H. C. Powell

Reduced nerve blood flow in edematous neuropathies: A biomechanical mechanism

Reduced nerve blood flow has been reported in two experimental neuropathies in which edema is an early and significant finding. While the mechanisms of fluid accumulation differed, both resulted in increases of endoneurial fluid pressure up to five times the normal value. As increased endoneurial fluid distends the nerve, the perineurial sheath resists expansion and endoneurial fluid pressure increases. The semirigid perineurium is also a conduit for regional nutritive vessels which provide the greater part of peripheral nerve blood flow. Engineering structural analysis of blood vessels entering the endoneurium suggests that moderate elevations in endoneurial fluid pressure can deform cylindrical vessels in the perineurium into elliptical shapes by creating circumferential elongation and longitudinal compression of the vessels which reduce the cross-sectional area of the lumen. We propose that the pathogenesis of reduced nerve blood flow in edematous neuropathies is linked to the unique structure of the peripheral nerve perineurium and the nerve vasculature, in particular to deformation of anastomotic vessels traversing it.

Cellular Pathology of the Nerve Microenvironment in Galactose Intoxication

The effect of chronic hyperglycemia and polyol pathway activation on the Schwann cell has not been resolved although injury to this cell has long been suspected in diabetic neuropathy. Hyperglycemia, resulting from galactose intoxication of four months duration, induces dose-dependent accumulations of endoneurial fluid sodium and chloride that are linked to polyol pathway activity and associated with dose-dependent increases in sciatic nerve water content, endoneurial fluid pressure and (Na+, K+)-ATPase activity. In order to understand the impact of these changes on the nerve microenvironment, cellular elements of the endoneurium were quantitatively and qualitatively assessed in rats receiving 0%, 10%, 20% or 40% galactose diets. After four months of galactose intoxication, dose-dependent changes in the size distribution of myelinated nerve fibers were apparent. A shift in size-frequency histograms of galactose-intoxicated animals towards smaller fibers was accompanied by a decrease in axon diameter and the volume fraction ratio of axon to myelinated nerve fibers. In the sciatic nerve of all 40% galactose-fed rats examined by electron microscopy, Schwann cells of myelinated fibers showed both reactive and degenerative changes. Demyelination was preceded by splitting at the intraperiod line. Remyelination was identified by axons with disproportionately thin myelin sheaths. Axonal dystrophy and degeneration were infrequently seen, but there was axonal regeneration. Dose-dependent increases in mast cell number were observed with degranulation apparent in rats receiving 20% and 40% galactose. Endothelial cell number and basal lamina thickness were increased in the endoneurial vessels of galactose-intoxicated rats. Increased cytoplasmic area and degenerative changes in pericytes were also noted. These observations indicate that significant morphologic changes accompany the hyperosmotic imbalance resulting from galactose intoxication of four months duration. Schwann cell injury and demyelination are present in a disorder linked to polyol metabolism since aldose reductase, the anabolic enzyme of the polyol pathway, is localized to this myelin-forming cell.

Response of Glia, Mast Cells and the Blood Brain Barrier, in Transgenic Mice Expressing Interleukin-3 in Astrocytes, an Experimental Model for CNS Demyelination

Transgenic mice overexpressing cytokines facilitate analysis of the effects of these immunomodulators on indigenous cells of the central nervous system. This study examines morphological aspects of demyelination and permeability changes, in a recently described transgenic model (termed GFAP‐IL3). GFAP‐IL3 mice develop progressive motor disease at approximately 5 months. Lesions identified after disease onset, showed activation of microglia, astroglial proliferation with phagocytosis of lipids, and immigration of macrophages and mast cells into neural parenchyma. Lymphocytes failed to appear until the later stages of the disease. Later, cerebellar and brain stem white matter contained focal demyelinating lesions with intense macrophage infiltration and a proliferative astrocytosis. Dystrophic axonal changes were noted, in addition to demyelination in heavily infiltrated lesions. Mast cells, variably present in the thalamus and meninges of wild type mice, were greatly increased at these sites in GFAP‐IL3 mice. Blood‐brain barrier (BBB) defects were documented with leakage of intravenously injected horseradish peroxidase. Mast cell infiltration into the CNS and their degranulation at the site of injury, may represent initial events in a spontaneous process of macrophage mediated demyelination in which glial cells and macrophages are both involved in the phagocytic process.

Necrotizing myopathy induced by overexpression of interferon-γ in transgenic mice

A transgenic mouse model has been established in which the cytokine interferon‐γ (IFN‐γ) is overexpressed through the action of the acetylcholine receptor epsilon promoter acting at the neuromuscular junction. While originally developed as a model for the study of the pathogenesis of myasthenia gravis, there are important differences from both human myasthenia gravis and its animal model, experimental autoimmune myasthenia gravis. By 4 months of age there was a well‐established inflammatory, predominantly necrotizing myopathy, with marked dystrophic calcification. Dystrophic and degenerative changes in terminal axons and adjacent Schwann cells were also apparent. The acetylcholine receptor was not the primary target of the inflammatory response, since at 10 weeks of age the receptor content was not decreased and antibodies were not detected bound to the receptor. The IFNγ transgenic mouse model may provide a clinically relevant model of necrotizing myopathy for investigation of the pathological changes associated with, and presumably precipitated by, overexpression of the proinflammatory cytokine interferon‐γ on the neuromuscular junction, intramuscular nerves and myofibers.

Elevated hemoglobin A1 in streptozotocin diabetic rats and in rats on sucrose and galactose-enriched diets

Increased hemoglobin A1 (HbA1) occurs in diabetes mellitus as a direct consequence of hyperglycemia, which causes postsynthetic modification of hemoglobin proportional to blood glucose concentration. Glycosylation of hemoglobin is demonstrable also in vitro, not only by glucose, but with other reducing sugars such as galactose. The purpose of this study was to show that increased HbA1 could be induced by feeding rats a diet enriched either by galactose or sucrose. A third experimental group consisted of rats made diabetic by injection of streptozotocin. Exposure to a diet of 40% galactose resulted in a diabetes-like state with polyuria, weight loss, muscle atrophy, fat depletion, bilateral cataracts, peripheral neuropathy and significantly elevated HbA1. Blood glucose levels were not elevated and after four months of galactose feeding there was hypoglycemia. Total HbA1 was determined by a microcolumn chromatographic procedure, recently made available for clinical use. Increased HbA1 was also found in rats which received a diet of 68% sucrose for several months. The highest values of HbA1 were determined in streptozotocin diabetic rats and there was significant correlation with plasma glucose concentration. Development of experimental models for increased HbA1 by dietary exposure makes it possible to relate increases in HbA1 to measured amounts of sugar included in the diet. Increased HbA1 in galactosemic and diabetic rats provides a biochemical marker which may be correlated with the evolving morphologic changes such as basement membrane thickening occurring in certain tissues in diabetes mellitus.