Size changes in single muscle fibers during fixation and embedding

Diaphragm muscle fibrosis involves changes in collagen organization with mechanical implications in Duchenne muscular dystrophy

In Duchenne muscular dystrophy (DMD), diaphragm muscle dysfunction results in respiratory insufficiency, a leading cause of death in patients. Increased muscle stiffness occurs with buildup of fibrotic tissue, characterized by excessive accumulation of extracellular matrix (ECM) components such as collagen, and prevents the diaphragm from achieving the excursion lengths required for respiration. However, changes in mechanical properties are not explained by collagen amount alone and we must consider the complex structure and mechanics of fibrotic tissue. The goals of our study were to 1) determine if and how collagen organization changes with the progression of DMD in diaphragm muscle tissue and 2) predict how collagen organization influences the mechanical properties of the ECM. We first visualized collagen structure with scanning electron microscopy (SEM) images and then developed an analysis framework to quantify collagen organization and generate image-based finite-element models. Image analysis revealed increased collagen fiber straightness and alignment in mdx over wild type (WT) at 3 mo (straightness: mdx = 0.976 ± 0.0108, WT = 0.887 ± 0.0309, alignment: mdx = 0.876 ± 0.0333, WT = 0.759 ± 0.0416) and 6 mo (straightness: mdx = 0.942 ± 0.0182, WT = 0.881 ± 0.0163, alignment: mdx = 0.840 ± 0.0315, WT = 0.759 ± 0.0368). Collagen fibers retained a transverse orientation relative to muscle fibers (70°-90°) in all groups. Mechanical models predicted an increase in the transverse relative to longitudinal (muscle fiber direction) stiffness, with stiffness ratio (transverse/longitudinal) increased in mdx over WT at 3 mo (mdx = 5.45 ± 2.04, WT = 1.97 ± 0.670) and 6 mo (mdx = 4.05 ± 0.985, WT = 1.96 ± 0.506). This study revealed changes in diaphragm ECM structure and mechanics during disease progression in the mdx muscular dystrophy mouse phenotype, highlighting the need to consider the role of collagen organization on diaphragm muscle function.NEW & NOTEWORTHY Scanning electron microscopy images of decellularized diaphragm muscle from WT and mdx, Duchenne muscular dystrophy model, mice revealed that collagen fibers in the epimysium are oriented transverse to muscle fibers, with age- and disease-dependent changes in collagen arrangement. Finite-element models generated from these images predicted that changes in collagen arrangement during disease progression influence the mechanical properties of the extracellular matrix. Thus, changes in collagen fiber-level structure are implicated on tissue-level properties during fibrosis.

The smallest known heliozoans are the Erebor lineage (nom. clad. n.) inside Microheliella maris (Eukaryota, Diaphoretickes), with the amendation of M. maris diagnosis and description of Berkeleyaesol magnus gen. nov., comb. nov. (Eukaryota, incertae sedis)

A new strain of planktonic heliozoans (ZI172) belonging to the genus Microheliella (the sister group of Cryptista in Diaphoretickes), closely related to the only one known strain of Microheliella maris (CCAP 1945/1), was studied with light microscopy and SSU rRNA gene sequencing. Morphometric data obtained from 127 cells and based on 254 measurements showed that this strain represents the smallest heliozoan (1.66-3.42 µm, av. 2.56 µm) in diameter known to date and one of the smallest free-living eukaryotes. We also did morphometry for strain CCAP 1945/1. Its cell body size is 3.20-6.47 µm (av. 4.15 µm; n=141; m=282). The secondary structures of hairpin 15 of the SSU rRNA molecules were reconstructed for ZI172 and CCAP 1945/1 and they were compared The possible biochemical explanation for the smaller size of the ZI172 strain, which is smaller than the CCAP 1945/1 strain, is discussed, including all published electron micrographs of CCAP 1945/1. The necessary taxonomic work is also carried out. The diagnosis of Microheliella maris is amended and the new infraspecific clade Erebor is described to include ZI172. The measurements and systematics of the enigmatic heliozoan 'Raphidiophrys' magna O'Donoghue 1922 (non 1921; the biggest known heliozoan) are also discussed and it is transferred to the new genus Berkeleyaesol.

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.

Spatial mapping of the collagen distribution in human and mouse tissues by force volume atomic force microscopy

Changes in the elastic properties of living tissues during normal development and in pathological processes are often due to modifications of the collagen component of the extracellular matrix at various length scales. Force volume AFM can precisely capture the mechanical properties of biological samples with force sensitivity and spatial resolution. The integration of AFM data with data of the molecular composition contributes to understanding the interplay between tissue biochemistry, organization and function. The detection of micrometer-size, heterogeneous domains at different elastic moduli in tissue sections by AFM has remained elusive so far, due to the lack of correlations with histological, optical and biochemical assessments. In this work, force volume AFM is used to identify collagen-enriched domains, naturally present in human and mouse tissues, by their elastic modulus. Collagen identification is obtained in a robust way and affordable timescales, through an optimal design of the sample preparation method and AFM parameters for faster scan with micrometer resolution. The choice of a separate reference sample stained for collagen allows correlating elastic modulus with collagen amount and position with high statistical significance. The proposed preparation method ensures safe handling of the tissue sections guarantees the preservation of their micromechanical characteristics over time and makes it much easier to perform correlation experiments with different biomarkers independently.

Micro-CT and histopathology methods to assess host response of aneurysms treated with shape memory polymer foam-coated coils versus bare metal coil occlusion devices

Recent studies utilizing shape memory polymer foams to coat embolizing coils have shown potential benefits over current aneurysm treatments. In the current study utilizing a rabbit-elastase aneurysm model, the performance of test article (foam-coated coil [FCC]) and control (bare platinum coils [BPCs]) devices were compared at 30, 90, and 180 days using micro-CT and histological assessments. The host response was measured by identifying the cells regionally present within the aneurysm, and assessing the degree of residual debris and connective tissue. The 3D reconstructions of aneurysms provided context for histologic findings, and aided in the overall aneurysm assessment. At all time points, >75% of the cells categorized in each aneurysm were associated with a bioactive yet biocompatible host response (vs. the remainder of cells that were associated with acute inflammation). The extracellular matrix exhibited a transition from residual fibrin at 30 days to a greater degree of connective tissue at 90 and 180 days. Although the control BPC-treated aneurysms exhibited a greater degree of connective tissue at the earliest time point examined (30 days), by 180 days, the FCC-treated aneurysms had more connective tissue and less debris overall than the control aneurysms. When considering cell types and extracellular matrix composition, the overall host response scores were significantly better in FCC-treated aneurysms at the later time point. Based on the results of these metrics, the FCC device may lead to an advanced tissue remodeling response over BPC occlusion devices.

Assessment of Primary Cell Wall Nanomechanical Properties in Internal Cells of Non-Fixed Maize Roots

The mechanical properties of cell walls play a vital role in plant development. Atomic-force microscopy (AFM) is widely used for characterization of these properties. However, only surface or isolated plant cells have been used for such investigations, at least as non-embedded samples. Theories that claim a restrictive role of a particular tissue in plant growth cannot be confirmed without direct measurement of the mechanical properties of internal tissue cell walls. Here we report an approach of assessing the nanomechanical properties of primary cell walls in the inner tissues of growing plant organs. The procedure does not include fixation, resin-embedding or drying of plant material. Vibratome-derived longitudinal and transverse sections of maize root were investigated by AFM in a liquid cell to track the changes of cell wall stiffness and elasticity accompanying elongation growth. Apparent Young's modulus values and stiffness of stele periclinal cell walls in the elongation zone of maize root were lower than in the meristem, i.e., cell walls became more elastic and less resistant to an applied force during their elongation. The trend was confirmed using either a sharp or spherical probe. The availability of such a method may promote our understanding of individual tissue roles in the plant growth processes.

Inertial picobalance reveals fast mass fluctuations in mammalian cells

The regulation of size, volume and mass in living cells is physiologically important, and dysregulation of these parameters gives rise to many diseases. Cell mass is largely determined by the amount of water, proteins, lipids, carbohydrates and nucleic acids present in a cell, and is tightly linked to metabolism, proliferation and gene expression. Technologies have emerged in recent years that make it possible to track the masses of single suspended cells and adherent cells. However, it has not been possible to track individual adherent cells in physiological conditions at the mass and time resolutions required to observe fast cellular dynamics. Here we introduce a cell balance (a 'picobalance'), based on an optically excited microresonator, that measures the total mass of single or multiple adherent cells in culture conditions over days with millisecond time resolution and picogram mass sensitivity. Using our technique, we observe that the mass of living mammalian cells fluctuates intrinsically by around one to four per cent over timescales of seconds throughout the cell cycle. Perturbation experiments link these mass fluctuations to the basic cellular processes of ATP synthesis and water transport. Furthermore, we show that growth and cell cycle progression are arrested in cells infected with vaccinia virus, but mass fluctuations continue until cell death. Our measurements suggest that all living cells show fast and subtle mass fluctuations throughout the cell cycle. As our cell balance is easy to handle and compatible with fluorescence microscopy, we anticipate that our approach will contribute to the understanding of cell mass regulation in various cell states and across timescales, which is important in areas including physiology, cancer research, stem-cell differentiation and drug discovery.

Extracellular space in mouse cerebellar cortex revealed by in vivo cryotechnique

Conventional methods of preparing tissue specimens for morphological investigation of the central nervous system suffer from inevitable artifacts caused by anoxia during the processing. In the present study we performed ultrastructural analyses of mouse cerebellar cortex using the in vivo cryotechnique (IVCr), which minimizes ischemic artifacts of target organs through direct cryofixation in vivo. In molecular and Purkinje cell layers of the mouse cerebellum prepared with IVCr, considerably large extracellular spaces (ECS) were detected among cellular profiles and synaptic clefts. The ECS obtained with IVCr without ischemia were larger than those obtained with IVCr after 8-minute ischemia or a conventional quick-freezing method with fresh resected tissues (FQF), but did not decrease with IVCr after 30-second ischemia. By contrast, the parallel fibers observed with IVCr without ischemia were slightly smaller than those after 30-second ischemia, and significantly smaller than those prepared with IVCr after 8-minute ischemia or FQF. ECS were frequently preserved around synaptic clefts, although the rest were totally or partially enclosed with closely apposed glial processes. The estimated sizes of the ECS around synaptic clefts did not differ between the opened and enclosed synapses, suggesting that the opened synapses might be temporarily surrounded by glial sheaths dynamically extending or retracting throughout the perisynaptic ECS. These findings indicate IVCr to be useful for some morphological analyses of ECS in the central nervous system. The appreciable ECS around synapses would allow morphological and functional changes of neuronal and glial cells dynamically involved in synaptic remodeling or signal transduction.

Aqueous diffusion pathways as a part of the ventricular cell ultrastructure

The physical organization of the ventricular myocyte includes barriers for the movement of objects of varying dimensions ranging from ions to solid particles. There are two kinds of diffusion in the cell: lateral (in membranes) and aqueous. Here we examine the size constraints of aqueous diffusion pathways and discuss their impact on cellular physiology. Calibrated gold nanoparticles were used to probe the accessibility of the entire transverse-axial tubular system (TATS), the sarcoplasm, and intracellular structures. The TATS tubules, although up to 300 nm in diameter, permitted only particles </=11 nm to enter. When calibrated nanoparticles were added to permeabilized ventricular cells, particles </=11 nm were found in the sarcoplasm. The distribution of nanoparticles in the cells allowed us to conclude that 1), the TATS and the sarcoplasm are accessible only for particles </=11 nm; 2), the gaps between T-tubules and junctional sarcoplasmic reticulum (jSR), jSR and mitochondria, and intermitochondrial contacts are inaccessible for particles with physical size >3 nm; 3), the mitochondrial voltage-dependent anion channel and the nuclear pore complex in ventricular cells could not be penetrated by particles >/=6 nm; and 4), there is a difference in size clearance between transversal and longitudinal sarcoplasmic diffusional pathways.

STRUCTURAL ASSESSMENT OF THE URETHRAL SPHINCTER IN WOMEN WITH URINARY RETENTION

PURPOSE

The pathophysiology of urinary retention in women is generally unknown but a subgroup of women with urinary retention have been diagnosed as having so-called primary disorder of sphincter relaxation on the basis of an abnormal urethral sphincter electromyogram. It was suggested this sphincter overactivity could lead to work hypertrophy of the urethral rhabdosphincter and in this study we looked for any evidence of such muscle fiber hypertrophy.

MATERIALS AND METHODS

In 9 women 18 to 45 years old (mean age 31.6) with urinary retention and overactive urethral sphincter electromyogram, light and electron microscopy were used to examine core needle biopsies of the urethral rhabdosphincter taken under transvaginal ultrasound control. Of the 9 patients only 5 biopsies processed for light microscopy and 4 processed for electron microscopy contained striated urethral muscle fibers. The results of these biopsies were compared to the morphology of a control specimen from a postmenopausal woman without a history of urinary retention.

RESULTS

On light microscopy the urethral rhabdosphincter fiber diameter did not differ among patients (mean average 7.6 mum), was less than that reported in the literature (15 to 20), but did not differ from that of the control (mean 9.9). In all patients electron microscopy showed excessive peripheral sarcoplasm with lipid and glycogen deposition, and sarcoplasmic accumulation of normal mitochondria. These ultrastructural abnormalities were not seen in the control.

CONCLUSIONS

To our knowledge this is the first morphological description of the urethral rhabdosphincter in a subgroup of women with urinary retention. Mean rhabdosphincter fiber diameter was approximately the same in patients and controls. This study does not support the previous theory that urethral sphincter overactivity in a subgroup of women with urinary retention leads to work hyperplasia of urethral rhabdosphincter fibers. An alternative hypothesis is suggested.

Plasticity of the transverse tubules following denervation and subsequent reinnervation in rat slow and fast muscle fibres

We have studied the effects of short term denervation followed by reinnervation on the ultrastructure of the membrane systems and on the content of and distribution of key proteins involved in calcium regulation of fast-twitch (FT) extensor digitorum longus (EDL) and slow-twitch (ST) soleus (SOL) muscle fibres. Ischiadic nerve freezing resulted in total lack of neuromuscular transmission for 3 days followed by a slow recovery, but no decline in twitch force elicited by direct stimulation. The latter measurements indicate no significant atrophy within this time frame. The membrane systems of skeletal muscle fibres were visualized using Ca92+)-K3Fe(CN)6-OsO4 techniques and observed using a high voltage electron microscope. [3H]nitrendipine binding was used to detect levels of dihydropyridine receptor (DHPR) expression. The Ca2+ pumping free sarcoplasmic reticulum domains were not affected by the denervation, but the Ca2+ release domains were dramatically increased, particularly in the FT-EDL muscle fibres. The increase is evidenced by a doubling up of the areas of contacts between SR and transverse (t-) tubules, so that in place of the normal triadic arrangement, pentadic and heptadic junctions, formed by multiple interacting layers of ST and t-tubules are seen. Frequency of pentads and heptads increases and declines in parallel to the denervation and reinnervation but with a delay. Immunofluorecence and electron microscopy observations show presence of DHPR and ryanodine receptor clusters at pentads and heptads junctions. A significant (P < 0.01) positive correlation between the level of [3H]nitrendipine binding component and the frequency pentads and heptads was observed in both the FT-EDL and ST-SOL muscle fibres indicating that overexpression of DHPRs accompanies the build up extra junctional contacts. The results indicate that denervation reversibly affects the domains of the membrane systems involved in excitation-contraction coupling.

Structure of kinetochore fibres in crane-fly spermatocytes after irradiation with an ultraviolet microbeam: Neither microtubules nor actin filaments remain in the irradiated region

We studied chromosome movement after kinetochore microtubules were severed. Severing a kinetochore fibre in living crane-fly spermatocytes with an ultraviolet microbeam creates a kinetochore stub, a birefringent remnant of the spindle fibre connected to the kinetochore and extending only to the edge of the irradiated region. After the irradiation, anaphase chromosomes either move poleward led by their stubs or temporarily stop moving. We examined actin and/or microtubules in irradiated cells by means of confocal fluorescence microscopy or serial-section reconstructions from electron microscopy. For each cell thus examined, chromosome movement had been recorded continuously until the moment of fixation. Kinetochore microtubules were completely severed by the ultraviolet microbeam in cells in which chromosomes continued to move poleward after the irradiation: none were seen in the irradiated regions. Similarly, actin filaments normally present in kinetochore fibres were severed by the ultraviolet microbeam irradiations: the irradiated regions contained no actin filaments and only local spots of non-filamentous actin. There was no difference in irradiated regions when the associated chromosomes continued to move versus when they stopped moving. Thus, one cannot explain motion with severed kinetochore microtubules in terms of either microtubules or actin-filaments bridging the irradiated region. The data seem to negate current models for anaphase chromosome movement and support a model in which poleward chromosome movement results from forces generated within the spindle matrix that propel kinetochore fibres or kinetochore stubs poleward.

Some recent advances on the study and understanding of the functional design of the avian lung: morphological and morphometric perspectives

The small highly aerobic avian species have morphometrically superior lungs while the large flightless ones have less well-refined lungs. Two parabronchial systems, i.e. the paleopulmo and neopulmo, occur in the lungs of relatively advanced birds. Although their evolution and development are not clear, understanding their presence is physiologically important particularly since the air- and blood flow patterns in them are different. Geometrically, the bulk air flow in the parabronchial lumen, i.e. in the longitudinal direction, and the flow of deoxygenated blood from the periphery, i.e. in a centripetal direction, are perpendicularly arranged to produce a cross-current relationship. Functionally, the blood capillaries in the avian lung constitute a multicapillary serial arterialization system. The amount of oxygen and carbon dioxide exchanged arises from many modest transactions that occur where air- and blood capillaries interface along the parabronchial lengths, an additive process that greatly enhances the respiratory efficiency. In some species of birds, an epithelial tumescence occurs at the terminal part of the extrapulmonary primary bronchi (EPPB). The swelling narrows the EPPB, conceivably allowing the shunting of inspired air across the openings of the medioventral secondary bronchi, i.e. inspiratory aerodynamic valving. The defence stratagems in the avian lung differ from those of mammals: fewer surface (free) macrophages (SMs) occur, the epithelial cells that line the atria and infundibula are phagocytic, a large population of subepithelial macrophages is present and pulmonary intravascular macrophages exist. This complex defence inventory may explain the paucity of SMs in the avian lung.

A qualitative and quantitative study of the lung of an ostrich,Struthio camelus

The ostrich lung, with its lack of interparabronchial septa, the presence of very shallow atria and exceptional morphometric refinement, structurally resembles those of small, energetic flying birds, whereas it also displays features characteristic of the flightless ratites in which the neopulmo is relatively poorly developed and a segmentum accelerans may be generally lacking. The large size of the bronchial system of the ostrich may help explain the unique shifts in the airflow pathways that must occur from resting to panting breathing, explaining its insensitivity to acid–base imbalance of the blood during sustained panting under thermal stress. The mass-specific volume of the lung is 39.1 cm3kg−1 and the volume density of the exchange tissue is remarkably high (78.31%). The blood–gas (tissue) barrier is relatively thick (0.56μm) but the plasma layer is very thin (0.14μm). In this flightless ratite bird, the mass-specific surface area of the tissue barrier (30.1 cm2g−1), the mass-specific anatomical diffusing capacity of the tissue barrier for oxygen (0.0022mlO2s−1Pa−1kg−1), the mass-specific volume of pulmonary capillary blood (6.25 cm3kg−1) and the mass-specific total anatomical diffusing capacity for oxygen (0.00073mlO2s−1Pa−1kg−1) are equivalent to or exceed those of much smaller highly aerobic volant birds. The distinctive morphological and morphometric features that seem to occur in the ostrich lung may explain how it achieves and maintains high aerobic capacities and endures long thermal panting without experiencing respiratory alkalosis.

Myofibril size variation along the length of extraocular muscle in rabbit and rat. I: orbital layer

It is generally assumed that a muscle fiber is structurally uniform along its length. That assumption is not consistent with the observed variation of myofibrillar profile size along the length of both singly innervated fibers (SIFs) and multiply innervated fibers (MIFs) in the orbital (outer) layer of extraocular muscle (EOM). Muscle fibers were reconstructed in serial sections along the orbital layer of rabbit and rat EOM. For both the SIFs and MIFs, myofibril profile size was smallest (narrowest) near the endplate. In the SIFs of rat, for example, the myofibril profiles were 28% wider at a distance of 1.5 mm from the endplate than at the endplate itself. Measures of profile size included the mean intercept length and the mean shortest path from test points within the profile to the profile boundary. The possible effect of sarcomere length variation was controlled by normalizing the myofibrillar profile size data to a constant spacing of the myosin filament lattice. This morphometric approach was also used to quantify the further increase of profile size that occurs in the end portions of the orbital MIFs where the myobrillar organization is typically ill-defined.

Analyzing renal glomeruli with the new stereology

The highly specialized architecture of the renal glomerulus is altered in a variety of disease states. Morphometric methods, including stereological methods, have been widely used to analyze these changes in both animal and human glomeruli. However, many of the methods available until recently were biased and provided incomplete information. The past few years have witnessed the development of a new generation of unbiased stereological methods. Another advantage of these new methods and strategies is that they are less influenced by technical artifacts than the traditional methods. This chapter describes how these new stereological methods can be used to quantify glomerular morphology. Parameters considered include glomerular number and volume; glomerular cell number and size; and the length, surface area, and number of glomerular capillaries. Methods for obtaining data for average glomeruli as well as individual glomeruli are described. Technical details are included wherever possible.

Capillaries measured in canine diaphragm by two methods

We have measured capillary distribution in costal and crural canine diaphragm using two methods: histochemical processing and perfusion fixation. Each of 18 dogs was deeply anesthetized, the abdomen opened, and the left inferior phrenic artery cannulated. The animal was heparinized and overdosed with pentobarbital. The right hemidiaphragm was frozen, either postexcision (Protocol 1) or intact with no preload (Protocol 2), for histochemical processing. The left hemidiaphragm was fixed by perfusion in situ using 2% glutaraldehyde, either with preload (Protocol 1) or without (Protocol 2). Costal and crural regions of each hemidiaphragm were sampled for analysis. Frozen samples were sectioned and processed for acid-stable (pH 4.0) ATPase activity; perfusion-fixed samples were postfixed, stained, embedded in Epon, and sectioned. Measurements were made using a digital imaging system. We found that muscle fibers had smaller cross-sectional areas in costal than in crural diaphragm; capillary-to-fiber ratio (C:F) did not differ by region and regional differences in capillary density could be attributed to differences in fiber size. Results depended critically on methodology. In perfusion-fixed muscle, fiber area was less, C:F was greater, and capillary density was greater than in histochemically-processed tissue. We conclude that capillary distribution is similar in costal vs. crural diaphragm and that perfusion fixation identifies capillaries more effectively than histochemistry.

Morphometric analysis of the developing, murine aneural soleus muscle

The pattern of organogenesis of the aneural soleus muscle of the 129ReJ mouse [rendered aneural by laser ablation of the lumbosacral spinal cord at 14 days in utero (during the period of primary myotube formation, but prior to the formation of secondary myotubes)] was evaluated quantitatively with spaced, serial ultrathin sections and computer-assisted morphometric analysis. Aneural muscles from 16- and 18-day gestation and newborn mice were analyzed to determine age-related changes in a number of parameters including: muscles' maximal girths, numbers of myotubes, myotube diameter distributions, and cluster frequency. Data were compared with a similar study of the organogenesis of the normal soleus muscle (Ontell et al: Am J Anat 181:279-288, 1988). Basic patterns of morphogenesis of the soleus muscle were unchanged by spinal cord ablation, and differences in development between the aneural and innervated muscles were quantitative rather than qualitative. At birth, the aneural muscle contained approximately 76% of the myotubes found in the innervated muscle (approximately 840 myotubes in the innervated muscle and approximately 640 in the aneural muscle). Evidence is presented consistent with the hypothesis that primary myotube formation is reduced by approximately 32% in the aneural muscles and that while extensive secondary myotube formation occurs (approximately 78% of the myotube present at birth in these muscles are secondary myotubes), there is a significant reduction in the number of secondary myotubes in aneural muscles. It is suggested that the reduced numbers of secondary myotubes may be related to the reduction in the number of primary myotubes, which are known to act as scaffolds for secondary myotube formation. The time course of secondary myotube formation and of cluster formation and cluster dispersal and the number of cells per cluster are similar in age-matched, innervated and aneural muscles. The absence of innervation has little effect on myotube growth until birth, when comparison of the myotube diameter distributions reveals a slight alteration in myotube diameter distributions of aneural as compared with innervated muscles.

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.

Formation of new quasi-crystalline ordered aggregates by gizzard myosin

Turkey gizzard myosin was found to self-assemble into new polymorphic forms as detected by thin-section electron microscopy. In high ionic strength buffers (0.3 mM KCl, pH 6.0), aggregates of sidepolar filaments were produced. Electron microscopy of thin sections revealed individual filaments with a 13.5 nm axial repeat. Under a number of conditions, with varying ionic strength, pH, MgCl2 and ATP, the filaments assembled through the head region with the tail portion projecting out radially from the aggregate. The regions corresponding to heads and tails within the aggregates were established by immunoelectron microscopy using anti-S1 and anti-LMM antibodies coupled to gold. These filaments often interacted to produce bilayer sheets, which, when cut perpendicular to the plane of the sheet, appeared as ladders. A hitherto unreported structure was obtained at 0.2 M KCl (pH 8.0): myosin aggregated to generate a three-dimensional quasi-crystalline lattice with a 270 nm period. In these aggregates, myosin was arranged in an antiparallel fashion, stacked on one another, producing ribbon-like strips stabilized through non-covalent interactions between heads, thereby producing a crystalline lattice. Neither Mg2+ nor ATP were required for this form. Phosphorylation of the regulatory light chains or the cleavage of the heavy chains at a single site in the head region prevented myosin from assembling in the 3-D lattice form. Generally, unphosphorylated myosin produced periodic paracrystals at low ionic strength in the presence of 10 mM MgCl2, but as the ionic strength was increased the regular 3-D lattice became the predominant form. Some paracrystalline forms could be obtained at high ionic strength without magnesium with phosphorylated myosin.

A quantitative study of myonuclear and satellite cell nuclear size in Duchenne's muscular dystrophy, polymyositis and normal human skeletal muscle

The dimensions of myonuclei and satellite cell nuclei in Duchenne's muscular dystrophy (DMD), polymyositis, and normal controls were compared in order to determine whether differences in satellite cell populations (previously reported by these and other authors) could be attributable to changes in the relative dimensions, thereby biasing the counts. The nuclei were measured directly from semithin resin sections using computerized measuring techniques, thereby avoiding errors due to photographic enlargement. In both the control and polymyositic groups, the satellite cell nuclei (8.30 microns and 8.81 microns respectively) were significantly shorter than the myonuclei (11.75 microns and 13.00 micron). Dystrophic myonuclei (10.98 microns) were significantly shorter than polymyositic myonuclei, but dystrophic satellite cell nuclei (11.62 microns) were significantly longer than both polymyositic and normal control satellite cell nuclei. The mean nuclear area in transverse sections was significantly greater in both myopathies than in the control material for both myonuclei and satellite cell nuclei. Myonuclei were significantly larger than satellite cell nuclei in all groups. When the values for the lengths of the nuclei were used to adjust previous estimates of satellite cell populations, it was found that earlier conclusions were still valid, i.e., that there is a significant increase in the number of satellite cells in the dystrophic muscle fibre population.

Comparison of the effects of critical point-drying and freeze-drying on cytoskeletons and microtubules

We have compared the effects of critical point-drying (CPD) and freeze-drying (FD) on the morphology of Triton-resistant cytoskeletons and microtubules by scanning (SEM) and transmission electron microscopy (TEM). In general, cytoskeletons attached to Formvar films suffer less structural damage than cells or cytoskeletons attached to glass, because the Formvar film absorbs some of the stress associated with shrinkage during drying. However, as seen in stereo-pair electron micrographs, the three-dimensional structure of cytoskeletons prepared by FD is better preserved and shows fewer artefacts than those prepared by CPD. CPD specimens are flatter, often have a concave and apparently collapsed nuclear matrix and show large cracks both in the perinuclear zone and through the cytoskeleton. At least some of the damage appears to be due to residual water in the CO2 used as the substitution fluid, because cytoskeletons dried with a water filter attached to the CPD apparatus show substantially less damage than those dried without the filter. Freeze-dried cytoskeletons consist mostly of unbroken, smooth filaments and have no perinuclear open space. Comparison of the effects of drying on the diameters of in vitro polymerized microtubules showed that the diameter of microtubules is reduced after drying, but that FD causes significantly less shrinkage than CPD. Addition of 0.2% tannic acid to the glutaraldehyde fixative significantly reduces the shrinkage of CPD microtubules, but has no effect on FD microtubules. The observations on microtubules support the hypothesis that drying-induced shrinkage is the result of both pressure and solvent evaporation and they indicate that tannic acid stabilizes samples against the former but not the latter.

Morphometric analysis of the developing mouse soleus muscle

The pattern of organogenesis of the soleus muscle of the 129 ReJ mouse was evaluated quantitatively using spaced, serial, ultrathin sections and computer-assisted morphometric analysis. Muscles from 14-, 16-, and 18-day in utero mice and muscles of 1- and 5-day-old mice were analyzed to determine age-related alterations in the maximal girth and length of the muscle, number of myotubes, cluster frequency, and the lengths and diameters of myotubes. Primary myotubes are found in the muscle at 14 days in utero. There is little de novo myotube formation between 14 and 16 days in utero, this interval being principally one of primary myotube growth and maturation. The interval between 16 and 18 days in utero is marked by extensive secondary myotube formation, with more myotubes being formed during this period than in any period studied. Morphometric data support the hypothesis that secondary generation myotubes use primary myotubes as a scaffold on which they are formed. Morphometric data also confirm the hypothesis that cluster formation and cluster dispersal occur concurrently during the prenatal period. Secondary myotubes continue to form until birth. At birth, the soleus muscle contains the adult number of myofibers. The first 5 days postnatally are marked by myofiber growth and maturation.

A quantitative analysis of rat adrenal chromaffin tissue: morphometric analysis at tissue and cellular level correlated with catecholamine content

A morphometric analysis of normal Wistar rat adrenal medulla following perfusion fixation and Araldite embedding, was correlated with catecholamine levels on fresh tissue, measured by high-performance liquid chromatography. The mean volume of whole adrenal is 13.2 mm3 and the mean medullary volume 1.3 mm3. Volume density estimates showed that the medulla is composed of 63% chromaffin tissue with an adrenaline to noradrenaline storing cell ratio of 4.4:1. The vasculature occupies 20%, neuronal tissue 5% and interstitial tissues 12% of the medulla. A comparison was made of cell volumes, cell numbers and volume and surface density estimates of cytoplasmic organelles in adrenaline and noradrenaline storing cells. The mean cell volume of adrenaline storing cells at 1300 micron3 is larger than that of noradrenaline storing cells at 980 micron3. A single adrenal medulla contains 4.4-5.7 X 10(5) adrenaline cells and 1.5-1.9 X 10(5) noradrenaline cells. Chromaffin granules account for approximately 30% of the volume of the cytoplasm; the numerical density of granules at different sites in the cell was calculated for adrenaline cells. The volume density of mitochondria (4%) and the surface density of mitochondrial membranes (the ratio of outer to inner membrane being approximately 1:2.3) were similar in both cell types. Rough endoplasmic reticulum was the only organelle to show a significant difference in volume and surface density between the two cell types. Adrenaline storing cells have stacks of rough endoplasmic reticulum which have two to three times the surface and volume densities of that found diffusely scattered throughout noradrenaline cells. The adrenaline content of an adrenaline storing cell is 0.14 X 10(-6) microM and that of a granule 3.0 X 10(-12) or 3.8 X 10(-12) mumoles depending on the method of calculation. The noradrenaline content of noradrenaline storing cells can only be calculated on the assumption that all noradrenaline is stored in this cell type though it is likely that some is contained within adrenaline cells. Based on this assumption the noradrenaline content is 0.17 X 10(-6) mumoles per cell and 5 X 10(-12) mumoles per granule. The present study provides baseline morphometric data on the rat adrenal medulla at tissue and cellular level correlated with amine levels in adrenaline and noradrenaline storing cells and granules.

Coordination of mural elements and myofilaments during arteriolar constriction

Arterioles undergo major morphological changes during vasoconstriction. We used transmission electron microscopy to study wall morphology in both dilated and constricted microvessels to understand the cellular basis of these changes. The relation between the orientation and density of myofilaments and the distribution of dense bodies was analyzed with respect to the level of microvessel tone. The data show a strong correlation between the degree of arteriolar constriction and both the orientation and density of myofilaments. In dilated arterioles, myofilament orientation was predominantly circumferential across the entire smooth muscle cell, averaging 84 +/- 2 degrees (SEM) relative to a radial reference line. In vessels constricted to 50% of their maximal diameter, myofilament orientation was dependent upon the location within the cell, being largely circumferential at the adventitial border (77 +/- 4 degrees) and shifting to a radial arrangement at the intimal border (36 +/- 5 degrees). The reorganization of myofilaments during constriction was associated with a decrease in myofilament density at the intimal-medial border of the smooth muscle cells. The decrease in myofilament density resulted from a selective withdrawal of myofilaments from periluminal areas where "ridges" had formed. Our observations suggest that an ordered distribution of membrane-associated dense bodies along the periluminal aspect of the smooth muscle cells is responsible for both the myofilament reorganization and ridge formation during vasoconstriction. Results of the present study are incorporated into a hypothetical model of arteriolar ultrastructure compatible with the mural reorganization observed during vasoconstriction.

The transverse tubular system of rat myocardium: its morphology and morphometry in the developing and adult animal

The three dimensional arrangements of the T system in the developing and adult animal were investigated by means of high voltage electron microscope stereoscopy using Golgi treated materials. The rat myocardial T system was composed of three major group elements: the transverse tubules, longitudinal tubules and flattened cisternae, which were classified according to their orientation and to their morphological features. It was found, as the growth of the rats proceeded, that the longitudinal tubules increased in number and that the transverse tubules were arranged more regularly and densely at the level of the z band. The flattened cisternae transiently increased in number during the 2-9 weeks, and then decreased gradually. Electron microscopy also revealed that all the transverse, longitudinal tubules and flattened cisternae of the T system had the chance of forming a coupling with the sarcoplasmic reticulum irrespective of its morphology and orientation to the myofibrils. Quantitative analysis of the rat T system from the stereo images indicated that the surface area (0.299 micron2/micron3) was considerably greater than previously reported.

Electric current flow in cell pairs isolated from adult rat hearts

Cell pairs were isolated from ventricles of adult rat hearts so as to study cell-to-cell coupling. Both cells of each pair were impaled with micro-electrodes connected to balanced bridge circuits. Rectangular current pulses were passed and the resulting voltage deflexions monitored. The data were analysed in terms of a delta configuration of three resistive elements, the resistances of the non-junctional membrane of cell 1 and cell 2 (rm, 1 and rm, 2), and the resistance of the nexal membrane (rn). The nexal membrane resistance was found to be insensitive to voltage gradients across the non-junctional membrane (range examined: -70 to -10 mV) and direction of current flow. The mean value of rn was 2.12 M omega ([K+]o = 12 mM). Taking into account morphological parameters, this corresponds to a specific nexal membrane resistance (Rn) of 0.1 omega cm2. Spontaneous uncoupling in which one cell remained polarized while the other one depolarized was never observed. The current-voltage relationship of the non-junctional membrane was found to be bell-shaped. The specific resistance (Rm) at the resting membrane potential (approximately -50 mV) was 3.2 k omega cm2 ([K+]o = 12 mM). Comparative studies performed on single cells revealed a similar relationship Rm versus Vm. Rm at the resting membrane potential (Vm approximately -50 mV) was 2.5 k omega cm2 ([K+]o = 12 mM). The specific capacitance of the non-junctional membrane (Cm) was determined from experiments on single cells. Cm was found to be independent of Vm (voltage range: -80 to 0 mV). The mean value of Cm was 1.66 microF/cm2 ([K+]o = 12 mM). For comparison, experiments on cell pairs and single cells were also carried out with [K+]o = 4 mM. The values obtained for Rn, Rm and Cm did not deviate significantly from those found with [K+]o = 12 mM.

Ultrastructural morphometric analysis of myocardium from dogs, rats, hamsters, mice, and from human hearts

Volume densities of mitochondria, myofibrils, and unspecified cytoplasm were measured by ultrastructural morphometry in myocardium from dogs, rats, hamsters, mice, and in biopsied tissue from human hearts. Human myocardium was composed of 23% mitochondria, 59% myofibrils, and 18% cytoplasm. Volume densities for mitochondria were 22% for dogs, 28% for rats and hamsters, and 32% for mice. Myofibrillar volume densities were highest in dogs with 63%, 57% for rats and hamsters, and 49% for mice. Differences were significant between all except man and dog, and rat and hamster. In an extensive analysis of canine myocardium, it could be shown that the quantitative composition of tissue from the left ventricular free wall (anterior, lateral, posterior) and the papillary muscles was identical. There were also no differences between subepi- and subendocardium as well as the midmyocardium. Volume densities from longitudinal sections were identical to those from transversal sections. Fixation with glutaraldehyde by perfusion or immersion provided identical results. There were no differences between volume densities in samples from the left ventricular free wall (anterior, lateral, and posterior) in rats, hamsters, and mice. It is concluded that each mammalian species is characterized by a very typical quantitative composition of the myocardium. The increase in mitochondrial volume correlated well with the increase in heart rate and oxygen consumption in smaller animals. These quantitative data are regarded as the morphological correlate of the differing functional capacity of hearts from different species.

Organogenesis of the mouse extensor digitorum logus muscle: a quantitative study

A quantitative analysis of the pattern of development and growth of the fetal extensor digitorum longus muscle of the 129 ReJ mouse was carried out in spaced, serial ultrathin sections with computer-assisted morphometry. Muscle from 12-, 14-, 16-, and 18-day in utero mice and from newborn and 5-day postnatal mice was analyzed to determine age-related changes in such factors as the maximal girth and length of the muscle, the number of myotubes, the "cluster" frequency, and the diameters and lengths of the myotubes and muscle units. A distinct temporal pattern of development was established. It was quantitatively determined that a delay less than or equal to 2 days occurs between the formation of primary myotubes (present at 12 days in utero) and secondary myotubes (present at 16 days in utero). By 16 days in utero, groups of myotubes, consisting of one primary myotube and a variable number of secondary myotubes, form "clusters" surrounded by a common basal lamina. Morphometric analyses of diameter distributions establish that most, if not all, secondary-generation myotubes are formed in association with larger, more mature myotubes. Quantitative data support the hypothesis (Ontell and Kozeka, 1984) that cluster formation and cluster dispersion occur simultaneously, beginning sometime between 16 and 18 days in utero. By 18 days in utero, the adult number of myofibers is present in the developing muscle mass. Analyses of lengths and diameters of the same fibers establish that the pattern of growth of the last-formed myotubes of the developing muscle mass is different from that of myotubes formed earlier in development.

Calcium and magnesium contents and volume of the terminal cisternae in caffeine-treated skeletal muscle

(a) The effects of caffeine on the composition and volume of the terminal cisternae (TC) of the sarcoplasmic reticulum (SR) in frog skeletal muscle were determined with rapid freezing, electron microscopy, and electron probe analysis. (b) Caffeine (5 mM) released approximately 65% of the Ca content of the TC in 1 min and 84% after 3 min. The release of Ca from the TC was associated with a highly significant increase in its Mg content. This increase in Mg was not reduced by valinomycin. There was also a small increase in the K content of the TC at 1 min, although not after 3 min of caffeine contracture. (c) On the basis of the increase in Mg content during caffeine contracture and during tetanus (Somlyo, A. V., H. Gonzalez-Serratos, H. Shuman, G. McClellan, and A. P. Somlyo, 1981, J. Cell Biol., 90:577-594), we suggest that both mechanisms of Ca release are associated with an increase in the Ca and Mg permeability of the SR membranes, the two ions possibly moving through a common channel. (d) There was a significant increase in the P content of the TC during caffeine contracture, while in tetanized muscle (see reference above) there was no increase in the P content of the TC. (e) Mitochondrial Ca content was significantly increased (at 1 and at 3 min) during caffeine contracture. Valinomycin (5 microM) blocked this mitochondrial Ca uptake. (f) The sustained Ca release caused by caffeine in situ contrasts with the transient Ca release observed in studies of fragmented SR preparations, and could be explained by mediation of the caffeine-induced Ca release by a second messenger produced more readily in intact muscle than in isolated SR. (g) The TC were not swollen in rapidly frozen, caffeine-treated muscles, in contrast to the swelling of the TC observed in conventionally fixed, caffeine-treated preparation, the latter finding being in agreement with previous studies. (h) The fractional volume of the TC in rapidly frozen control (resting) frog semitendinosus muscles (approximately 2.1%) was less than the volume (approximately 2.5%) after glutaraldehyde-osmium fixation.

Influence of fixative osmolality on the morphometric determination of extracellular space in normal and reperfused ischaemic myocardium

Morphometric determination of extracellular space in control and post-ischaemic reperfused rabbit myocardium was evaluated using two fixatives differing in their composition and total osmolality. Measurement of control extracellular space in an isotonic fixative (294 mOsm/kg water) was 20.8% and in a hypertonic fixative (1816 mOsm/kg water) was 22.2%. These values were not statistically different. Ischaemic durations of 15, 30, 60 and 90 min, followed by an equivalent period of reperfusion, created significant increases in extracellular space. The size of the extracellular space determined by both fixatives was found to be the same. Total fixative osmolality does not appear to influence morphometric evaluation of the extracellular space in control tissue or in tissue damaged by ischaemia and reperfusion.

Relationship of the vestibular hair cells to magnetic particles in the otolith of the guitarfish sacculus

Magnetite particles were systematically localized within the otolithic mass as a conspicuous curved band in the most ventral region of the guitarfish sacculus. The magnetite band overlaid two separated segments of neuroepithelium in the macula, a long segment oriented parallel to the longitudinal body axis, and a short segment oriented in a transverse plane. The exogenous magnetite particles differed from the endogenous otoconia both in their capacity of orienting to magnetic fields, and their difference in mass due to the higher atomic weight of iron. In addition to the normal gravistatic function of the sacculus, two additional receptor functions are hypothesized based upon the differences between the endogenous and exogenous otoconia. A geomagnetic field could induce magnetite displacements detectable by the hair cells for purposes of geomagnetic orientation. Alternatively, the greater atomic weights of magnetite, relative to that of otoconia, could result in gravitational and linear acceleration, which differed in different regions of the macula.

Myofilament diameters: an ultrastructural re-evaluation

In situ, ultrastructural measurements of diameters of contractile filaments in skeletal and heart muscle differ considerably from those previously reported. Past measurements have been made in thin, transverse epoxy sections that were non-specifically stained with heavy metal salts to overcome background scattering of epoxy polymer. In our images from transverse de-embedded sections, the hexagonal lattice has some considerable differences from that seen in epoxy sections. Muscle samples from rat atrium and frog sartorius were fixed, dehydrated, embedded in polyethylene glycol, and sectioned. Sections were de-embedded in graded polyethylene glycol/ethanol, mounted on coated grids, critical point dried, and viewed in the electron microscope without staining. The backbone diameters of thick filaments were measured in the M band region and have an average value, after correction for shrinkage, of 25 nm. Thin filament diameters range from 6.5-9.5 nm. In regions of overlap of thin and thick filaments, the thick filament profiles varied from circular to asymmetric; diameters range up to 36 nm and yield eccentricity ratios varying from 1.5 to 1.0 (circular profiles). Portions of thick filaments touch or partially envelope neighboring thin filaments. The relative contributions of cytoskeletal components to these images of overlap regions remains to be determined, but the backbone diameters in glycerinated frog sartorius are not significantly different from control samples. The present results are consistent with those reported for rotary shadowed thick filaments; from recent experiments in muscles whose myofilament lattice is osmotically compressed; and with estimates of A band mass. This lattice geometry yields relatively low surface-to-surface distances between filaments. Steric considerations and their implications for cross bridge theory are discussed.

Problems associated with the morphometric measurement of transverse skeletal muscle fibers: I. Analysis of frozen sections

The aim of this study was to evaluate the effects of a number of reported procedures which precede freezing on the cross sectional area of skeletal muscle fibers. Probable sources of variation were recognised as: Within blocks, same animal, same procedure. Variability in the size of muscle fibers from area-based measurements was determined for the anterior head of the biceps femoris muscle of the rat. Fibers were measured within areas selected at random from sections taken from the same block of tissue. Variance within and between sections was subsequently established. Between blocks from rats having undergone the same procedure. Variability in the size of muscle fibers measured within areas selected at random from sections taken from different blocks of tissue within the same animal. Between rats, same or equivalent procedure. Fibers were measured from randomly selected areas within sections taken from blocks of tissue derived from different animals. The muscles were subjected to the same or equivalent treatment prior to freezing. Between procedures. Restraining muscles prior to and during freezing did not significantly affect the results except the procedure involving holding the muscle between two pairs of forceps. However, the variability was high within and between sections and blocks from the same muscle. It is postulated that the variability is of technical rather than biological origin and may be the result of small focal contractions which occur along the fibers during freezing.

The ultrastructure of the cardiac Purkinje strand in the dog: a morphometric analysis

Purkinje strands from both ventricles of adult mongrel dogs were excised, and electrical properties were studied by the voltage-clamp technique. The strands were then examined with light and electron microscopy and structural properties were analysed by morphometric techniques. The canine Purkinje strand contains (by volume) about 28% myocyte and 55% dense outer connective tissue. The remainder of the volume is taken up by the inner shell of loosely packed connective tissue within 10 microns of a myocyte membrane. These volume fractions vary considerably from one strand to another. Clefts less than 10 microns wide occupy 18% of the myocyte volume and clefts less than 1 micron wide occupy 1%. The membrane surface area of the myocytes can be divided into three categories by reference to the size of the adjacent cleft. About 47.8% of the membrane surface area faces clefts wider than 1 micron, another 22.2% faces clefts between 0.1 and 1 micron wide, and the final 30% faces clefts less than 0.1 micron wide. The surface area facing the narrowest clefts (less than 0.1 micron wide) is divided between nexuses 3%, desmosomes 10%, and unspecialized membrane 17% (each figure is expressed as a percentage of the total surface area of myocyte membrane). The canine Purkinje strand has a more favourable anatomy than the sheep Purkinje strand for most physiological experiments. We expect that the complicating effects of series resistance and change in the concentration of extracellular ions will be much smaller than in sheep strands, but still not negligible.

Oxygen transport in resting and contracting hamster cremaster muscles: experimental and theoretical microvascular studies

Intravital microscopy of the superfused cremaster muscle was used to measure the density, diameter, length, hematocrit, red cell velocity, and red cell flux in capillaries of the pentobarbital-anesthetized hamster. Oxygen microelectrodes were used to measure oxygen tension (Po2) at a position 75-100 micrometers deep in the muscle between the venous ends of capillaries and, very importantly, at the superfusate-muscle interface. These parameters were measured in resting and contracting muscles and under three values of superfusate Po2: low (8mm Hg), medium (40 mm Hg), and high (75 mm Hg). These data were complete enough to be useful input parameters in a recently developed mathematical model of oxygen transport in exposed tissue (A. S. Popel, 1981, Math. Biosci. 55, 231-246). The model indicated that with high superfusate Po2, oxygen was supplied to the resting muscle almost exclusively from the superfusate because of the vasoconstriction and reduced blood flow. Oxygen consumption of the resting muscle was estimated to be 0.4 ml O2/100 ml tissue X min, assuming muscle oxygen consumption was uniform and independent of Po2 above 1 mm Hg. The estimated rise in oxygen consumption with exercise was four to eight times resting muscle values, which agrees with previously published data. Also, the model predicted an inlet capillary Po2 of 27 mm Hg with a low superfusate Po2, which is consistent with the few available direct measurements. The model emphasized that with measurement of the Po2 at the superfusate-tissue interface, the complex O2 transport effects of the superfusate can be accurately characterized. Measurement of this and other parameters of the model leads to a potentially useful prediction of the Po2 distribution within tissues under a variety of conditions.

Effect of chloride withdrawal on the geometry of the T-tubules in amphibian and mammalian muscle

The relative chloride permeabilities of the T-tubule membranes in mammalian (rat sternomastoid) and amphibian (toad sartorius) skeletal muscle fibers have been assessed from the change in volume of the T-tubules during chloride withdrawal from fibers exposed to low extracellular chloride concentrations. A 3.5- to 4.2-fold increase in T-tubule volume was found in mammalian fibers, and this was shown to be independent of the composition of the low chloride solution or the nature of the fixative used in preparation for electron microscopy. The increase in T-tubule volume was transient and was inhibited by factors which block chloride conductance, i.e., low pH, 2,4-dichlorophenoxyacetic acid, and nitrate ions. A small increase (1.48-fold) in T-tubule volume was seen in amphibian fibers when chloride ions were replaced by sulphate ions. No increase in volume was observed in amphibian T-tubules when methyl sulphate ions replaced chloride ions. The results support the idea that the chloride permeability of the T-tubule membrane is significantly higher in mammalian fibers than in amphibian fibers.

The contractile properties, histochemistry, ultrastructure and electrophysiology of the cricothyroid and posterior cricoarytenoid muscles in the rat

The contractile, histochemical, morphological and electrophysiological properties of two rat laryngeal muscles, the cricothyroid and posterior cricoarytenoid, have been measured. Both muscles act during respiration to maintain upper airway patency and an even distribution of air in the lungs. The cricothyroid and posterior cricoarytenoid are fast-twitch muscles, having contraction times of 3.4 and 7.2 ms respectively, high myosin ATPase activity, abundant sarcoplasmic reticulum (with average volumes of 9% and 15%, respectively, of the fibre volume) and T-system membrane (with average areas of 0.4 and 0.5 micron 2 micron -3 of fibre). The large areas of T-tubule membrane are reflected in the average specific membrane capacities of 6.5 muF cm-2 to 10.5 muF cm-2, which are high considering the small diameter of the fibres (20-30 micron). Of the two muscles, the posterior cricoarytenoid has the faster contraction time and the more abundant sarcoplasmic reticulum content. In addition, the posterior cricoarytenoid is less resistant to fatigue and demonstrates lower succinic dehydrogenase activity. The fatigability of this muscle, coupled with its general lack of functional reserve, suggest that its failure may contribute to upper airway obstruction during respiratory distress.