Four weeks after labeling myelin lipids with an intraneural injection of 3H-acetate, sciatic nerves were crushed, and the distribution of radiolabeled myelin lipids was followed by autoradiography from 1 d to 10 weeks later. Just prior to crush, silver grains were localized to the myelin sheath. Three days after crush, axons were degenerating and myelin sheaths were breaking down; silver grains appeared over lipid droplets within Schwann cells, fibroblasts, and macrophages. One week after crush the basal-lamina-delimited Schwann-cell tubes (Bungner bands) contained myelin debris, and some tubes already contained regenerating axons. Schwann cells were often displaced to the periphery of the tubes by phagocytes containing heavily labeled myelin debris; extratubal macrophages within the endoneurium contained labeled lipid droplets but no myelin debris. Two weeks after nerve crush silver grains were associated with newly formed myelin around regenerating axons. Many extratubal endoneurial macrophages now contained labeled myelin debris and lipid droplets. By 3 weeks myelination of regenerating axons was advanced, and the myelin sheaths were well labeled. Extratubal macrophages had become the major labeled structure within the nerve because they contained large amounts of labeled myelin debris and lipid droplets. From 4 to 10 weeks after nerve crush the new myelin sheaths continued to thicken and to be well labeled. Debris- laden extratubal macrophages remained the major site of labeled material within the endoneurium. Our results confirm that there is reutilization of myelin cholesterol by Schwann cells to form new myelin, and indicate that some lipid catabolism takes place in Schwann cells and endoneurial fibroblasts prior to infiltration of the nerve by macrophages. However, most of the myelin debris is phagocytized by macrophages within 1–2 weeks following nerve injury. These debris-laden macrophages persist within the nerve for many weeks, indicating that much of the salvaged cholesterol is not reutilized for myelin regeneration.

The 90,674 wildland fires that burned 2.9 million ha at an estimated suppression cost of $1.6 billion in the United States during the 2000 fire season demonstrated that forest fuel loading has become a hazard to life, property, and ecosystem health as a result of past fire exclusion policies and practices. The fire regime at any given location in these regions is a result of complex interactions between forest biomass, topography, ignitions, and weather. Forest structure and biomass are important aspects in determining current and future fire regimes. Efforts to quantify live and dead forest biomass at the local to regional scale has been hindered by the uncertainty surrounding the measurement and modeling of forest ecosystem processes and fluxes. The interaction of elevated CO 2 with climate, soil nutrients, and other forest management factors that affect forest growth and fuel loading will play a major role in determining future forest stand growth and the distribution of species across the southern United States. The use of satellite image analysis has been tested for timely and accurate measurement of spatially explicit land use change and is well suited for use in inventory and monitoring of forest carbon. The incorporation of Landsat Thematic Mapper data coupled with a physiologically based productivity model (PnET), soil water holding capacity, and historic and projected climatic data provides an opportunity to enhance field plot based forest inventory and monitoring methodologies. We use periodic forest inventory data from the USDA Forest Service's Forest Inventory and Analysis (FIA) project to obtain estimates of forest area and type to generate estimates of carbon storage for evergreen, deciduous, and mixed forest classes for use in an assessment of remotely sensed forest cover at the regional scale for the southern United States. The displays of net primary productivity (NPP) generated from the PnET model show areas of high and low forest carbon storage potential and their spatial relationship to other landscape features for the southern United States. At the regional scale, predicted annual NPP in 1992 ranged from 836 to 2181 g/m 2 /year for evergreen forests and 769-2634 g/m 2 /year for deciduous forests with a regional mean for all forest land of 1448 g/m 2 /year. Prediction of annual NPP in 2050 ranged from 913 to 2076 g/m 2 /year for evergreen forest types to 1214-2376 g/m 2 /year for deciduous forest types with a regional mean for all forest land of 1659 g/m 2 /year. The changes in forest productivity from 1992 to 2050 are shown to display potential areas of increased or decreased forest biomass. This methodology addresses the need for spatially quantifying forest carbon in the terrestrial biosphere to assess forest productivity and wildland fire fuels. #

We investigated the temporal course of blood-nerve barrier (BNB) breakdown during the evolution of tellurium neuropathy, ricin neuropathy, and Wallerian degeneration following nerve transection or nerve crush. Blood-nerve barrier permeability was assessed with a 4,000-molecular weight fluoresceinated dextran from three days to 19 weeks after onset of neuropathy. Blood-nerve barrier breakdown was present during the first two weeks in all four models of neuropathy. Restoration of the BNB to the dextran began within four weeks and was complete by 14 weeks in tellurium neuropathy, a model of demyelinating neuropathy characterized by rapid remyelination, and after nerve crush, a model of Wallerian degeneration characterized by rapid axonal regeneration into distal stump. In contrast, there was persistence of BNB breakdown beyond 14 weeks in ricin neuropathy, a model of neuropathy with no axonal regeneration or remyelination, and after nerve transection, a model of Wallerian degeneration characterized by minimal axonal regeneration into distal stump. We conclude from these data that alterations in the BNB over the course of neuropathy differ among various types of neuropathy, and that these alterations are dependent on the form of nerve fiber injury. The lack of regenerating or remyelinating axons in ricin neuropathy and after nerve transection may be responsible for the persistent BNB breakdown found in these neuropathies.

The frequency of demyelinated fibers in mixed nerve and cutaneous nerve and the relationship of the frequency of demyelination to internodal length were assessed in a model of tellurium neuropathy in the rat. Twenty-day-old Long-Evans rats were fed chow containing 1.25% elemental tellurium for seven days and subsequently killed at 34 or 41 days of age. Teased-fiber preparations revealed a higher frequency of demyelinated fibers in sciatic nerve (mixed nerve) than in sural nerve (cutaneous nerve). The frequency of demyelinated fibers was positively associated with internodal length in both nerves. The type of nerve (mixed or cutaneous) was not a significant predictor of the frequency of demyelinated fibers once internodal length had been taken into account. These data indicate that there is a hierarchy of vulnerability within the population of myelinating Schwann cells to tellurium toxicity, and that this hierarchy is related to internodal length. The hierarchy of vulnerability may reflect intrinsic differences among Schwann cells, such as the volume of myelin each cell is synthesizing and maintaining, or a gradient of unrecognized axonal abnormalities.

Exposure of weanling rats to a diet containing elemental tellurium results in a peripheral neuropathy characterized by segmental demyelination and minimal axonal degeneration. One of the earliest ultrastructural abnormalities in tellurium neuropathy is an increased number of cytoplasmic lipid droplets in myelinating Schwann cells. The pathogenesis of these lipid droplets was investigated using light and electron microscopic autoradiography. Nerve lipids were either "prelabeled" with [3H]acetate via in vivo intraneural injection 3 days before a 2-day exposure to tellurium, or "postlabeled" via in vivo intraneural injection or in vitro incubation with [3H]acetate following a 2-day exposure to tellurium. In the prelabeled nerves, myelin became heavily labeled, but the tellurium-induced cytoplasmic lipid droplets were rarely labeled. In the postlabeled nerves, the tellurium-induced cytoplasmic lipid droplets were the most heavily labeled structures within the nerve. These data indicate that the tellurium-induced lipid droplets in Schwann cells are derived from newly synthesized lipid rather than from the early breakdown and internalization of myelin lipids. The earliest biochemical abnormality observed in tellurium neuropathy is an inhibition of cholesterol synthesis at the squalene epoxidase step. This leads to an accumulation of squalene within the nerve. We conclude that the cytoplasmic lipid droplets in Schwann cells contain this accumulated lipid.