Optical disector counting in cryosections and vibratome sections underestimates particle numbers: effects of tissue quality

Horizontal deformation of skeletal muscle thick sections visualised by confocal microscopy

Certain morphological parameters of the skeletal muscle tissue can be better understood via 3D considerations. Fluorescent confocal microscopy of thick tissue sections is a well-established method for visualising and measuring skeletal muscle fibres and surrounding capillaries in 3D. However, thick tissue sections are prone to deformations which may significantly influence some stereological and morphometric results like muscle fibre diameter and capillary length, but not dimensionless parameters like object number and Euler-Poincaré characteristics. To better understand this phenomenon, we studied the horizontal deformation of thick (100 µm) transverse skeletal muscle sections, by comparing the muscle fibre diameters measured on thick sections to muscle fibre diameters measured on thin (10 µm) sections of the same sample. Diameter changes were further correlated with shrinkage in the Z direction (axial shrinkage) and deviation of the muscle fibre preferential axis from the Z-axis. We showed that the thick sections dilated in horizontal and shrunk in Z direction, and that the magnitude of horizontal dilation was associated with the magnitude of shrinkage in the Z direction. The latter was more pronounced in transversely than obliquely cut tissue sections. The results emphasise that even when shrinkage in the Z direction can be corrected using calibration, it is important to optimise histological protocols to minimise the Z-axis collapse that could cause horizontal dilation. LAY DESCRIPTION: In skeletal muscle research, 3D analysis is especially important for studying the microvasculature. Laser scanning confocal microscopy of skeletal muscle thick tissue sections is a well-established method for visualising and measuring skeletal muscle fibres and surrounding capillaries in 3D. However, such sections are prone to deformations which may significantly influence the study results. To better understand this phenomenon, we studied the horizontal deformation of thick transverse skeletal muscle sections. We compared the average muscle fibre diameters measured on thick skeletal muscle sections, thin fixed skeletal muscle sections and immunohistochemically stained thin skeletal muscle sections with the muscle fibre diameters measured on thin native skeletal muscle sections of the same sample, with the latter condition serving as the standard diameters (ie the control condition). We further studied the association among muscle fibre diameter changes, shrinkage of the thick skeletal muscle sections in the Z direction and their sectioning angle. We showed that the thick skeletal muscle sections dilated in the horizontal direction and shrunk in the Z direction, and that the magnitude of horizontal dilation was associated with the magnitude of shrinkage in Z direction. The shrinkage in the Z direction was more pronounced in transversely than obliquely cut tissue sections. These results emphasise that even when shrinkage in the Z direction can be corrected using Z-axis calibration, it is very important to optimise histological protocols to minimise the Z-axis collapse that could cause horizontal dilation in order to enhance the integrity of study results.

Is there section deformation resulting in differential change of nuclear numerical densities along the z axis of thick methacrylate or paraffin sections?

The optical disector, a three-dimensional counting frame or probe in stereology, is often positioned in the middle (depth) of a thick section for unbiased nuclear counting. Using 30-40 μm thick methacrylate or paraffin sections for nuclear counting of neurons with the optical disector, however, some studies showed markedly higher nuclear densities at 10% of the section thickness near the top or bottom surface of the section, suggestive of deformation of section along its z axis and thus affecting the number estimation. To verify the findings, this study obtained two sets of 12-14 methacrylate sections (average thicknesses 21.7 and 29.4 μm) and two sets of 12 paraffin sections (average thicknesses 13.8 and 29.2 μm) from mature rat testes. Each section was used to count round spermatid nuclei in the seminiferous epithelium densely packed with the cells, using 3-4 consecutive disectors placed vertically (along the z axis of the section) from the top surface of the section, through the whole section thickness (two sets of methacrylate and paraffin sections) or in 80-83% of the thickness (other sections). The results demonstrated that, overall, there were no considerable nonuniform changes of the nuclear densities along the z axis of the sections.

A concise review of optical, physical and isotropic fractionator techniques in neuroscience studies, including recent developments

Stereology is a collection of methods which makes it possible to produce interpretations about actual three-dimensional features of objects based on data obtained from their two-dimensional sections or images. Quantitative morphological studies of the central nervous system have undergone significant development. In particular, new approaches known as design-based methods have been successfully applied to neuromorphological research. The morphology of macroscopic and microscopic structures, numbers of cells in organs and structures, and geometrical features such as length, volume, surface area and volume components of the organ concerned can be estimated in an unbiased manner using stereological techniques. The most practical and simplest stereological method is the fractionator technique, one of the most widely used methods for total particle number estimation. This review summarizes fractionator methods in theory and in practice. The most important feature of the methods is the simplicity of its application and underlying reasoning. Although there are three different types of the fractionator method, physical, optical and isotropic (biochemical), the logic underlying its applications remains the same. The fractionator method is one of the strongest and best options among available methods for estimation of the total number of cells in a given structure or organ. The second part of this review focuses on recent developments in stereology, including how to deal with lost caps, with tissue section deformation and shrinkage, and discusses issues of calibration, particle identification, and the role of stereology in the era of a non-histological alternative to counting of cells, the isotropic fractionator (brain soup technique).

Maximizing Explanatory Power in Stereological Data Collection: A Protocol for Reliably Integrating Optical Fractionator and Multiple Immunofluorescence Techniques

With the promise of greater reliability and replicability of estimates, stereological techniques have revolutionized data collection in the neurosciences. At the same time, improvements in immunohistochemistry and fluorescence imaging technologies have facilitated easy application of immunofluorescence protocols, allowing for isolation of multiple target proteins in one tissue sample. Combining multiple immunofluorescence labeling with stereological data collection can provide a powerful tool to maximize explanatory power and efficiency, while minimizing tissue use. Multiple cell classes, subtypes of larger populations, or different cell states can be quantified in one case and even in one sampling run. Here, we present a protocol integrating stereological data collection and multiple immunofluorescence using commonly employed widefield epifluorescence filter sets, optimized for blue (DAPI), green (FITC), and far red (CY5) channels. Our stereological protocol has been designed to accommodate the challenges of fluorescence imaging to overcome limitations like fixed filter sets, photobleaching, and uneven immunolabeling. To enhance fluorescence signal for stereological sampling, our immunolabeling protocol utilizes both high temperature antigen retrieval to improve primary antibody binding and secondary antibodies conjugated to optimally stable fluorophores. To illustrate the utility of this approach, we estimated the number of Ctip2 immunoreactive subcerebral projection neurons and NeuN immunoreactive neurons in rat cerebral cortex at postnatal day 10. We used DAPI (blue) to define the neocortex, anti-NeuN (far red) to identify neurons, and co-labeling of anti-Ctip2 (green) and anti-NeuN (far red) to isolate only subcerebral projection neurons. Our protocol resulted in estimates with low sampling error (CE < 0.05) and high intrarater reliability (ICC > 0.98) that fall within the range of published values, attesting to its efficacy. We show our immunofluorescence techniques can be used to reliably identify other cell types, e.g., different glial cell classes, to highlight the broader applications of our approach. The flexibility of the technique, increasingly reduced costs of fluorescence technologies, and savings in experimental time and tissue use make this approach valuable for neuroscientists interested in incorporating stereology to ask precise neurophysiological and neuroanatomical questions.

Myths and truths about the cellular composition of the human brain: A review of influential concepts

Over the last 50 years, quantitative methodology has made important contributions to our understanding of the cellular composition of the human brain. Not all of the concepts that emerged from quantitative studies have turned out to be true. Here, I examine the history and current status of some of the most influential notions. This includes claims of how many cells compose the human brain, and how different cell types contribute and in what ratios. Additional concepts entail whether we lose significant numbers of neurons with normal aging, whether chronic alcohol abuse contributes to cortical neuron loss, whether there are significant differences in the quantitative composition of cerebral cortex between male and female brains, whether superior intelligence in humans correlates with larger numbers of brain cells, and whether there are secular (generational) changes in neuron number. Do changes in cell number or changes in ratios of cell types accompany certain diseases, and should all counting methods, even the theoretically unbiased ones, be validated and calibrated? I here examine the origin and the current status of major influential concepts, and I review the evidence and arguments that have led to either confirmation or refutation of such concepts. I discuss the circumstances, assumptions and mindsets that perpetuated erroneous views, and the types of technological advances that have, in some cases, challenged longstanding ideas. I will acknowledge the roles of key proponents of influential concepts in the sometimes convoluted path towards recognition of the true cellular composition of the human brain.

The search for true numbers of neurons and glial cells in the human brain: A review of 150 years of cell counting

For half a century, the human brain was believed to contain about 100 billion neurons and one trillion glial cells, with a glia:neuron ratio of 10:1. A new counting method, the isotropic fractionator, has challenged the notion that glia outnumber neurons and revived a question that was widely thought to have been resolved. The recently validated isotropic fractionator demonstrates a glia:neuron ratio of less than 1:1 and a total number of less than 100 billion glial cells in the human brain. A survey of original evidence shows that histological data always supported a 1:1 ratio of glia to neurons in the entire human brain, and a range of 40-130 billion glial cells. We review how the claim of one trillion glial cells originated, was perpetuated, and eventually refuted. We compile how numbers of neurons and glial cells in the adult human brain were reported and we examine the reasons for an erroneous consensus about the relative abundance of glial cells in human brains that persisted for half a century. Our review includes a brief history of cell counting in human brains, types of counting methods that were and are employed, ranges of previous estimates, and the current status of knowledge about the number of cells. We also discuss implications and consequences of the new insights into true numbers of glial cells in the human brain, and the promise and potential impact of the newly validated isotropic fractionator for reliable quantification of glia and neurons in neurological and psychiatric diseases. J. Comp. Neurol. 524:3865-3895, 2016. © 2016 Wiley Periodicals, Inc.

The search for true numbers of neurons and glial cells in the human brain: A review of 150 years of cell counting

For half a century, the human brain was believed to contain about 100 billion neurons and one trillion glial cells, with a glia:neuron ratio of 10:1. A new counting method, the isotropic fractionator, has challenged the notion that glia outnumber neurons and revived a question that was widely thought to have been resolved. The recently validated isotropic fractionator demonstrates a glia:neuron ratio of less than 1:1 and a total number of less than 100 billion glial cells in the human brain. A survey of original evidence shows that histological data always supported a 1:1 ratio of glia to neurons in the entire human brain, and a range of 40-130 billion glial cells. We review how the claim of one trillion glial cells originated, was perpetuated, and eventually refuted. We compile how numbers of neurons and glial cells in the adult human brain were reported and we examine the reasons for an erroneous consensus about the relative abundance of glial cells in human brains that persisted for half a century. Our review includes a brief history of cell counting in human brains, types of counting methods that were and are employed, ranges of previous estimates, and the current status of knowledge about the number of cells. We also discuss implications and consequences of the new insights into true numbers of glial cells in the human brain, and the promise and potential impact of the newly validated isotropic fractionator for reliable quantification of glia and neurons in neurological and psychiatric diseases. J. Comp. Neurol. 524:3865-3895, 2016. © 2016 Wiley Periodicals, Inc.

Primary olfactory cortex in autism and epilepsy: increased glial cells in autism

Autism Spectrum Disorder is characterized by sensory anomalies including impaired olfactory identification. Between 5 and 46 percent of individuals with autism have a clinical diagnosis of epilepsy. Primary olfactory cortex (piriform cortex) is central to olfactory identification and is an epileptogenic structure. Cytoarchitectural changes in olfactory cortex may underlie olfactory differences seen in autism. Primary olfactory cortex was sampled from 17 post-mortem autism cases with and without epilepsy, 11 epilepsy cases without autism and 11 typically developed cases. Stereological and neuropathological methods were used to quantify glial, pyramidal and non-pyramidal cell densities in layers of the piriform as well as identify pathological differences in this area and its neighbouring region, the olfactory tubercle. We found increased layer II glial cell densities in autism with and without epilepsy, which were negatively correlated with age and positively correlated with levels of corpora amylacea in layer I. These changes were also associated with greater symptom severity and did not extend to the olfactory tubercle. Glial cell organization may follow an altered trajectory of development with age in autism. The findings are consistent with other studies implicating increased glial cells in the autism brain. Altered cytoarchitecture may contribute to sensory deficits observed in affected individuals. This study provides evidence that autism is linked to alterations in the cytoarchitectural structure that underlies primary sensory processes and is not restricted to heteromodal ("higher") cognitive centers.

Thick methacrylate sections devoid of lost caps simplify stereological quantifications based on the optical fractionator design

In neuroscience, the optical fractionator technique is frequently used for unbiased cell number estimations. Although unbiased in theory, the practical application of the technique is often biased by the necessity of introducing a guard zone at one side of the disector to counter lost caps and/or optical limitations. Restricting the disector within the section thickness potentially introduces bias in two ways. First, the need to measure section thickness in order to obtain the disector height/section thickness fraction is challenging since both microcator measurements, microtome block advance, and measurements on re-embedded sections are potentially biased. Second, disector placement is not uniform random within the section thickness resulting in a bias in most sections with inhomogeneous cell distribution along the z axis. Re-embedded 2-hydroxyethylmethacrylate (hereafter methacrylate) sections were inspected for lost caps to evaluate the possibility of whole section thickness counting with the optical fractionator technique and hippocampal granular cell nucleoli density differences along the z axis were assessed with a z axis analysis. No lost caps were found in the examined re-embedded tissue and an inhomogeneous cell distribution through the section thickness was observed. In thick methacrylate sections devoid of lost caps sampling through the entire section thickness could be an acceptable alternative to the use of guard zones and the consequent biases associated with section thickness measurement and non-random placement of disectors.

Cryostat Slice Irregularities May Introduce Bias in Tissue Thickness Estimation: Relevance for Cell Counting Methods

Stereological techniques using the optical disectors require estimation of final section thickness, but frozen tissue irregularities may interfere with this estimation. Cryostat slices from rodent nerve tissues (dorsal root ganglia, spinal cord, and brain), cut at 16, 40, and 50 μm, were digitized with a confocal microscope and visualized through 3D software. Geometric section thickness of tissue (T geom) was defined as tissue volume/area. Maximal section thicknesses (T max), from the top to the bottom of the section, were measured in a random sample of vertical ZX planes. Irregularities were mostly related to blood vessels traversing the tissue and neuronal somas protruding over the cut surfaces, with other neuron profiles showing a fragmented appearance. Irregularities contributed to increasing the distance between the tops and bottoms of slices sectioned in different laboratories. Significant differences were found between T max and T geom for all thickness studies and counting frames (p<0.01). The T geom/T max average rate was 68.4-85.7% in volumes around cell profiles (∼600-1,200 μm2) and 83.3-91.8% in subcellular samples (∼25-160 μm2). Confocal microscopy may help to assess tissue irregularities, which might lead to an overestimation of tissue volume if section thickness is estimated by focusing on the top and bottom of the sections.

Histological analysis of neurodegeneration in the mouse brain

Neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) are characterized by chronic and progressive neuronal loss. Being able to detect and quantify neurodegeneration is the first step to identify mechanisms underlying neuronal cell death and to develop novel therapeutic strategies. In this chapter, we describe a practical method for detecting and quantifying neurodegeneration in adult and aging mouse brains based on protocols developed in our laboratory over the last decade. We include protocols on sample preparation, immunohistochemical analysis, and stereological methods for counting neurons using examples of AD and PD mouse models. We also describe how to use Fluoro-Jade staining and terminal deoxynucleotidyl transferase dUTP nick end labeling to detect degenerating neurons and apoptotic cells, respectively, and how to use specific proteins as early markers of neurodegeneration.

Validation of the isotropic fractionator: comparison with unbiased stereology and DNA extraction for quantification of glial cells

BACKGROUND

The "isotropic fractionator" (IF) is a novel cell counting technique that homogenizes fixed tissue, recovers cell nuclei in solution, and samples and quantifies nuclei by extrapolation. Studies using this technique indicate that the ratio of glia to neurons in the human brain is approximately 1:1 rather than the 10:1 or 50:1 ratio previously assumed. Although some results obtained with the IF have been similar to those obtained by stereology, the IF has never been calibrated or validated. It is conceivable that only a fraction of glial cell nuclei are recovered intact or recognized after the homogenization step.

NEW METHOD

To rule out this simple explanation for the claim of a 1:1 glia-neuron ratio, we compared cell numbers obtained from adjacent, weight-normalized samples of human and macaque monkey white matter using three techniques: the IF, unbiased stereology of histological sections in exhaustively sectioned samples, and cell numbers calculated from DNA extraction.

RESULTS AND COMPARISON OF METHODS

In primate forebrains, the IF yielded 73,000-90,000 nuclei/mg white matter, unbiased stereology yielded 75,000-92,000 nuclei/mg, with coefficients of error ranging from 0.013 to 0.063, while DNA extraction yielded only 4000-23,000 nuclei/mg in fixed white matter tissues.

CONCLUSIONS

Since the IF revealed about 100% of the numbers produced by unbiased stereology, there is no significant underestimate of glial cells. This confirms the notion that the human brain overall contains glial cells and neurons with a ratio of about 1:1 - far from the originally assumed ratio of 10:1 in favor of glial cells.

Neuronal populations in the basolateral nuclei of the amygdala are differentially increased in humans compared with apes: a stereological study

In human and nonhuman primates, the amygdala is known to play critical roles in emotional and social behavior. Anatomically, individual amygdaloid nuclei are connected with many neural systems that are either differentially expanded or conserved over the course of primate evolution. To address amygdala evolution in humans and our closest living relatives, the apes, we used design-based stereological methods to obtain neuron counts for the amygdala and each of four major amygdaloid nuclei (the lateral, basal, accessory basal, and central nuclei) in humans, all great ape species, lesser apes, and one monkey species. Our goal was to determine whether there were significant differences in the number or percent of neurons distributed to individual nuclei among species. Additionally, regression analyses were performed on independent contrast data to determine whether any individual species deviated from allometric trends. There were two major findings. In humans, the lateral nucleus contained the highest number of neurons in the amygdala, whereas in apes the basal nucleus contained the highest number of neurons. Additionally, the human lateral nucleus contained 59% more neurons than predicted by allometric regressions on nonhuman primate data. Based on the largest sample ever analyzed in a comparative study of the hominoid amygdala, our findings suggest that an emphasis on the lateral nucleus is the main characteristic of amygdala specialization over the course of human evolution.

Distribution of Particles in the Z-axis of Tissue Sections: Relevance for Counting Methods

The distribution of particles in the z-axis of thick tissue sections has gained considerable attention, primarily because of implications for the accuracy of modern stereological counting methods. Three major types of artifacts can affect these sections: loss of particles from the surfaces of tissue sections (lost caps), homogeneous collapse in the z-axis, and differential deformation in the z-axis. Initially it was assumed that thick sections were not compromised by differential shrinkage or compression (differential uniform deformation). Studies in the last decade showed that such artifacts are common and that they depend on embedding media and sectioning devices. Paraffin, glycolmethacrylate and vibratome sections are affected by this artifact, but not celloidin sections or cryostat-derived cryosections. Differential distribution of particles in the z-axis is likely due to compression of the surface areas (margins) during sectioning, resulting in differential particle densities in the core and margin of tissue sections. This deformation of tissue sections can be rapidly assessed by measuring the position of particles in the z-axis. The analysis is complicated by potential secondary effects on section surfaces through loss of particles, the so-called "lost caps" phenomenon. Secondary effects necessitate the use of guard spaces, while their use in case of primary effects (compression due to sectioning) would enhance the artifact's impact on bias. Symmetric versus asymmetric patterns of z-axis distortion can give clues to distinguish primary and secondary effects. Studies that use the optical disector need to take these parameters into account to minimize biases.

Analysis of differential shrinkage in frozen brain sections and its implications for the use of guard zones in stereology

Increasing numbers of neuroanatomists are using stereological methods, and unbiased stereological estimation rules recommend the use of guard zones with the optical disector method to count objects of interest within a volume. Although these methods are statistically unbiased, we believe there is a need to explore sources of systematic bias (e.g., effects of tissue processing and sectioning) that may be affecting estimates of object number. Toward this end, we evaluated neuron distribution through, and tissue shrinkage in, non-embedded tissue cut on a freezing microtome. Our data show that in the x- and y-planes there are minimal changes in tissue area during tissue processing, sectioning, and staining. In the z-axis (perpendicular to the cutting surface), however, sections shrink to ∼25% of the cut thickness. This z-axis shrinkage was quite variable between sections (coefficient of variation about 10%) but stable within the same section (coefficient of variation about 3%). Lastly, individual particle densities are non-uniform through the thickness of the section when the densities should have been uniform. We advise experimenters to use a new protocol, a modified optical disector, for estimation when objects to be counted are marked such that the x-, y-, and z-coordinates are recorded through the full thickness of a section and guard zones are applied post data collection based on the characteristics of the object distribution along the z-axis. It is likely that individual experiments with different embedding materials and histological processing steps could require guard zones of varying sizes, or none at all, depending on object distribution in the z-axis.

Canal cristae growth and fiber extension to the outer hair cells of the mouse ear require Prox1 activity

BACKGROUND

The homeobox gene Prox1 is required for lens, retina, pancreas, liver, and lymphatic vasculature development and is expressed in inner ear supporting cells and neurons.

METHODOLOGY/PRINCIPAL FINDINGS

We have investigated the role of Prox1 in the developing mouse ear taking advantage of available standard and conditional Prox1 mutant mouse strains using Tg(Pax2-Cre) and Tg(Nes-Cre). A severe reduction in the size of the canal cristae but not of other vestibular organs or the cochlea was identified in the E18.5 Prox1(Flox/Flox); Tg(Pax2-Cre) mutant ear. In these mutant embryos, hair cell differentiated; however, their distribution pattern was slightly disorganized in the cochlea where the growth of type II nerve fibers to outer hair cells along Prox1 expressing supporting cells was severely disrupted. In the case of Nestin-Cre, we found that newborn Prox1(Flox/Flox); Tg(Nestin-Cre) exhibit only a disorganized innervation of outer hair cells despite apparently normal cellular differentiation of the organ of Corti, suggesting a cell-autonomous function of Prox1 in neurons.

CONCLUSIONS/SIGNIFICANCE

These results identify a dual role of Prox1 during inner ear development; growth of the canal cristae and fiber guidance of Type II fibers along supporting cells in the cochlea.

Calibration of the stereological estimation of the number of myelinated axons in the rat sciatic nerve: a multicenter study

Several sources of variability can affect stereological estimates. Here we measured the impact of potential sources of variability on numerical stereological estimates of myelinated axons in the adult rat sciatic nerve. Besides biological variation, parameters tested included two variations of stereological methods (unbiased counting frame versus 2D-disector), two sampling schemes (few large versus frequent small sampling boxes), and workstations with varying degrees of sophistication. All estimates were validated against exhaustive counts of the same nerve cross sections to obtain calibrated true numbers of myelinated axons (gold standard). In addition, we quantified errors in particle identification by comparing light microscopic and electron microscopic images of selected consecutive sections. Biological variation was 15.6%. There was no significant difference between the two stereological approaches or workstations used, but sampling schemes with few large samples yielded larger differences (20.7+/-3.7% SEM) of estimates from true values, while frequent small samples showed significantly smaller differences (12.7+/-1.9% SEM). Particle identification was accurate in 94% of cases (range: 89-98%). The most common identification error was due to profiles of Schwann cell nuclei mimicking profiles of small myelinated nerve fibers. We recommend sampling frequent small rather than few large areas, and conclude that workstations with basic stereological equipment are sufficient to obtain accurate estimates. Electron microscopic verification showed that particle misidentification had a surprisingly variable and large impact of up to 11%, corresponding to 2/3 of the biological variation (15.6%). Thus, errors in particle identification require further attention, and we provide a simple nerve fiber recognition test to assist investigators with self-testing and training.

The universal detection of antigens from one skin biopsy specimen

BACKGROUND

Immunohistochemistry is an important tool in dermatology but is limited. Certain antigens can only be preserved in formalin-fixed paraffin-embedded sections, while others can only be detected on frozen sections, resulting in situations where two biopsies are needed. We aimed to develop a technique for universal detection of different antigens out of just one biopsy specimen.

METHODS

Single biopsies were obtained from lesional skin of patients with psoriasis. Standard sample procedures for frozen and paraffin-embedded sections were used. To convert frozen tissue into paraffin-embedded sections, the biopsy specimen was disposed of the embedding medium and subsequently fixed in 10% neutral buffered formalin. We applied various antigen retrieval techniques with alkaline solutions. The differential expression of keratin 10, keratin 15, CD3, CD26 and human beta defensin-2 (HBD-2) was examined using immunohistochemical staining.

RESULTS

We showed that keratin 10 and 15 can be stained on both frozen and paraffin-embedded sections. Staining of paraffin-embedded sections required unmasking with trypsin and Tris-buffered saline Tween solution, respectively. CD3 and CD26 can only be detected on frozen sections, while HBD-2 can only be detected on paraffin-embedded sections.

CONCLUSION

We have described a straightforward technique that gives us the opportunity to use just one biopsy specimen to obtain frozen sections as well as paraffin-embedded sections.

Dimensions and morphology of the cornea in three strains of mice

PURPOSE

To use a histologic approach to obtain dimensional and morphologic information on the cornea in three commonly used strains of mice.

METHODS

Adult mice (three each of 129/SVJ, C57BL/6, and BALB/c) were euthanatized, and the eyes were enucleated, immersed in 2% glutaraldehyde fixative, and prepared for light and transmission electron microscopy. The full corneal, epithelial, stromal, and posterior limiting lamina (PLL) with endothelium thicknesses were measured at the same location centrally and peripherally.

RESULTS

All three strains showed a statistically significant (P < 0.001) decrease in overall thickness in the peripheral compared with the central cornea. The decrease was due to a reduced thickness of both the epithelium and the stroma. The stroma and epithelium contributed to approximately two thirds and one third of the total corneal thickness, respectively. The epithelium had the classic stratified layout and consisted of 13.00 +/- 1.41 layers centrally versus 10.33 +/- 1.37 peripherally. Some adaptation of stromal tissue was found immediately adjacent to the epithelial basement membrane, but a clearly defined anterior limiting lamina did not exist. The stroma was organized into lamellae but lacked the anterior branching and interweaving reported in humans and had unmyelinated nerve fibers within micrometers of the endothelium. The PLL was 2.17 +/- 0.3 microm thick and was divided into pre- and postnatal layers, with striated bodies in the postnatal portion.

CONCLUSIONS

This study demonstrated that in the three strains of mice examined, the cornea becomes significantly thinner toward the periphery. Dimensionally, proportionally, and anatomically the three strains used appeared to be similar. However, morphologic differences were observed compared with other mammals, and awareness of these differences is important when using the mouse as an animal model applicable to the human.

Chapter 5: Methods and protocols in peripheral nerve regeneration experimental research: part II-morphological techniques

This paper critically overviews the main procedures used for carrying out morphological analysis of peripheral nerve fibers in light, confocal, and electron microscopy. In particular, this paper emphasizes the importance of osmium tetroxide post-fixation as a useful procedure to be adopted independently from the embedding medium. In order to facilitate the use of any described techniques, all protocols are presented in full details. The pros and cons for each method are critically addressed and practical indications on the different imaging approaches are reported. Moreover, the basic rules of morpho-quantitative stereological analysis of nerve fibers are described addressing the important concepts of design-based sampling and the disector. Finally, a comparison of stereological analysis on myelinated nerve fibers between paraffin- and resin-embedded rat radial nerves is reported showing that different embedding procedures might influence the distribution of size parameters.

Optical disector counting in cryosections and vibratome sections underestimates particle numbers: effects of tissue quality

Optical disector counting is currently applied most often to cryosections, followed in frequency by resin-embedded tissues, paraffin, and vibratome sections. The preservation quality of these embedding options differs considerably; yet, the effect of tissue morphology on numerical estimates is unknown. We tested whether different embedding media significantly influence numerical estimates in optical disector counting, using the previously calibrated trochlear motor nucleus of hatchling chickens. Animals were perfusion-fixed with paraformaldehyde (PFA) only or in addition with glutaraldehyde (GA), or by Methacarn immersion fixation. Brains were prepared for paraffin, cryo-, vibratome- or celloidin sectioning. Complete penetration of the thionin stain was verified by z-axis analysis. Neuronal nuclei were counted using an unbiased counting rule, numbers were averaged for each group and compared by ANOVA. In paraffin sections, 906 +/- 12 (SEM) neurons were counted, similar to previous calibrated data series, and results obtained from fixation with Methacarn or PFA were statistically indistinguishable. In celloidin sections, 912 +/- 28 neurons were counted-not statistically different from paraffin. In cryosections, 812 +/- 12 neurons were counted (underestimate of 10.4%) when fixed with PFA only, but 867 +/- 17 neurons were counted when fixed with PFA and GA. Vibratome sections had the most serious aberration with 729 +/- 31 neurons-a deficit of 20%. Thus, our analysis shows that PFA-fixed cryosections and vibratome sections result in a substantial numerical deficit. The addition of GA to the PFA fixative significantly improved counts in cryosections. These results may explain, in part, the significant numerical differences reported from different labs and should help investigators select optimal conditions for quantitative morphological studies.