurial tubes with focal swellings occupied by macrophages or myelin debris, together with fine unmyelinated and small myelinated regenerating axons. Strict centrifugal progression of myelination of regenerating axons was not observed.

Filamentous contacts: the ultrastructure and three-dimensional organization of specialized non-synaptic interneuronal appositions in thalamic relay nuclei

Abstract.Filamentous contacts are non-synaptic interneuronal junctions characteristic of thalamic relay nuclei. Symmetrical filamentous contacts occur between two dendrites, two somata or a dendrite and a soma; asymmetrical filamentous contacts occur between axon terminals and dendrites, or occasionally somata, chiefly between the large specific afferent axon terminals of the synaptic glomeruli and the shafts of relay cell dendrites. Both are arranged as extensive net-like (reticular) specializations. The strands of the network enclose fenestrae of variable shape and size and, in perpendicular thin sections, appear as stretches of slightly widened intercellular space containing an electron-dense material and bounded by plasma membranes, the cytoplasmic surfaces of which are coated by electron-dense material into which microfilaments appear to insert. The lamina of cytoplasmic material in dendrites and somata is thicker than that in axon terminals and contains distinct electron-dense sub-units. Regular synaptic junctions may be situated like islands within the territory of an asymmetrical filamentous contact, and small spot-like close membrane appositions resembling gap junctions are occasionally seen in the fenestrae adjacent to the strands of both varieties of contact. Bundles of neurofilaments running in different directions, but in a plane parallel to the plasma membrane, are prominent on either side of the symmetrical filamentous contact and on the dendritic side of the asymmetrical variety. The agranular reticulum also exhibits differences between the contact types. Because of their highly specialized ultrastructure and specific distribution, filamentous contacts probably do not serve a purely adhesive function. Their possible role in the establishment and maintenance of orderly connections between cells is discussed but not favoured. Filamentous contacts probably mediate some form of intercellular communication, possibly involving gap junctions.

Microtubule Fascicles in the Stem Processes of Cultured Sensory Ganglion Cells

SummaryMicrotubule fascicles, resembling those characterizing the initial segment of multipolar neurons, have been observed by electron microscopy within and close to the origin of the stem process of some unipolar ganglion cells in explant cultures of embryonic chick dorsal root ganglia. Each fascicle comprised 2–6 closely spaced parallel microtubules linked by electron dense cross-bridges. Since similar observations have been made on stem processes in vivo, the possibility that linked microtubules occur commonly in this site is considered. The observations are discussed in relation to a possible correlation between the presence of microtubule fascicles and the initiation of action potentials.

Spine-like astrocytic protrusions into large axon terminals.

SummaryElectron microscopic studies and three-dimensional graphic reconstructions from serial sections have shown that the large axon terminals of synaptic glomeruli in the ventrobasal nucleus of the rat are invaginated by spine-like protrusions from the astrocyte processes surrounding the glomeruli (“micro-trophospongium”). The astrocytic protrusions are similar in dimensions and internal morphology to the synapse-bearing dendritic excrescences that also invaginate the large axon terminals. Consequently astrocytic protrusions may be overlooked, or confused with dendritic excrescences sectioned at a non-synaptic level. The intimate neuronal—neuroglial relationship at such large axon terminals may reflect ion-exchange or metabolic interactions between the astrocytes and the axon terminal.