Harmonization of diagnostic terminology used during the histopathologic analysis of rodent tissue sections from nonclinical toxicity studies will improve the consistency of data sets produced by laboratories located around the world. The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a cooperative enterprise of 4 major societies of toxicologic pathology to develop a globally accepted standard vocabulary for proliferative and nonproliferative lesions in rodents. A prior manuscript (Toxicol Pathol 2012;40[4 Suppl]:87S-157S) defined multiple diagnostic terms for toxicant-induced lesions, common spontaneous and age-related changes, and principal confounding artifacts in the rat and mouse central nervous system (CNS) and peripheral nervous system (PNS). The current article defines 9 new diagnostic terms and updates 2 previous terms for findings in the rodent CNS and PNS, the need for which has become evident in the years since the publication of the initial INHAND nomenclature for findings in rodent neural tissues. The nomenclature presented in this document is also available electronically on the Internet at the goRENI website (http://www.goreni.org/).

Toxicologic neuropathology for the peripheral nervous system (PNS) is a vital but often underappreciated element of basic translational research and safety assessment. Evaluation of the PNS may be complicated by unfamiliarity with normal nerve and ganglion biology, which differs to some degree among species; the presence of confounding artifacts related to suboptimal sampling and processing; and limited experience with differentiating such artifacts from genuine disease manifestations and incidental background changes. This compilation of key PNS neurobiology, neuropathology, and neurotoxicology references is designed to allow pathologists and toxicologists to readily access essential information that is needed to enhance their proficiency in evaluating and interpreting toxic changes in PNS tissues from many species.

Nerve fiber teasing is a sensitive technique utilized in diagnostic neuropathology practice, laboratory research, and animal toxicity studies for characterizing changes in single myelinated nerve fibers over extended distances. In animal toxicity studies, a nerve portion (approximately 10 mm in length) is stained with Sudan black for 24 to 48 hours and then transferred into a drop of viscous medium (eg, glycerin) mounted on an adhesive-coated glass slide, positioning it such that the proximodistal orientation is known. Individual fibers are removed using fine forceps while the sample is viewed under a stereomicroscope. In general, lesions can be identified during teasing, but more detailed characterization and photodocumentation is undertaken once nerve fibers have been dried and coverslipped. Nerve fiber teasing is particularly useful for distinguishing early stages of axonal degeneration (which presents as ovoid fiber fragments in the midinternodal region) from segmental demyelination (which presents as loss of original myelin segments and their replacement by thinner, shorter segments in the absence of axonal damage). The slow, laborious nature of nerve fiber teasing dictates that the technique will be employed on a few samples as an auxiliary method to better define the pathogenesis of nerve lesions first identified by conventional histopathologic assessment.

Two beagle dog strains were used in a 14-day intrathecal infusion study for a small molecule test article. A moderate number of Renaut bodies (RBs) were observed in the sciatic nerves of control and test article–treated adult animals as early as 1 day after test article infusion (ie, 5 days after catheter implantation in the lumbar cistern). In most cases, the sciatic nerve was affected unilaterally, apparently in association with extended lateral recumbency on one side. The lighter beagle strain (Marshall), and especially the females (which weighed less than age-matched Marshall males), developed more RBs. In contrast, neither females nor males of the larger strain (Harlan) developed any nerve lesions. These data support the hypothesis that RBs develop following mechanical stress to sciatic nerves, suggest that this change may develop fairly quickly following an insult, and demonstrate that different dog strains exhibit strain-specific nerve changes.

The ability to differentiate among normal structures, procedural and processing artifacts, spontaneous background changes, and test article–related effects in the peripheral nervous system (PNS) is essential for interpreting microscopic features of ganglia and nerves evaluated in animal species commonly used in toxicity studies evaluating regulated products and chemicals. This atlas provides images of findings that may be encountered in ganglia and nerves of animal species commonly used in product discovery and development. Most atlas images are of tissues from control animals that were processed using routine methods (ie, immersion fixation in neutral-buffered 10% formalin, embedding in paraffin, sectioning at 5 µm, and staining with hematoxylin and eosin) since these preparations are traditionally applied to study materials produced during most animal toxicity studies. A few images are of tissues processed using special procedures (ie, immersion or perfusion fixation using methanol-free 4% formaldehyde, postfixation in glutaraldehyde and osmium, embedding in hard plastic resin, sectioning at 1 µm, and staining with toluidine blue), since these preparations promote better stabilization of lipids and thus optimal resolution of myelin sheaths. Together, this compilation provides a useful resource for discriminating among normal structures, procedure- and processing-related artifacts, incidental background changes, and treatment-induced findings that may be seen in PNS tissues of laboratory animals.

Peripheral nervous system (PNS) toxicity is surveyed inconsistently in nonclinical general toxicity studies. These Society of Toxicologic Pathology "best practice" recommendations are designed to ensure consistent, efficient, and effective sampling, processing, and evaluation of PNS tissues for four different situations encountered during nonclinical general toxicity (screening) and dedicated neurotoxicity studies. For toxicity studies where neurotoxicity is unknown or not anticipated (situation 1), PNS evaluation may be limited to one sensorimotor spinal nerve. If somatic PNS neurotoxicity is suspected (situation 2), analysis minimally should include three spinal nerves, multiple dorsal root ganglia, and a trigeminal ganglion. If autonomic PNS neuropathy is suspected (situation 3), parasympathetic and sympathetic ganglia should be assessed. For dedicated neurotoxicity studies where a neurotoxic effect is expected (situation 4), PNS sampling follows the strategy for situations 2 and/or 3, as dictated by functional or other compound/target-specific data. For all situations, bilateral sampling with unilateral processing is acceptable. For situations 1-3, PNS is processed conventionally (immersion in buffered formalin, paraffin embedding, and hematoxylin and eosin staining). For situation 4 (and situations 2 and 3 if resources and timing permit), perfusion fixation with methanol-free fixative is recommended. Where PNS neurotoxicity is suspected or likely, at least one (situations 2 and 3) or two (situation 4) nerve cross sections should be postfixed with glutaraldehyde and osmium before hard plastic resin embedding; soft plastic embedding is not a suitable substitute for hard plastic. Special methods may be used if warranted to further characterize PNS findings. Initial PNS analysis should be informed, not masked ("blinded"). Institutions may adapt these recommendations to fit their specific programmatic requirements but may need to explain in project documentation the rationale for their chosen PNS sampling, processing, and evaluation strategy.

The representative areas for examination of the mouse peripheral nervous system are the spinal cord, containing central components of the peripheral nervous system that needs to be examined at least at cervical and lumbar level, the sciatic and the tibial nerve. Skeletal muscle samples should include the soleus muscle and the quadriceps femoris or long digital extensor, as well as the medial gastrocnemius. Examination can be extended to the thoracic spinal cord, lumbar dorsal root ganglia and spinal nerve roots, as well as the plantar nerve, and other areas of interest. Perfusion fixation is considered optimal for the nervous system; however, immersion fixation allows producing microscopic sections of excellent quality as well. Paraffin-embedded, hematoxylin and eosin-stained sections can be made from all areas, save for small nerves such as the tibial or plantar nerve, which are examined with advantage in hard plastic sections. It is possible to produce hard plastic sections also of the vertebral column, including the spinal cord, dorsal root ganglia and nerve roots. For special investigations, mice can be fixed in toto, decalcified, embedded and sectioned to reveal the areas of interest. In the mouse peripheral nerves, myelination progresses until the adult age. In aging peripheral nerves there is axonal atrophy, degeneration, nerve fiber loss, increase of collagen and sporadic demyelination, especially radiculoneuropathy. The dorsal root ganglia of untreated control animals show frequent cytoplasmic vacuolation. Axonal degeneration is distally, primary demyelination proximally accentuated. Mouse is not very sensitive to peripheral neurotoxicity: to induce toxic peripheral neuropathy mostly parenteral administration and/or newborn animals are used. Naturally occurring infection affecting the spinal cord and peripheral nerves is Theiler's encephalomyelitis virus inducing acute poliomyelitis or chronic demyelination. Any experimental results are to be assessed taking into account spontaneous, age-related, background changes.

The representative areas for examination of the mouse peripheral nervous system are the spinal cord, containing central components of the peripheral nervous system that needs to be examined at least at cervical and lumbar level, the sciatic and the tibial nerve. Skeletal muscle samples should include the soleus muscle and the quadriceps femoris or long digital extensor, as well as the medial gastrocnemius. Examination can be extended to the thoracic spinal cord, lumbar dorsal root ganglia and spinal nerve roots, as well as the plantar nerve, and other areas of interest. Perfusion fixation is considered optimal for the nervous system; however, immersion fixation allows producing microscopic sections of excellent quality as well. Paraffin-embedded, hematoxylin and eosin-stained sections can be made from all areas, save for small nerves such as the tibial or plantar nerve, which are examined with advantage in hard plastic sections. It is possible to produce hard plastic sections also of the vertebral column, including the spinal cord, dorsal root ganglia and nerve roots. For special investigations, mice can be fixed in toto, decalcified, embedded and sectioned to reveal the areas of interest. In the mouse peripheral nerves, myelination progresses until the adult age. In aging peripheral nerves there is axonal atrophy, degeneration, nerve fiber loss, increase of collagen and sporadic demyelination, especially radiculoneuropathy. The dorsal root ganglia of untreated control animals show frequent cytoplasmic vacuolation. Axonal degeneration is distally, primary demyelination proximally accentuated. Mouse is not very sensitive to peripheral neurotoxicity: to induce toxic peripheral neuropathy mostly parenteral administration and/or newborn animals are used. Naturally occurring infection affecting the spinal cord and peripheral nerves is Theiler's encephalomyelitis virus inducing acute poliomyelitis or chronic demyelination. Any experimental results are to be assessed taking into account spontaneous, age-related, background changes.

Investigations in toxicologic neuropathology are complex undertakings because of the intricate spatial and temporal diversity in the anatomic, functional, and molecular organization of the central and peripheral nervous systems. This compilation of toxicologic neuropathology resources has been designed to consolidate a broad range of useful neurobiology, neuropathology, and neurotoxicology resources in a single reference. This collection will increase familiarity with the basic knowledge, skills, and tools required for the proficient practice of toxicologic neuropathology and should help to improve the analysis and interpretation of pathology data sets from neural tissues in toxicology studies.

The continuing education course on Developmental Neurotoxicity Testing (DNT) was designed to communicate current practices for DNT neuropathology, describe promising innovations in quantitative analysis and noninvasive imaging, and facilitate a discussion among experienced neuropathologists and regulatory scientists regarding suitable DNT practices. Conventional DNT neuropathology endpoints are qualitative histopathology and morphometric endpoints of particularly vulnerable sites (e. g., cerebral, cerebellar, or hippocampal thickness). Novel imaging and stereology measurements hold promise for automated analysis of factors that cannot be effectively examined in routinely processed specimens (e. g., cell numbers, fiber tract integrity). The panel recommended that dedicated DNT neuropathology data sets be acquired on a minimum of 8 sections (for qualitative assessment) or 3 sections (for quantitative linear and stereological analyses) using a small battery of stains to examine neurons and myelin. Where guidelines permit discretion, immersion fixation is acceptable for younger animals (postnatal day 22 or earlier), and peripheral nerves may be embedded in paraffin. Frequent concerns regarding DNT data sets include false-negative outcomes due to processing difficulties (e. g., lack of concordance among sections from different animals) and insensitive analytical endpoints (e. g., qualitative evaluation) as well as false-positive results arising from overinterpretation or misreading by inexperienced pathologists.

Neoplasms of the nervous system, whether spontaneous or induced, are infrequent in laboratory rodents and very rare in other laboratory animal species. The morphology of neural tumors depends on the intrinsic functions and properties of the cell type, the interactions between the neoplasm and surrounding normal tissue, and regressive changes. The incidence of neural neoplasms varies with sex, location, and age of tumor onset. Although the onset of spontaneous tumor development cannot be established in routine oncogenicity studies, calculations using the time of diagnosis (day of death) have revealed significant differences in tumor biology among different rat strains. In the central nervous system, granular cell tumors (a meningioma variant), followed by glial tumors, are the most common neoplasms in rats, whereas glial cell tumors are observed most frequently in mice. Central nervous system tumors usually affect the brain rather than the spinal cord. Other than adrenal gland pheochromocytomas, the most common neoplasms of the peripheral nervous system are schwannomas. Neural tumors may develop in the central nervous system and peripheral nervous system from other cell lineages (including extraneural elements like adipose tissue and lymphocytes), but such lesions are very rare in laboratory animals.

This session at the 2010 joint symposium of the Society of Toxicologic Pathology (STP) and the International Federation of Societies of Toxicologic Pathologists (IFSTP) explored modern neuropathology methods for assessing the neurotoxicologic potential of xenobiotics. Conventional techniques to optimally prepare and evaluate the central and peripheral neural tissues while minimizing artifact were reviewed, and optimal schemes were set forth for evaluation of the nervous system during both routine (i.e., general toxicity) studies and enhanced (i.e., specialized neurotoxicity) studies. Stereology was introduced as the most appropriate means of examining the possible impact of toxicants on neural cell numbers. A focused discussion on brain sampling took place among a panel of expert neuroscientists (anatomists and pathologists) and the audience regarding the proper balance between sufficient sampling and cost- and time-effectiveness of the analysis. No consensus was reached on section orientation (coronal sections of both sides vs. a parasagittal longitudinal section with several unilateral hemisections from the contralateral side), but most panelists favored sampling at least 8 sections (or approximately double to triple the current complement) in routine toxicity studies.

This session was designed to provide current information on what the organizers consider to be special problems related to toxicology of the central nervous system, in particular, issues that confront pathologists dealing with issues of toxicity from pharmaceuticals, agrochemicals, and environmental contaminants. The panel of speakers included accomplished academics, researchers, and industrial pathologists with wide respect and recognition by their peers. Topics covered were the blood brain barrier (BBB), the cerebrospinal fluid (CSF) barrier, the circumventricular organs (CVOs), direct delivery of agents to the brain, the dilemma of neuronal necrosis, and finally, new approaches to the assessment of synaptic connections. Dr. Colin Willis of Arizona State University (Tempe, AZ) provided current information on the BBB, the regulated interface that mediates selective transcellular transport of nutrients and essential components from the blood into the brain parenchyma (Willis, Leach, et al. 2004; Willis, Nolan, et al. 2004; Willis and Davis 2008). Despite the increasing body of evidence for the involvement of glia in maintaining the BBB, few studies have specifically addressed glial/endothelial/extracellular matrix interactions and the mechanisms by which glia may regulate the BBB. Imaging studies to assess BBB integrity were presented using gadolinium and fluorescent dextran tracers (4–70 kDa) as magnetic resonance imaging (MRI) probes. Parallel experiments used confocal and electron microscopy to examine the neurovascular unit. These studies, inducing astrocyte loss by treatment with the glial toxicant 3-chloropropanediol, demonstrated the critical role of astrocytes in regulating the integrity of BBB, changes in the expression of tight and adherens junction proteins in vascular endothelial cells as well as vascular permeability, and a cascade of events involving microglial activation, inflammatory response, and remodeling of the extracellular matrix. Laminin (basal lamina) and macrophages play a role in restoring astrocytic population, although it is uncertain whether the macrophages are recruited from the microglia or elsewhere. Comparisons were drawn between regions with robust BBB and the area postrema, a CVO that lacks BBB properties. Dr. Conrad Johanson, Director of Neurosurgery Research at the Alpert Medical School at Brown University (Providence, RI), enhanced our understanding of the relationship of the CSF to the ependyma, choroid plexus, and the CVOs (Johanson

Neuropathology analyses as end points during nonclinical efficacy and toxicity studies are challenging and require trained personnel and particular equipment to achieve optimal results. Accordingly, many regulatory agencies have produced explicit guidelines for designing and performing neuropathology assessments for nonclinical studies. This compilation of international regulatory guidance for toxicologic neuropathology end points represents a set of criteria recommended for general toxicity studies and specialized neurotoxicity studies that should facilitate the efforts of individuals who plan, perform, analyze, and report neuropathology evaluations in nonclinical toxicity studies.

Bordering the ventricular cerebrospinal fluid (CSF) are epithelial cells of choroid plexus (CP), ependyma and circumventricular organs (CVOs) that contain homeostatic transporters for mediating secretion/reabsorption. The distributional pathway ("nexus") of CP-CSF-ependyma-brain furnishes peptides, hormones, and micronutrients to periventricular regions. In disease/toxicity, this nexus becomes a conduit for infectious and xenobiotic agents. The sleeping sickness trypanosome (a protozoan) disrupts CP and downstream CSF-brain. Piperamide is anti-trypanosomic but distorts CP epithelial ultrastructure by engendering hydropic vacuoles; this reflects phospholipidosis and altered lysosomal metabolism. CP swelling by vacuolation may occlude CSF flow. Toxic drug tools delineate injuries to choroidal compartments: cyclophosphamide (vasculature), methylcellulose (interstitium), and piperazine (epithelium). Structurally perturbed CP allows solutes to penetrate the ventricles. There, CSF-borne pathogens and xenobiotics may permeate the ependyma to harm neurogenic stem cell niches. Amoscanate, an anti-helmintic, potently injures rodent ependyma. Ependymal/brain regions near CP are vulnerable to CSF-borne toxicants; this proximity factor links regional barrier breakdown to nearby periventricular pathology. Diverse diseases (e.g., African sleeping sickness, multiple sclerosis) take early root in choroidal, circumventricular, or perivascular loci. Toxicokinetics informs on pathogen, anti-parasitic agent, and auto-antibody distribution along the CSF nexus. CVOs are susceptible to plasma-borne toxicants/pathogens. Countering the physico-chemical and pathogenic insults to the homeostasis-mediating ventricle-bordering cells sustains brain health and fluid balance.

Capsule summary: The identification of homozygous FAS mutations with normal or residual protein expression expands the spectrum of autoimmune lymphoproliferative syndrome types and should prompt clinicians to search for such patients particularly in highly endogamous populations.