Faculty Bibliography

The perineurium around frog dorsal root ganglia consists of layers of flattened cells separated by extracellular connective tissue elements. The number of layers is smaller than that in the perineurium around adjacent peripheral nerves, and some of the layers are discontinuous, but in both cases, cells in the same layer overlap and form tight junctions with each other, sometimes accompanied by desmosomes or gap junctions. In freeze-fracture replicas the tight junctions between perineurial cells around peripheral nerves consist of 13-91 strands (mean: approximately 38). Some of these are parallel to the cell borders and some are oblique, forming elaborate meshworks. The overall width of each junction averages approximately 12 microns. In contrast, the tight junctions between perineurial cells around ganglia are much narrower, averaging approximately 2 microns in width, and they consist of only 1-14 strands (mean: approximately 6) with few anastomoses and many free ends. These structural differences provide a morphological basis for a less complete diffusion barrier around dorsal root ganglia

Ultrastructural changes in nodal and paranodal regions of myelinated mouse and rat optic nerve fibers were followed between 4 h and 28 days during the course of Wallerian degeneration. In the mouse, axoplasmic changes, including accumulation of organelles and segregation of microtubules, were detectable 4 h after transection, and progressed to a maximum level on day 4, at which time many axons were markedly swollen. Dense axoplasm was seen as early as 16 h and was a common feature of degenerating axoplasm at later times. Paranodal changes, which first appeared as early as 16 h after injury, included detachment of terminal loops of myelin from the axolemma, disconnection of terminal loops from compact myelin lamellae and broadening of terminal loops, or separation of the loops from each other, resulting in paranodal elongation. In freeze-fracture replicas, the E-face of the axolemma showed the normal particle distribution as late as days 3-5. By day 8, however, the nodal particles were patchy and the overall nodal particle density was reduced to approximately half normal. Some normal-looking fibers were present at all stages examined, but their number had declined to about half the total population on day 5 and to less than 10% on day 11. In the rat, the overall sequence of events and time course were comparable to those in the mouse. Thus, the morphological changes found follow approximately the same sequence as that described previously in frog nerves, but progress more rapidly in the mouse and rat

The generalized tonic seizures that occur in myelin-deficient rats can be eliminated temporarily by spinal cord injury or spinal anesthesia. These observations imply that the seizures in this mutant can be triggered by activity in the spinal cord. The results are consistent with an earlier proposal that axons in myelin-deficient CNS fiber tracts can interact to produce abnormal excitation

Myelinated axons are highly differentiated in the vicinity of the node of Ranvier, both structurally and with respect to ion channel distribution. Evidence is reviewed showing that axonal differentiation depends upon two distinct types of interaction between glial cells and the axolemma, one at the node itself, with astrocyte processes, and the second, more extensive one, in the paranodal region, with oligodendrocyte processes. In the peripheral nervous system, Schwann cells fulfill both roles. Glial or Schwann cell abnormalities, due to genetic deficiencies, diseases or experimental procedures, result in corresponding abnormalities in the axolemma and can have devastating effects on nerve fiber function. An example, the myelin-deficient mutant rat, is presented, and the defects underlying the profound and ultimately lethal neurological abnormalities seen in this mutant are discussed in relation to abnormalities in its axoglial interactions

The nodal and paranodal regions of myelinated peripheral nerve fibers in frogs were examined at sequential times (1-24 days) during Wallerian degeneration. In the region up to 3 mm distal to the transection, paranodal demyelination and axoplasmic degeneration became apparent on day 4 and progressed to involve most of the nodes by day 8. The E-fracture face of the axolemma showed a patchy distribution of nodal particles and some paranodal demyelination on days 4 and 6. On day 8, nodal particles were evenly distributed at low concentration and the adjacent demyelinated paranodal regions showed a corresponding increase in particle density, suggesting redistribution of the nodal particles. The sequence of changes seen in comparable to that in Wallerian degeneration of central nervous system (CNS) fibers but progressed more rapidly in the peripheral nervous system (PNS). In addition a higher proportion of PNS fibers shows pathological changes at corresponding time periods

Although the myelin-deficient rat displays a gross deficiency of myelin in the CNS, occasional myelin segments of moderate thickness can be found. The typical lamellar pattern, consisting of alternating major dense and intermediate lines, is present in some regions of such segments, but the pattern is abnormal elsewhere. Redundant folds are common, and astrocyte processes occur frequently between the myelin sheath and axolemma or within the sheath. In the paranodal region, myelin lamellae occasionally form a palisade of 'terminal loops' against the axolemma, but discrete transverse bands occur only rarely and regular arrays of transverse bands over an extended length have not been seen. 'Reversed' paranodal junctions occur more often. Here the outermost layer of myelin, instead of being closest to the node, is furthest from it, and successive layers form terminal loops that approach the node progressively. These loops face away from the axon and do not contact it. At paranodal junctions of this kind only the innermost loop, or a small number of inner loops, adjoins the axolemma and, as a result, the size of the paranodal axoglial junction is markedly restricted. These defects in the paranodal junction may underlie the intrusion of astrocyte processes from either end of a myelin segment into the internodal periaxonal space and between myelin lamellae. Thus, one of the normal functions of the paranodal junction may be to restrict extension of astrocyte processes into and beneath myelin segments. The myelin-deficient rat also exhibits node-like specializations of the axolemma in association with glial cell processes

We adapted immunocytochemical methods for localization of laminin to examine its disposition in neural tissue at the ultrastructural level. In dorsal root ganglia, laminin was found in basal laminae of the satellite and Schwann cells ensheathing neuronal perikarya and nerve fibers, respectively, and around blood vessels. Within the basal lamina, the immunostain was found in the lamina lucida and lamina densa. Occasional immunostained coated pits were identified in satellite and Schwann cells, but virtually no intracellular label was seen even in freeze-thawed/detergent-permeabilized specimens. In the perineurium, only the basal lamina of the inward-facing surface of the inner-most cell layer was usually stained.

Ultrastructural changes in the nodal and paranodal regions of myelinated nerve fibres of frog optic nerves were studied during early stages of Wallerian degeneration. The earliest changes seen include retraction of paranodal loops of myelin from the axolemma and disconnection of paranodal myelin loops from myelin lamellae. These paranodal changes are asymmetric around the node and may be more advanced on either the proximal or distal side. Axoplasmic changes, including segregation of microtubules from neurofilaments, disorientation of microtubules and accumulation of abnormal organelles at nodes, appear shortly. In some axons the 'undercoating' along the widened nodal surfaces becomes patchy, and blebs appear in the nodal axolemma. In freeze-fracture replicas a mixture of particle clusters and particle-free areas appears in both E- and P-faces of the nodal axolemma. Blebs remain particle free. Initially, E-face particles remain segregated to the node and are present only at much lower concentrations in the demyelinated paranodal axolemma, suggesting that they are not freely mobile at this stage. Nodal E-face particles begin to decrease on day 5 associated with an increase in particles at the adjacent demyelinated paranode, and by day 11 the particle distribution is uniformly low over the entire extent of the nodal and demyelinated paranodal axolemma. If nodal E-face particles represent sodium channels, as has been proposed, the sequence of changes in Wallerian degeneration would be compatible with a gradual redistribution of nodal sodium channels into the demyelinated paranode.

The structure of the satellite cell sheath of frog dorsal root ganglion cells was studied in thin sections and freeze-fracture replicas. The sheath around the cell body is composed of thin satellite cell lamellae closely applied to the neuronal plasma membrane. At the axon hillock the sheath divides into outer and inner components separated by a broad space containing a distinctive extracellular matrix and occasional flattened satellite cell processes. The sheath around the initial segment is usually multilayered but less compact than that around the cell body, and in some places it exhibits node-like interruptions. Apart from occasional particle groupings characteristic of tight junctions and gap junctions, the satellite cells display homogeneously distributed intramembranous particles in both fracture faces in all regions of the sheath.