FIXATION OF THE CENTRAL NERVOUS SYSTEM FOR ELECTRON MICROSCOPY BY ALDEHYDE PERFUSION. II. EFFECT OF OSMOLARITY, PH OF PERFUSATE, AND FIXATIVE CONCENTRATION

Developmental origin of oligodendrocytes determines their function in the adult brain

In the mouse embryonic forebrain, developmentally distinct oligodendrocyte progenitor cell populations and their progeny, oligodendrocytes, emerge from three distinct regions in a spatiotemporal gradient from ventral to dorsal. However, the functional importance of this oligodendrocyte developmental heterogeneity is unknown. Using a genetic strategy to ablate dorsally derived oligodendrocyte lineage cells (OLCs), we show here that the areas in which dorsally derived OLCs normally reside in the adult central nervous system become populated and myelinated by OLCs of ventral origin. These ectopic oligodendrocytes (eOLs) have a distinctive gene expression profile as well as subtle myelination abnormalities. The failure of eOLs to fully assume the role of the original dorsally derived cells results in locomotor and cognitive deficits in the adult animal. This study reveals the importance of developmental heterogeneity within the oligodendrocyte lineage and its importance for homeostatic brain function.

Sarcomere, troponin, and myosin X-ray diffraction signals can be resolved in single cardiomyocytes

Cardiac function relies on the autonomous molecular contraction mechanisms in the ventricular wall. Contraction is driven by ordered motor proteins acting in parallel to generate a macroscopic force. The averaged structure can be investigated by diffraction from model tissues such as trabecular and papillary cardiac muscle using collimated synchrotron beams, offering high resolution in reciprocal space. In the ventricular wall, however, the muscle tissue is compartmentalized into smaller branched cardiomyocytes, with a higher degree of disorder. We show that X-ray diffraction is now also capable of resolving the structural organization of actomyosin in single isolated cardiomyocytes of the ventricular wall. In addition to the hexagonal arrangement of thick and thin filaments, the diffraction signal of the hydrated and fixated cardiomyocytes was sufficient to reveal the myosin motor repeat (M3), the troponin complex repeat (Tn), and the sarcomere length. The sarcomere length signal comprised up to 13 diffraction orders, which were used to compute the sarcomere density profile based on Fourier synthesis. The Tn and M3 spacings were found in the same range as previously reported for other muscle types. The approach opens up a pathway to record the structural dynamics of living cells during the contraction cycle, toward a more complete understanding of cardiac muscle function.

Rehydration of Freeze Substituted Brain Tissue for Pre-embedding Immunoelectron Microscopy

Electron microscopy (EM) volume reconstruction is a powerful tool for investigating the fundamental structure of brain circuits, but the full potential of this technique is limited by the difficulty of integrating molecular information. High quality ultrastructural preservation is necessary for EM reconstruction, and intact, highly contrasted cell membranes are essential for following small neuronal processes through serial sections. Unfortunately, the antibody labeling methods used to identify most endogenous molecules result in compromised morphology, especially of membranes. Cryofixation can produce superior morphological preservation and has the additional advantage of allowing indefinite storage of valuable samples. We have developed a method based on cryofixation that allows sensitive immunolabeling of endogenous molecules, preserves excellent ultrastructure, and is compatible with high-contrast staining for serial EM reconstruction.

WNT11/ROR2 signaling is associated with tumor invasion and poor survival in breast cancer

BACKGROUND

Breast cancer has been associated with activation of the WNT signaling pathway, although no driver mutations in WNT genes have been found yet. Instead, a high expression of the alternative WNT receptor ROR2 was observed, in particular in breast cancer brain metastases. However, its respective ligand and downstream signaling in this context remained unknown.

METHODS

We modulated the expression of ROR2 in human breast cancer cells and characterized their gene and protein expression by RNA-Seq, qRT-PCR, immunoblots and reverse phase protein array (RPPA) combined with network analyses to understand the molecular basis of ROR2 signaling in breast cancer. Using co-immunoprecipitations, we verified the interaction of ROR2 with the identified ligand, WNT11. The functional consequences of WNT11/ROR2 signaling for tumor cell aggressiveness were assessed by microscopy, impedance sensing as well as viability and invasion assays. To evaluate the translational significance of our findings, we performed gene set enrichment, expression and survival analyses on human breast cancer brain metastases.

RESULTS

We found ROR2 to be highly expressed in aggressive breast tumors and associated with worse metastasis-free survival. ROR2 overexpression induced a BRCAness-like phenotype in a cell-context specific manner and rendered cells resistant to PARP inhibition. High levels of ROR2 were furthermore associated with defects in cell morphology and cell-cell-contacts leading to increased tumor invasiveness. On a molecular level, ROR2 overexpression upregulated several non-canonical WNT ligands, in particular WNT11. Co-immunoprecipitation confirmed that WNT11 indeed interacts with the cysteine-rich domain of ROR2 and triggers its invasion-promoting signaling via RHO/ROCK. Knockdown of WNT11 reversed the pro-invasive phenotype and the cellular changes in ROR2-overexpressing cells.

CONCLUSIONS

Taken together, our study revealed a novel auto-stimulatory loop in which ROR2 triggers the expression of its own ligand, WNT11, resulting in enhanced tumor invasion associated with breast cancer metastasis.

The Golgi Outpost Protein TPPP Nucleates Microtubules and Is Critical for Myelination

Oligodendrocytes extend elaborate microtubule arbors that contact up to 50 axon segments per cell, then spiral around myelin sheaths, penetrating from outer to inner layers. However, how they establish this complex cytoarchitecture is unclear. Here, we show that oligodendrocytes contain Golgi outposts, an organelle that can function as an acentrosomal microtubule-organizing center (MTOC). We identify a specific marker for Golgi outposts-TPPP (tubulin polymerization promoting protein)-that we use to purify this organelle and characterize its proteome. In in vitro cell-free assays, recombinant TPPP nucleates microtubules. Primary oligodendrocytes from Tppp knockout (KO) mice have aberrant microtubule branching, mixed microtubule polarity, and shorter myelin sheaths when cultured on 3-dimensional (3D) microfibers. Tppp KO mice exhibit hypomyelination with shorter, thinner myelin sheaths and motor coordination deficits. Together, our data demonstrate that microtubule nucleation outside the cell body at Golgi outposts by TPPP is critical for elongation of the myelin sheath.

Perfusion fixation in brain banking: a systematic review

Background

Perfusing fixatives through the cerebrovascular system is the gold standard approach in animals to prepare brain tissue for spatial biomolecular profiling, circuit tracing, and ultrastructural studies such as connectomics. Translating these discoveries to humans requires examination of postmortem autopsy brain tissue. Yet banked brain tissue is routinely prepared using immersion fixation, which is a significant barrier to optimal preservation of tissue architecture. The challenges involved in adopting perfusion fixation in brain banks and the extent to which it improves histology quality are not well defined.

Methodology

We searched four databases to identify studies that have performed perfusion fixation in human brain tissue and screened the references of the eligible studies to identify further studies. From the included studies, we extracted data about the methods that they used, as well as any data comparing perfusion fixation to immersion fixation. The protocol was preregistered at the Open Science Framework: https://osf.io/cv3ys/.

Results

We screened 4489 abstracts, 214 full-text publications, and identified 35 studies that met our inclusion criteria, which collectively reported on the perfusion fixation of 558 human brains. We identified a wide variety of approaches to perfusion fixation, including perfusion fixation of the brain in situ and ex situ, perfusion fixation through different sets of blood vessels, and perfusion fixation with different washout solutions, fixatives, perfusion pressures, and postfixation tissue processing methods. Through a qualitative synthesis of data comparing the outcomes of perfusion and immersion fixation, we found moderate confidence evidence showing that perfusion fixation results in equal or greater subjective histology quality compared to immersion fixation of relatively large volumes of brain tissue, in an equal or shorter amount of time.

Conclusions

This manuscript serves as a resource for investigators interested in building upon the methods and results of previous research in designing their own perfusion fixation studies in human brains or other large animal brains. We also suggest several future research directions, such as comparing the in situ and ex situ approaches to perfusion fixation, studying the efficacy of different washout solutions, and elucidating the types of brain donors in which perfusion fixation is likely to result in higher fixation quality than immersion fixation.

Electronic supplementary material

The online version of this article (10.1186/s40478-019-0799-y) contains supplementary material, which is available to authorized users.

Myelinating Glia-Specific Deletion of Fbxo7 in Mice Triggers Axonal Degeneration in the Central Nervous System Together with Peripheral Neuropathy

Myelination of axons facilitates the rapid propagation of electrical signals and the long-term integrity of axons. The ubiquitin-proteasome system is essential for proper protein homeostasis, which is particularly crucial for interactions of postmitotic cells. In our study, we examined how the E3 ubiquitin ligase FBXO7-SCF (SKP1, Cul1, F-box protein) expressed in myelinating cells affects the axon-myelin unit. Deletion of Fbxo7 in oligodendrocytes and Schwann cells in mice using the Cnp1-Cre driver line led to motor impairment due to hindlimb paresis. It did not result in apoptosis of myelinating cells, nor did it affect the proper myelination of axons or lead to demyelination. It however triggered axonal degeneration in the CNS and resulted in the severe degeneration of axons in the PNS, inducing a full-blown neuropathy. Both the CNS and PNS displayed inflammation, while the PNS was also characterized by fibrosis, massive infiltration of macrophages, and edema. Tamoxifen-induced deletion of Fbxo7, after myelination using the Plp1-CreERT2 line, led to a small number of degenerated axons and hence a very mild peripheral neuropathy. Interestingly, loss of Fbxo7 also resulted in reduced proteasome activity in Schwann cells but not in cerebellar granule neurons, indicating a specific sensitivity of the former cell type. Together, our results demonstrate an essential role for FBXO7 in myelinating cells to support associated axons, which is fundamental to the proper developmental establishment and the long-term integrity of the axon-myelin unit.SIGNIFICANCE STATEMENT The myelination of axons facilitates the fast propagation of electrical signals and the trophic support of the myelin-axon unit. Here, we report that deletion of Fbxo7 in myelinating cells in mice triggered motor impairment but had no effect on myelin biogenesis. Loss of Fbxo7 in myelinating glia, however, led to axonal degeneration in the CNS and peripheral neuropathy of the axonal type. In addition, we found that Schwann cells were particularly sensitive to Fbxo7 deficiency reflected by reduced proteasome activity. Based on these findings, we conclude that Fbxo7 is essential for the support of the axon-myelin unit and long-term axonal health.

X-ray diffraction imaging of cardiac cells and tissue

With the development of advanced focusing optics for x-rays, we can now use x-ray beams with spot sizes in the micro- or nanometer range to scan cells and large areas of tissues and continuously record the diffraction signals. From this data, x-ray scattering maps or so-called x-ray darkfield images are computed showing how different types of cells or regions of tissues differ in their diffraction intensity. At the same time a diffraction pattern is available for each scan point which encodes the local nanostructure, averaged over many contributing constituents illuminated by the beam. In this work we have exploited these new capabilities of scanning x-ray diffraction to investigate cardiac muscle cells as well as cardiac tissue. We give examples of how cardiac cells, especially living, cultured cells, can be prepared to be compatible with the instrumentation constraints of nano- or micro-diffraction instruments. Furthermore, we show how the developmental stage, ranging from neonatal to adult cells, as well as the final preparation state of the cardiomyocytes influences the recorded scattering signal and how these diffraction signals compare to the structure of a fully developed cardiac muscle.

Axon and Myelin Morphology in Animal and Human Spinal Cord

Characterizing precisely the microstructure of axons, their density, size and myelination is of interest for the neuroscientific community, for example to help maximize the outcome of studies on white matter (WM) pathologies of the spinal cord (SC). The existence of a comprehensive and structured database of axonal measurements in healthy and disease models could help the validation of results obtained by different researchers. The purpose of this article is to provide such a database of healthy SC WM, to discuss the potential sources of variability and to suggest avenues for robust and accurate quantification of axon morphometry based on novel acquisition and processing techniques. The article is organized in three sections. The first section reviews morphometric results across species according to range of densities and counts of myelinated axons, axon diameter and myelin thickness, and characteristics of unmyelinated axons in different regions. The second section discusses the sources of variability across studies, such as age, sex, spinal pathways, spinal levels, statistical power and terminology in regard to tracts and protocols. The third section presents new techniques and perspectives that could benefit histology studies. For example, coherent anti-stokes Raman spectroscopy (CARS) imaging can provide sub-micrometric resolution without the need for fixation and staining, while slide scanners and stitching algorithms can provide full cross-sectional area of SC. In combination with these acquisition techniques, automatic segmentation algorithms for delineating axons and myelin sheath can help provide large-scale statistics on axon morphometry.

The Cannabinoid CB1/CB2 Agonist WIN55212.2 Promotes Oligodendrocyte Differentiation In Vitro and Neuroprotection During the Cuprizone-Induced Central Nervous System Demyelination

Aim and methods

Different types of insults to the CNS lead to axon demyelination. Remyelination occurs when the CNS attempts to recover from myelin loss and requires the activation of oligodendrocyte precursor cells. With the rationale that CB1 receptor is expressed in oligodendrocytes and marijuana consumption alters CNS myelination, we study the effects of the cannabinoid agonist WIN55212.2 in (1) an in vitro model of oligodendrocyte differentiation and (2) the cuprizone model for demyelination.

Results

The synthetic cannabinoid agonist WIN55212.2 at 1 μM increased the myelin basic protein mRNA and protein expression in vitro. During cuprizone‐induced acute demyelination, the administration of 0.5 mg/kg WIN55212.2 confers more myelinated axons, increased the expression of retinoid X receptor alpha, and declined nogo receptor expression. Controversially, 1 mg/kg of the drug increased the number of demyelinated axons and reduced the expression of nerve growth factor inducible, calreticulin and myelin‐related genes coupling specifically with a decrease in 2′,3′‐cyclic nucleotide 3′ phosphodiesterase expression.

Conclusion

The cannabinoid agonist WIN55212.2 promotes oligodendrocyte differentiation in vitro. Moreover, 0.5 mg/kg of the drug confers neuroprotection during cuprizone‐induced demyelination, while 1 mg/kg aggravates the demyelination process.

Correlative in vivo 2-photon imaging and focused ion beam scanning electron microscopy: 3D analysis of neuronal ultrastructure

This protocol describes how dendrites and axons, imaged in vivo, can subsequently be analyzed in 3D using focused ion beam scanning electron microscopy (FIBSEM). The fluorescent structures are identified after chemical fixation and their position highlighted using the 2-photon laser to burn fiducial marks around the region. Once the section has been stained and resin embedded, a small block is trimmed close to these marks. Serially aligned EM images are acquired through this region, using FIBSEM, and the neurites of interest then reconstructed semi-automatically using the Ilastik software (ilastik.org). This fast and reliable imaging and reconstruction technique avoids the use of specific labels to identify the features of interest in the electron microscope and optimizes their preservation for high-quality imaging and 3D analysis.

Glutaraldehyde fixation of central nervous tissue: an electron microscopic evaluation in the isolated rabbit retina

The isolated rabbit retina was studied electron microscopically after fixation with a 3% solution of glutaraldehyde in a 0.05 M Sørensen's phosphate buffer. In radial sections, the inner segments, nuclei, and synapses of the photoreceptor cells seemed similar in size to those from retinas that had been fixed in an isotonic solution containing 1 % crystalline osmium tetroxide in the incubating medium used for the isolation procedure. However, when the number of comparable structures was greatly increased by viewing them in tangential sections, the cellular shrinkage and mitochondrial swelling produced by this widely used, hypertonic, glutaraldehyde fixative were obvious.

Automatic morphometry of nerve histological sections

A method for the automatic segmentation, recognition and measurement of neuronal myelinated fibers in nerve histological sections is presented. In this method, the fiber parameters i.e. perimeter, area, position of the fiber and myelin sheath thickness are automatically computed. Obliquity of the sections may be taken into account. First, the image is thresholded to provide a coarse classification between myelin and non-myelin pixels. Next, the resulting binary image is further simplified using connected morphological operators. By applying semantic rules to the zonal graph axon candidates are identified. Those are either isolated or still connected. Then, separation of connected fibers is performed by evaluating myelin sheath thickness around each candidate area with an Euclidean distance transformation. Finally, properties of each detected fiber are computed and false positives are removed. The accuracy of the method is assessed by evaluating missed detection, false positive ratio and comparing the results to the manual procedure with sampling. In the evaluated nerve surface, a 0.9% of false positives was found, along with 6.36% of missed detections. The resulting histograms show strong correlation with those obtained by manual measure. The noise introduced by this method is significantly lower than the intrinsic sampling variability. This automatic method constitutes an original tool for morphometrical analysis.

Practical aspects of neuropathology: a technical guide for working with the nervous system

Toxicologic pathologists are evaluating tissues from the central and peripheral nervous systems with increasing frequency. This change is being driven by recently established regulatory guidelines and intense interest in developing pharmaceutical compounds to treat various nervous system disorders. However, morphologic evaluation of the nervous system by light or electron microscopy requires special understanding and effort. Here, we review the general concepts of fixation for the nervous system, explain perfusion procedures for optimal preservation, and provide information on handling tissues to avoid artifacts. In general, fixation with aldehydes is recommended for nervous tissue (a combination of paraformaldehyde and glutaraldehyde is preferred). Electron microscopic studies require fixatives of the highest purity possible, typically paraformaldehyde prepared fresh from powder mixed with high-grade glutaraldehyde. The final osmolality of the solution should be slightly hypertonic, in the range of 400-600 mOsmol. Slight hypertonicity is very important and will facilitate maintenance of vascular distention during whole-body perfusion, which is the best method for producing high-quality tissue preparations. Special effort is necessary for handling nervous tissue in a way that minimizes artifacts because chemical fixation is not completed immediately following the perfusion. These technical details should help toxicologic pathologists in their efforts to work with the nervous system, thereby increasing their effectiveness in supporting safety characterization of new test materials undergoing toxicologic assessments.

Fixation of human detrusor smooth muscle cells: role of osmolarity and magnesium ions on the ultrastructural morphology

Magnesium ions added to fixatives for processing to Transmission Electron Microscopy (TEM) have been claimed to cause relaxation of detrusor smooth muscle cells [1]. This should facilitate the morphologic evaluation of the tissue. However, magnesium ions are osmotically active and their addition may cause the fixative to become hypertonic to the tissue. To ascertain whether the presence of magnesium ions causes significant changes compared to those found where the osmolarity is raised without the presence of magnesium, human detrusor specimens were fixed in glutaraldehyde to which increasing amounts of MgCl2 or NaCl were added in different concentrations. With the addition of increasing amounts of MgCl2 and NaCl, the osmolarity of the fixative increased, causing significant changes in the morphology and morphometry of the tissue. The intercellular distances increased, the cells shrank and the shape of the cells changed from smooth and rounded to spiky and angulated. With regard to its muscle-relaxing effect, it was not possible to distinguish the specimens fixed in magnesium-containing fixatives from those without. In this study it was not possible to prove any relaxing effect of magnesium ions added to the fixative. On the contrary the magnesium ions caused an increase in the osmolarity, with significant changes in both the morphometry and the morphology of the human detrusor smooth muscle cells.

Improvements in the technique of vascular perfusion-fixation employing a fluorocarbon-containing perfusate and a peristaltic pump controlled by pressure feedback

A new improved technique for whole-body perfusion-fixation of rats and other small animals is described. The driving force is a peristaltic pump which is feedback regulated by a pressure transducer that monitors the blood-perfusion pressure in the left ventricle of the heart. The primary perfusate-fixative is composed of a blood substitute--13.3% oxygenated fluorocarbon FC-75--in 0.05 M cacodylate buffer (pH 7.4) with a 2% glutaraldehyde. The secondary perfusate-fixative is composed of 2% glutaraldehyde in 0.05 M cacodylate buffer (pH 7.4) with 20 mM CaCl2. A double-barrelled, self-holding cannula is used to cannulate the heart; the outer and inner barrels of the cannula are connected to the peristaltic pump and to the pressure transducer, respectively. The tissue oxygen tension in the rat is monitored by a subcutaneous oxygen electrode. Measurements showed that tissue hypoxia/anoxia did not develop before or during the perfusion-fixation. Thus, the technique permits study of specimens which do not exhibit fixation gradients and do not contain cells fixed in a state of asphyxia. This is substantiated by electron micrographs of cells from different organs, revealing new fine structural elements. By adding oxygenated fluorocarbon to glutaraldehyde perfusate-fixatives, enough oxygen is made accessible for cellular respiration as well as for the oxygen-consuming chemical reactions of glutaraldehyde with the tissue. Data on anaesthesia, operative manoeuvres, mechanical components of the system, preparation of fixatives and flow of the perfusate-fixatives are furnished and discussed.

Changes in size and dendritic arborization patterns of adult cat spinal alpha-motoneurons following permanent axotomy

This study was performed to analyse quantitatively the changes in dimensions and dendritic branching patterns of adult cat spinal alpha-motoneurons following permanent axotomy, i.e., in a situation in which the transected motoraxons are prevented from reinnervating their peripheral target muscle. After transection and ligation of the medial gastrocnemius nerve of adult cats, homonymous alpha-motoneurons were intracellularly labelled with horseradish peroxidase and subjected to quantitative light microscopic analyses. The cell bodies and proximal dendrites were studied at 3, 6, and 12 weeks after the axotomy. An initial increase in cell body size at 3 weeks was followed by a gradual return towards normal values. The mean diameter of the stem dendrites was decreased at all time periods studied, and the combined diameter of the stem dendrites was reduced at 12 weeks after the axotomy. Entire dendritic trees were reconstructed at 12 weeks postoperatively, and the regression equations describing the correlations between dendritic stem diameter, on one hand, and the size of the entire dendrite, on the other, were used to calculate the total dendritic length, volume, and membrane area of whole axotomized motoneurons. The dendritic branching patterns were also analysed. In comparison with normal medial gastrocnemius alpha-motoneurons, the dendritic membrane area and volume of the axotomized cells had decreased by 36% and 29%, respectively, at 12 weeks after the axotomy. This reduction in dendritic size was due to a loss of preterminal and terminal dendritic segments. Abnormal dendritic elongations were observed in 2 of 16 completely reconstructed dendrites.

Dimensional changes in cells and tissues during specimen preparation for the electron microscope

Studies on dimensional changes incurred during preparation of tissue specimens for the transmission and scanning electron microscopes are reviewed, with emphasis on quantitative measurements pertinent to morphometry and three-dimensional reconstruction. The scope of the review includes fixation, dehydration, plastic embedment, critical-point drying, and freeze-drying. Recommendations are presented for monitoring dimensional changes; a strategy for the choice of method of specimen preparation is outlined.

Marek's disease virus-induced transient paralysis in chickens: demonstration of vasogenic brain oedema by an immunohistochemical method

The presence of vasogenic brain oedema and its distribution in Marek's disease virus (MDV)-induced transient paralysis (TP) were determined in genetically resistant and susceptible inbred White Leghorn chickens. MDV-inoculated TP-susceptible chickens with nervous signs (9 days post-inoculation) had severe vacuolation of cerebellar white matter and associated diffuse leakage of albumin and IgG. The serum protein leakage was associated morphologically with a vasculitis and intramural pseudocyst formation in the walls of blood vessels cuffed by mononuclear cells. This transient vasculitis and resulting vasogenic oedema coincided with the temporary neurological signs seen in TP-susceptible chickens. The vasculitis and vasogenic oedema were not present in brain tissue from recovered MDV-inoculated TP-susceptible chickens, MDV-inoculated TP-resistant chickens, or uninoculated control chickens from either line.

Preservation of the ependyma of the mouse spinal cord by various vascular perfusion procedures

Seventy-two mice were used to find out which of 13 vascular perfusion procedures gave best structural preservation of the spinal cord ependyma and central canal lumen. Best results were obtained by 3% glutaraldehyde in Tyrodes solution with 50% of normal NaCl amount, 0.06 M sucrose and 2% dextran T-40 (556 mOsmol, pH 7.2, 0-4 degrees C). This was perfused by a peristaltic pump at 40 ml/min for 10 min through a cannula inserted in the ascending aorta. No advantage was seen by heparin pretreatment or adding a prefixation rinse. With good tissue preservation the central canal was found to be round to oval in cross-sectional profile and almost free of intraluminal material.

Marek's disease virus-induced transient paralysis in chickens: alterations in brain density

The presence of brain edema and its distribution in Marek's disease virus (MDV)-induced transient paralysis (TP) was determined in genetically resistant and susceptible inbred White Leghorn chickens. Density measurements on samples from the cerebellum, cerebral cortex, optic chiasm, and optic lobe were determined on continuous density Percoll gradients. At 9 days post inoculation of MDV, the cerebellums from the susceptible line of chickens were significantly less dense than cerebellums from the resistant line or the controls of either line. The cerebellar edema, as measured by tissue density, coincided with the transient neurological symptoms and was not present in brain tissue from clinically recovered TP birds.

Differential adhesivity of neuroepithelial cells and pioneering circumferential axons

To determine the initial growth pattern of pioneering axons and investigate the factors that may influence their guidance, the lateral margin of a stage 16+ chick brachial spinal cord was examined in serial thin sections. The specimen was prepared with hypertonic fixative which partially shrank the tissue and increased extracellular space. The retention of surface contact after shrinkage was used as an index of the relative adhesiveness between cells in situ. Six axons and growth cones were found within the reconstructed tissue; five were oriented dorsoventrally and one apparent motor neuron growth cone was oriented radially. The five circumferential axons originated from presumptive interneurons distributed in a dispersed pattern along the neural tube lateral wall. Four terminated with growth cones, and each extended a short distance (less than 30 microns) ventrally along the outer margin. No contact was found between these nonfasciculating axons or growth cones. Thus, the earliest intracentral axons appear to grow dorsoventrally from the outset with no appreciable wandering. Morphological features that may indicate their mechanism of guidance, including preformed cellular guides, extracellular channels, contact with basal lamina, and intercellular junctions were not found. The preferential retention of surface contact between adjacent endfeet, as well as between pioneering circumferential axons and neuroepithelial cells, suggests that these particular surfaces are mutually adherent. These findings are consistent with a proposed dorsal-to-ventral adhesive gradient mechanism of circumferential axonal guidance.

Fixation of neural tissue for electron microscopy with an electronically controlled perfusion pump

Many attempts to improve the perfusion of mammalian tissues aim at changes of the osmotic pressure. We describe a method for fixation of nervous tissues controlling both the hydrostatic pressure and the flow rate of a perfusion solution. The constancy of these parameters is guaranteed by an electronically controlled perfusion pump. Thus, a more uniform and complete preservation can be achieved. Further advantages of this method include provision for a rapid succession of rinsing and fixation solution and a continuous control of the hydrostatic pressure during perfusion.

Morphometric studies of sural nerve in childhood

Sural nerve myelinated fiber density and myelinated fiber diameter distribution have been examined in 27 control subjects, ranging in age from 1 day to 59 years. Total transverse fascicular area was measured in 10 of the subjects. There is an exponential decline in myelinated fiber density from birth to adult life. The predicted normal density (D) at any age may be derived from the formula D = (1 X 10(3]/(0.0699 + 0.00725 square root t). The distribution of myelinated fiber diameters is unimodal in the first 4 months of life, and there is a definite bimodal distribution by 2 years of age. Total transverse fascicular area of sural nerve increases progressively from values of about 0.25 mm2 in the first week to about 0.82 mm2 at 9 years. Control values for sural nerve morphometry in childhood are essential for accurate interpretation of biopsies in patients with peripheral neuropathy.

Changes in extracellular space in optic nerves of regenerative and nonregenerative animals after hypertonic fixation: implications for axon regeneration

The optic nerves of some regenerative and nonregenerative animals were compared using electron microscopy, after hypertonic perfusion. Optic axons and glial cell processes separated more readily in regenerative animals, creating large extracellular spaces. In mammals, cell processes remained in close proximity. These findings may indicate lesser adhesion between cell processes in optic nerves of regenerative animals, a characteristic that could allow "potential" space for axon regrowth after nerve injury.

Vascular reactions to perfusion fixation

Vascular reactions to fixation with a 2 x 5% w/v glutaraldehyde fixative were studied in a constant flow perfusion system on isolated rabbit intestines and hind limb preparations. Reactions induced by the fixative were compared to reactions induced by noradrenaline in each single preparation. Changes in vascular resistance were assessed from continuous recordings of arterial and venous pressures and perfusion flow. Changes in vascular filtration and macromolecular permeability during fixation were approximated from recordings of flow and measurements of concentrations of dextran in the perfusate at the arterial inlet and at the venous outlet. Our results indicate that the vascular bed of the preparations studied can be reliably fixed in a high resistance state of smooth muscle activity induced by noradrenaline as well as in a low resistance state in the absence of noradrenaline, provided that hydrodynamic parameters are strictly controlled. The vascular filtration decreases during fixation. In optimally perfused preparations 5-10 min are required to fully saturate the binding capacity of the tissue for glutaraldehyde. Data on viscosity and colloid osmotic pressure of Tyrode solution containing dextran T-70 in various concentrations, and of various aldehyde-containing fixatives in 0 . 1 M sodium phosphate buffer with and without dextran added, are given.

The central-peripheral transition zone of cervical spinal nerve roots in Jimpy mutant and normal mice. Light- and electron-microscopic study

Comparative morphological and ultrastructural investigations on the cervical dorsal and ventral central-peripheral transition zones (CPTZs) of Jimpys and control mice have been performed at early and advanced myelination stages. After postnatal development a characteristic cone-shaped glial outgrowth extends into the proximal part of the dorsal roots, while the ventral roots exhibit short Schwann cell and peripheral nervous tissue invaginations into the spinal cord at the ventral root-spinal cord junction in both animal groups. In Jimpys, although there is marked central myelin deficiency and absence of oligodendroglial development on the CNS side, the normal general aspect of the CPTZs is maintained. Previously postulated astrocytic and neuroaxonal abnormalities in the mutants do not alter the central-peripheral borderline, and Schwann cell migration from the spinal nerve roots into the cord does not occur.

Invasive squamous cell carcinoma of the uterine cervix. VII. Influence of vehicle osmolarity on biopsy quality

A histopathologic grading system for more reliable determination of the prognosis in individual patients with squamous cell carcinoma of the uterine cervix necessitates better sampling and fixation techniques. Fixation in 10 per cent formalin and 0.17 mol phosphate buffer at pH 7.0, with an effective osmolarity of 350 mmol/l, gave optimal stainability, and minimized tissue shrinkage and sectioning artefacts. Vessels remained open, which facilitated the analysis of vascular invasion. A newly designed biopsy forceps yielded large and representative specimens with a minimum of pressure distortion.

Microscopy of the dentine of enamel-free areas of rat molar teeth

Dentine of the enamel-free areas (EFA) of first mandibular molar teeth of rats aged between 20-150 days was examined by scanning electron microscopy. Demineralized sections were stained with Gram-Twort's stain and examined for dental caries. In newly erupted teeth, a few dentinal tubules opened on the EFA surface. As teeth became worn down, extensive areas of EFA showed numerous patent tubules. In primary dentine, no occlusion or partial occlusion of tubules was apparent at any age except at the occlusal surface. It is postulated that this surface narrowing was caused either by dentine debris or salivary deposits. The only other covering of tubule openings appeared to be a thin salivary pellicle which in older animals was plaque-like. Patent tubules in primary dentine contained odontoblast processes which even in worn teeth extended to EFA surfaces. Processes in adjacent enamel-covered dentine also extended to the enamel-dentine junction. Only a few examples of bacterial invasion of EFA dentine were seen: in younger animals bacteria occupied cracks in the EFA surface, but in older animals they also occupied tubules. The EFA dentine does not appear to respond to attrition by infilling of the tubules; the persistence of vital odontoblast processes in worn teeth suggests that dead tract formation is not extensive. Continuous occlusal wear may restrict the progress of caries in EFA but odontoblasts in the rat may be also physiologically and immunologically involved in restricting the ingress of bacteria into otherwise poorly protected EFA.

Electron dense granules and the role of buffers: artefacts from fixation with glutaraldehyde and osmium tetroxide

Electron dense granules may appear in tissues after glutaraldehyde prefixation and osmium tetroxide postfixation. In order to determine the conditions under which the granules are formed various vehicles in pre- and post-fixatives were tested on lymph node, thymus and heart. If granules appeared they were found in all cell types investigated, particularly in the nuclei. There was no difference in the distribution of the granules in the different compartments of these organs. The granules probably consist of complexes of glutaraldehyde, osmium and phosphate. The ultimate phosphate concentration in the tissues during the postfixation was shown to determine whether or not the artefacts appeared. Local conditions in the tissues also contributed to the appearance of the granules. It is concluded that phosphate buffers can be used in the double fixation procedure, but to avoid the granules in lymph node, thymus and heart, a concentration of 0.1 mol/l or less phosphate should be used. For brain and kidney other conditions apply.

Localization of methylene blue paramolybdate in vitally stained nerves

Methylene blue taken up by living neurons can be preserved for electron microscopy in a fixative containing osmium tetroxide and ammonium paramolybdate at pH 5.2. Paramolybdate is the buffer, precipitating agent and main osmotic ingredient; it does not function as an electron stain unless methylene blue is present. The low pH keeps the dye/paramolybdate complex from dissolving. Neither the low pH nor drastic dehydration from water to absolute ethanol harm the tissue. The staining mechanism involves cationic methylene blue associating with anionic structures such as microtubules and neurofilaments in the living cell; during fixation paramolybdate forms a precipitate with the dye at the staining sites. This fixative does not preserve microtubules unless they are first vitally stained.

Ultrastructure of synapses in the lateral line canal organ

The ultrastructure of afferent and efferent synapses on hair cells in the lateral line canal organ of hair cells in the lateral line canal organ of the fish Lota lota was studied, utilizing various fixation and staining techniques. New information was obtained about membrane-associated material, such as the presynaptic body and postsynaptic densities, by examining glutaral-dehyde-fixed material not subjected to osmication. Contrast was instead obtained either by section staining with uranyl acetate and lead citrate or by block impregnation with phosphotungstic acid (PTA). PTA-staining enhances the postsynaptic membrane of both the afferent and efferent synapses. It also stains the presynaptic dense projections at the efferent synapse and in a differential fashion, the afferent synaptic body. Secretion staining reveals a substructure in the feed of the afferent synaptic body, faintly seen but masked in osmicated tissue.

The occurrence of cytoplasmic lamellar bodies in normal and pathologic conditions

Lamellar bodies are stacks of parallel cisternae of approximately regular spacing derived from ergastoplasm. Their occurrence was studied in various nervous and epithelial cells of different animal species in both normal and diseased subjects. This organelle appears to be, at least for some cell types, a normal cytoplasmic feature. The number of lamellar bodies increases in some pathologic conditions and when the cell metabolism is elevated.

The influence of fixation procedure, embedding medium and section thickness on morphometric data in thyroid gland

In this study, the effects of fixation procedures, embedding medium and section thickness on stereological measurements of normal thyroid were analysed. The following conclusions were drawn: A) the use of a single section for the analysis of a lobe is sufficient if this section is located in the central part of the lobe. B) fixation and embedding with glutaraldehyde-Epon leads to a larger shrinkage than Bouin-paraplast, but the difference between the two procedures is not significant. C) osmium post-fixation reduces the shrinkage induced by glutaraldehyde and lowers the axial deformation produced by sectioning. D) Bouin's fixative and paraplast embedding induce considerable shrinkage of the interstitial tissue. The shrinkage obtained with glutaraldehyde-Epon is less. However, it is still not known whether this difference is due to the fixative, or to the embedding procedure or to both. E) only in glutaraldehyde and osmium-fixed material, embedded in Epon, can follicles and colloids be assumed to be spherical in shape without significant errors.

The efficiency of aldehyde fixation for electron microscopy: stabilization of rat brain tissue to withstand osmotic stress

Rat brains were fixed either with glutaraldehyde (GA) or formaldehyde (FA). After 20 min or 24 h fixation the osmotic sensitivity of the tissue was tested by immersion in (a) distilled water (b) 0.15 M or (c) 0.3 M cacodylate buffer. GA-fixed material retained some sensitivity to osmotic stress after 20 min fixation but was entirely resistant after 24 h fixation. Ultrastructural preservation was good after only 20 min GA-fixation, provided that the subsequent treatment was with isotonic solutions. The fixation with FA was less efficient and slower. Dark neurons and other artifacts were commonly seen after the 20 min fixation with FA. Prolongation of the FA-fixation overnight gave markedly better preservation, but however, never equivalent to that with GA.

The use of osmolality as an aid to establishing consistent fixation quality: studies on the kidney of the domestic fowl

1. Using the proximal convoluted tubule of the fowl kidney as indicator tissue the immersion application of fixatives of differing composition and osmolality was studied. 2. The best results were obtained with Dalton's buffered osmium tetroxide and with sodium cacodylate buffered glutaraldehyde followed by osmium post-fixation. 3. The most satisfactory component concentrations in both cases were those which most reproduced a total osmolality close to that of fowl plasma. With both types of fixation the ultrastructural image of the kidney tubule was sensitive to changes in fixative osmolality and differences were clearly identified over a 30 to 50 mOsm range. 4. With glutaraldehyde/osmium double fixation it was found that it was the total osmolality of the glutaraldehyde buffer combination that was to be considered in a study of osmolotic effects, not the osolality of the buffer vehicle alone. 5. The 30 to 50 mOsm difference in fixative osmolality that was able to be detected by studying the image of the kidney tubule was of the order of magnitude commonly experienced as experimental error in the laboratory as a result of changes in reagent batches or error in making up the solutions. The measurement of osmolality therefore provides an ideal quality control to the laboratory make-up of fixative solutions.

Ultrastructural demonstration of amine granules in the adrenal medullary cells of the rat using acid permanganate fixation

Potassium permanganate fixative is usually employed at pH 7.0. At this pH the amines in the granules of the adrenal medullary cells do not react with permanganate. When the pH was adjusted to 5.0, electron dense precipitates were seen in the amine granules of part of the medullary cells, probably noradrenalin containing cells.

A comparison of the effects of three widely used glutaraldehyde fixatives on cellular volume and structure. A TEM, SEM, Volumetric and Cytochemical Study

The effects of three widely used glutaraldehyde-based fixatives on cellular volume and structure have been studied utilizing TEM, SEM, time-lapse micrography during the fixation procedure, volumetry and demonstration of the lysosomal enzyme acid phosphatase. The cells used were in vitro cultivated human glia and glioma cells and suspensions of isolated rat liver parenchymal cells. The fixatives compared were the following: 2 per cent glutaraldehyde (GA) in 0.1 M Na-cacodylate-HCL buffer (cac) with 0.1 M sucrose (pH 7.2); total osmolality (T) 510 mOsmol; vehicle osmolality (V) 300 mOsm, 2 per cent GA in 0.1 M cac (pH 7.2; T = 410 mOsmol; V = 200 mOsmol) and 1.5 per cent GA in 0.067 M cac with 0.033 M sucrose (pH 7.2; T = 320 mOsmol; V = 170 mOsmol). It was found that the fixative with a vehicle osmolality of 300 mOsmol gave results which were interpreted as ideal while the two fixatives were hypotonic vehicles resulted in changes which were easily demonstrated during volumetry, time-lapse micrography, SEM and cytochemistry. However, the differences observed in the TEM were less obvious and difficult to interpret, the major alternations being changes in the configuration of the ER in the liver cells. In conclusion, our findings show that even small variations in the composition of a glutaraldehyde fixative can result in structural changes which do not correspond to the functional morphology of a living cell. Such changes make correct interpretation of micrographs difficult.

Kinetics of horseradish peroxidase migration through cerebral cortex

This study provides a semiquantitative description of the migration of the extracellular space marker, horseradish peroxidase (HRP), through the cerebral cortex. Following a continuous subarachnoid infusion of HRP, this marker was fixed rapidly within the cortical extracellular space (ECS) by intravascular aldehyde perfusion fixation. Microscopic measurements of the maximum depth of penetration of HRP into the cortex perpendicular to the pial surface were taken from coronal whole brain sections of rabbits that had been exposed to HRP for varying periods of time. The depth of penetration plotted as a function of time of exposure to HRP produced a 'diffusion profile'. The failure of experimental points to conform to an ideal diffusion curve indicates that simple diffusion alone is an inadequate explanation of the rate of movement of large molecular solutes from the subarachnoid space through the cerebral cortical ECS. The complex pattern of migration velocity of HRP through cortex may be due to alterations of flux of this solute which result from variations in the volume of ECS in different cortical laminae and to the presence of bulk flow of extracellular fluid in the deeper cortical regions.

Cellular reactions to small cerebral stab wounds in the rat frontal lobe. An ultrastructural study

Small stab wounds were made in the frontal lobe of adult rats by the insertion of a glass capillary or a steel needle with a diameter of 50 mum. Macroscopically, there was a minute lesion filled with red blood cells. Ultrastructurally, there was a prominent edema with 200 mum from the wound during the first 3 days. There was an invasion of neutrophilic leukocytes, monocytes, and pericytes during the first postoperative week and a subsequent phagocytosis of necrotic devris during the first 2 weeks. Neurons within 150-200 mum from the wound degenerated and became phagocytized by macrophages. Astrocytes showed a transitional swelling, later followed by an accumulation of glycogen and filaments. Oligodendrocytes as well as astrocytes took part in phagocytosis. The most prominent early reaction of oligodendrocytes was the formation of multiple vacuoles and, in less injured cells, pinocytotic vesicles. Many cells with a morphology intermediate between the usual appearances of astrocytes and oligodendrocytes were seen. Some of these cells were similar to "third type of neuroglial cells." It was concluded that the healing of this type of small stab wounds occurred in a very regular way. Morpholocially, very little necrosis was seen, but there was a profound reactivity both in invading hematogeneous phagocytes, i.e., neutrophilic leukocytes and monocytes, and in neuroglial cells. Thus, the present type of trauma might be considered a useful model for studying events in healing of brain injuries.

Changes in red blood cell volume on fixation in glutaraldehyde solutions

Light scattering (nephelometry) was used to determine directly the change in volume of red blood cells immersed in a variety of buffer and fixative solutions. Cells immersed in saline or phosphate buffer solutions showed a change in volume that reflected the osmolarity of the solution, shrinkage taking place in hypertonic solutions and swelling and haemolysis occurring in strongly hypotonic solutions. On the other hand, while there was considerable shrinkage in hypertonic glutaraldehyde solutions, swelling was more restricted and haemolysis was prevented in the weaker glutaraldehyde solutions. Thus, while glutaraldehyde exerts a definite osmotic effect on cells in fixative solutions, the magnitude of this effect seems to be limited by its direct action as a fixative.