The effect of perinatal undernutrition on perineurial diffusion barrier to exogenous protein. An experimental study on rat sciatic nerve

Glucose transporters in rat peripheral nerve: paranodal expression of GLUT1 and GLUT3

Peripheral nerve depends on glucose oxidation to energize the repolarization of excitable axonal membranes following impulse conduction, hence requiring high-energy demands by the axon at the node of Ranvier. To enter the axon at this site, glucose must be transported from the endoneurial space across Schwann cell plasma membranes and the axolemma. Such transport is likely to be mediated by facilitative glucose transporters. Although immunohistochemical studies of peripheral nerves have detected high levels of the transporter GLUT1 in endoneurial capillaries and perineurium, localization of glucose transporters to Schwann cells or peripheral axons in vivo has not been documented. In this study, we demonstrate that the GLUT1 transporter is expressed in the plasma membrane and cytoplasm of myelinating Schwann cells around the nodes of Ranvier and in the Schmidt-Lanterman incisures, making them potential sites of transcellular glucose transport. No GLUT1 was detected in axonal membranes. GLUT3 mRNA was expressed only at low levels, but GLUT3 polypeptide was barely detected by immunocytochemistry or immunoblotting in peripheral nerve from young adult rats. However, in 13-month-old rats, GLUT3 polypeptide was present in myelinated fibers, endoneurial capillaries, and perineurium. In myelinated fibers, GLUT3 appeared to be preferentially expressed in the paranodal regions of Schwann cells and nodal axons, but was also present in the internodal aspects of these structures. The results of the present study suggest that both Schwann cell GLUT1 and axonal and Schwann cell GLUT3 are involved in the transport of glucose into the metabolically active regions of peripheral axons.

A freeze-fracture study of the perineurium in normal and protein-deprived rats

Observations have been made using the freeze-fracture replication technique on the perineurium of normal and protein-deprived rats in which its permeability barrier function is known to be deficient. The perineurial cells of young normal rats possessed belt-like tight junctions (zonulae occludentes) at the borders and maculae occludentes at sites remote from their borders. In older rats, the zonulae occludentes were more prominent and the maculae occludentes relatively less frequent. No abnormalities were detected in the tight junctions of young rats with early induction of protein deficiency but this may have been related to sampling problems. In older severely protein-deficient animals, although many of the tight junctions were normal, some were abnormal and contained focal regions of dispersed strands. The density of caveolae in the surface membrane of the perineurial cells of older rats with severe protein deficiency was significantly greater than in the control animals. This provides support for the view that the pinocytotic-like vesicles of perineurial cells are involved in transport of substances across the cells. The increased numbers of caveolae in the protein deficient rats may reflect increased transcellular traffic. There were considerable differences in the density of P-face IMPs between the different perineurial lamellae, but the results did not allow a decision to be made as to whether there was a polarization of the cells between their endoneurial and epineurial aspects. No differences were detected in the density of P-face IMPs between the young control and protein-deprived rats. In the perineurium of the older rats with protein deficiency, IMP density was significantly greater in the E face than in the controls but not different in the P face. The delay in the development of enzymatic activity in the perineurium of protein-deficient rats that has been demonstrated histochemically is therefore not paralleled by a reduction in IMPs.

Morphometric-interferometric investigations of neurons in the medial thalamic nucleus of newborn rats following maternal treatment with ethanol

The nucleus and cell size, as well as the dry weight and protein contents of neurons in the medial thalamic nucleus of newborn rats, exposed to ethanol in utero, were examined on the first day of postnatal life. Comparison was made with a control group of animals, not exposed to ethanol. No significant changes in the protein content and dry weight values were observed. The perikarya of neurons from animals exposed to ethanol were significantly larger than those of the controls. The findings indicate that the nucleus and perikaryon are affected differently by alcohol, following intrauterine exposure.

Capsules of duck muscle spindles

Duck muscle spindles show a large outer capsule enclosing a number of intrafusal muscle fibres which are individually encircled by an inner capsule. The outer capsule consists of a multilayer of 4-6 flattened cells with elongated nuclei, and are usually lined by a basement membrane. They resemble and are linked with perineural cells of the nerve bundle that innervates the spindle. There is overlapping and interdigitation between branching processes of these cells. Some apposing surfaces develop junctional complexes in the form of hemi-desmosomes and desmosomes. The cytoplasmic content shows numerous pinocytotic vesicles, some polyribosomes, lamellated cisternae of rough endoplasmic reticulum and microfilaments. The inner capsule consists of cells showing, at the mid-equatorial region, a large ovoid nucleus and extensive branching of thin and long processes that overlap, forming numerous layers around each intrafusal muscle fibre. Bundles of collagen fibrils in a crescent-shaped configuration occur circumferentially between the multilayer of processes and the muscle fibre. Some linkages between the inner and the outer capsule cell processes provide a network that subdivides the large periaxial space into compartments. There is no basement material lining the inner capsule cells and the processes. Some cytoplasmic area occurs around the ovoid nucleus and the cytoplasm varies in density, showing some dilated short profiles of rough endoplasmic reticulum, a few pinocytotic vesicles and microfilaments.

Barriers and transport properties of the perineurium. An ultrastructural study with 125I-labeled albuminin and horseradish peroxidase in normal and protein-deprived rats

The permeability properties of the perineurium in sciatic nerves of 12-week-old rats were studied. The penetration of 125I-labeled albumin and horseradish peroxidase into the perineurium was investigated electronmicroscopically 10, 30, and 120 min after the local extraneural application of the tracers. The autoradiographic study included age-matched protein-deprived rats. It was concluded that the perineurium acted as a diffusion barrier but also permitted a slow passage of the macromolecules into the endoneurium. The result indicates that this penetration to some extent is due to vesicular transport across the perineurial cells. The significance of these barrier and transport properties of the perineurium is discussed. No obvious differences in perineurial permeability between normal and protein-deprived rats were obtained.

Effects of local extraneural application of diphtheria toxin on the sciatic nerves of normal and protein deprived rats

Diphtheria toxin was locally administered around the sciatic nerves of normal and protein deprived rats aged 3, 6, 12, and 26 weeks in order to investigate the permeability of the barriers enveloping the nerves. At all ages the rats developed a reversible hind limb paralysis linked to a severe segmental demyelination, indicating passage of the toxin into the endoneurium. From 6 weeks of age the pattern of reaction differed between the protein deprived and control rats. The differences are interpreted as being partly dependent on less efficient protective barriers of the sciatic nerves of the protein deprived rats.

The perineurial and blood-nerve barriers in experimental diabetes

The permeabilities of the blood-nerve barrier and the perineurial barrier were investigated in alloxan and streptozotocin diabetic rats and in the mutant diabetic mouse [C57BL/Ks(db/db)]. In the mouse model both fluorescence and electron microscopic techniques were employed using Evans blue albumin (EBA) and horseradish peroxidase (HRP), respectively, as exogeneous tracers. In the rat models only HRP was used. Tracers were applied locally around the sciatic nerve in order to investigate the perineurial barrier, and systemically to detect changes in the blood nerve barrier. None of the models was found to show increased permeability across either of the barriers.

Barriers of peripheral nerve towards exogenous peroxidase in normal and protein deprived rats

The protective barriers of the sciatic nerve in protein deprived and normal rats have been studied. Horseradish peroxidase (HRP) was applied extra-neurally in rats aged between 4 and 26 weeks. The spread of the tracer was investigated 2 h, 24 h, and 3 days after application. Passage of tracer into the endoneurium was found in all rats but decreased with age. Within the endoneurium, HRP was phagocytized by normally occurring endoneurial histiocytes, which constitute part of the protective barrier system of peripheral nerves. The functional development of the nerve protective barriers was retarded in protein deprived rats. Observations made in this study support the concept of a transperineurial vesicular transport of macromolecules.

Impaired development of the rat perineurium by undernutrition. An enzyme histochemical study

Enzyme activity of adenosinetriphosphatase, NADH2-tetrazolium reductase and lactate dehydrogenase was studied in the rat perineurium 10, 20, 30, and 90 days post partum. The effect of under-nutrition was shown by a lack of enzyme activity in rats 30 days post partum in contrast to control animals, 30 days also corresponds to the time when, in normal rats, the perineurial diffusion barrier to exogenous macromolecules becomes established. In undernourished rats this diffusion barrier does not appear. At 90 days post partum raised enzyme activities indicate a high metabolic turnover in the perineurium even in animals with undernutrition.

The distribution of peroxidases in the sciatic nerves of normal and hexachlorophene intoxicated developing rats

Horseradish peroxidase (mol. wt. 40 000) or microperoxidase (mol. wt. 1900) were injected over the sciatic nerve of normal or hexachlorophene (HCP) intoxicated developing rats (3,7,14 and 21 days). Light and electron microscopic studies of nerves after histochemical staining for peroxidase revealed: a) the perineurial barrier to the two peroxidases was established in 21 day normal and HCP intoxicated rats; b) in animals 3-14 days, the perineurial barrier to both peroxidases was not formed and peroxidase staining was observed in the periaxonal space and in the space between paranodal loops of myelin; c) intramyelinic vacuoles, induced by HCP in animals 7-14 days did not show peroxidase staining. HCP-induced intramyelinic vacuolation is due to the separation of the myelin lamellae at the intraperiod line; although these vacuoles are potential extensions of the extracellular space, they are not stained with the extracellular markers horseradish or microperoxidase.

Calcification of the perineurium. A case report

Necropsy of a patient who died of uremia complicating juvenile diabetes revealed selective calcification of the perineurial sheaths in the sciatic nerves. The calcium phosphate deposits were limited to the outer layers of the perineurium while the innermost lamellae were free. Structural analysis, including electron diffraction and X-ray microanalysis, showed electron-dense, spicular deposits, which were composed mainly of finely crystallized hydroxyapatite. The end-stage diabetic nephropathy of the patient was associated with an extremely high calcium phosphorus ion product known to favour metastatic calcification. The mechanism of the selective localization of the calcium phosphate deposits to the outer layers of the perineurial sheaths is discussed with reference to the structure and suggested barrier function of the perineurium in regard to phosphate ions.