Bimodal collagen fibril diameter distributions direct age-related variations in tendon resilience and resistance to rupture

Scaling relationships have been formulated to investigate the influence of collagen fibril diameter (D) on age-related variations in the strain energy density of tendon. Transmission electron microscopy was used to quantify D in tail tendon from 1.7- to 35.3-mo-old (C57BL/6) male mice. Frequency histograms of D for all age groups were modeled as two normally distributed subpopulations with smaller (D(D1)) and larger (D(D2)) mean Ds, respectively. Both D(D1) and D(D2) increase from 1.6 to 4.0 mo but decrease thereafter. From tensile tests to rupture, two strain energy densities were calculated: 1) u(E) [from initial loading until the yield stress (σ(Y))], which contributes primarily to tendon resilience, and 2) u(F) [from σ(Y) through the maximum stress (σ(U)) until rupture], which relates primarily to resistance of the tendons to rupture. As measured by the normalized strain energy densities u(E)/σ(Y) and u(F)/σ(U), both the resilience and resistance to rupture increase with increasing age and peak at 23.0 and 4.0 mo, respectively, before decreasing thereafter. Multiple regression analysis reveals that increases in u(E)/σ(Y) (resilience energy) are associated with decreases in D(D1) and increases in D(D2), whereas u(F)/σ(U) (rupture energy) is associated with increases in D(D1) alone. These findings support a model where age-related variations in tendon resilience and resistance to rupture can be directed by subtle changes in the bimodal distribution of Ds.

Electron microscope 3D reconstruction of branched collagen fibrils in vivo

The ability of tendon to withstand tensile forces is largely attributable to an extracellular matrix containing parallel collagen fibrils organized into fascicles. A major belief is that force is transmitted between collagen fibrils via interactions of molecules at the fibril surface. However, there is existing evidence (reviewed here) for persistent connections between fibrils formed by interfibrillar fusion. Furthermore, in vitro studies have shown the ability of the ends of fibrils to fuse together. In this study, we show using serial section electron microscopy of embryonic mouse-tail tendon further evidence for interfibril fusion in vivo. We showed: (1) fibrils fused via Y-shaped branches without disruption of the 67 nm D-periodicity, (2) the frequency of the branches was approximately 1:20 000 D-periods, and (3) the small angle of the Y ranged from 4 degrees to 10 degrees, indicating a structure-based mechanism of branch formation. The regular occurrence of Y-shaped branches between collagen fibrils suggests direct force transmission between fibrils. Furthermore, the formation of the Y-shaped branches by tip-to-shaft fusion would explain the paucity of fibril tips in vivo.

Ageing changes in the tensile properties of tendons: influence of collagen fibril volume fraction

Connective tissues are biological composites comprising of collagen fibrils embedded in (and reinforcing) the hydrated proteoglycan-rich (PG) gel within the extracellular matrices (ECMs). Age-related changes to the mechanical properties of tissues are often associated with changes to the structure of the ECM, namely, fibril diameter. However, quantitative attempts to correlate fibril diameter to mechanical properties have yielded inconclusive evidence. Here, we described a novel approach that was based on the rule of mixtures for fiber composites to evaluate the dependence of age-related changes in tendon tensile strength (sigma) and stiffness (E) on the collagen fibril cross-sectional area fraction (rho), which is related to the fibril volume fraction. Tail tendons from C57BL6 mice from age groups 1.6-35.3 months old were stretched to failure to determine sigma and E. Parallel measurements of rho as a function of age were made using transmission electron microscopy. Mathematical models (rule of mixtures) of fibrils reinforcing a PG gel in tendons were used to investigate the influence of rho on ageing changes in sigma and E. The magnitudes of sigma, E, and rho increased rapidly from 1.6 months to 4.0 months (P-values <0.05) before reaching a constant (age independent) from 4.0 months to 29.0 months (P-values >0.05); this trend continued for E and rho (P-values >0.05) from 29.0 months to 35.3 months, but not for sigma, which decreased gradually (P-values <0.05). Linear regression analysis revealed that age-related changes in sigma and E correlated positively to rho (P-values <0.05). Collagen fibril cross-sectional area fraction rho is a significant predictor of ageing changes in sigma and E in the tail tendons of C57BL6 mice.

Three-dimensional reconstructions of extracellular matrix polymers using automated electron tomography

The extracellular matrix is an intricate network of macromolecules which provides support for cells and a framework for tissues. The detailed structure and organisation of most matrix polymers is poorly understood. These polymers have a complex ultrastructure, and it has proved a major challenge both to define their structural organisation and to relate this to their biological function. However, new approaches using automated electron tomography are beginning to reveal important insights into the molecular assembly and structural organisation of two of the most abundant polymer systems in the extracellular matrix. We have generated three-dimensional reconstructions of collagen fibrils from bovine cornea and fibrillin microfibrils from ciliary zonules. Analysis of these data has provided new insights into the organisation and function of these large macromolecular assemblies.

Corneal collagen fibril structure in three dimensions: Structural insights into fibril assembly, mechanical properties, and tissue organization

The ability of the cornea to transmit light while being mechanically resilient is directly attributable to the formation of an extracellular matrix containing orthogonal sheets of collagen fibrils. The detailed structure of the fibrils and how this structure underpins the mechanical properties and organization of the cornea is understood poorly. In this study, we used automated electron tomography to study the three-dimensional organization of molecules in corneal collagen fibrils. The reconstructions show that the collagen molecules in the 36-nm diameter collagen fibrils are organized into microfibrils (approximately 4-nm diameter) that are tilted by approximately 15 degrees to the fibril long axis in a right-handed helix. An unexpected finding was that the microfibrils exhibit a constant-tilt angle independent of radial position within the fibril. This feature suggests that microfibrils in concentric layers are not always parallel to each other and cannot retain the same neighbors between layers. Analysis of the lateral structure shows that the microfibrils exhibit regions of order and disorder within the 67-nm axial repeat of collagen fibrils. Furthermore, the microfibrils are ordered at three specific regions of the axial repeat of collagen fibrils that correspond to the N- and C-telopeptides and the d-band of the gap zone. The reconstructions also show macromolecules binding to the fibril surface at sites that correspond precisely to where the microfibrils are most orderly.

Collagen fibril organisation in mammalian vitreous by freeze etch/rotary shadowing electron microscopy

Mammalian vitreous gel contains two major network-forming polymeric systems: long, thin fibrils comprising predominantly type II collagen and a meshwork of hyaluronan. The gel structure is maintained primarily by the collagen component, but little is known about the mechanisms of spacing of the collagen fibrils and of interactions between fibrils to form a stable network. In this study we have applied the technique of freeze etching/rotary shadowing electron microscopy in order to reveal the fibrillar network in central, cortical and basal vitreous and to understand the structural relationship between the collagen fibrils. The fibrils were arranged side by side in narrow bundles that frequently branched to link one bundle to another. Only a minor part of the fibrillar network consisted of segments that had a diameter of a single fibril (16.4nm mean diameter). In addition, three morphologically distinct filamentous structures were observed that appeared to form links within the collagen fibrillar network: short, single interlinking filaments of 7.0nm mean diameter, network-forming filaments of 6.7nm mean diameter, and longer filaments of 8.2nm mean diameter. All three types of filamentous structure were removed by digestion of the vitreous gels with Streptomyces hyaluronan lyase prior to freeze etching, indicating that these structures contain or are stabilised by hyaluronan. These filamentous structures may contribute to the structural stability of the vitreous gel.

STEM/TEM studies of collagen fibril assembly

Quantitative scanning transmission electron microscopy (STEM), implemented on a conventional transmission electron microscope with STEM-attachment, has been a primary tool in our laboratory for the quantitative analysis of collagen fibril assembly in vivo and in vitro. Using this technique, a precise measurement of mass per unit length can be made at regular intervals along a fibril to generate an axial mass distribution (AMD). This in turn allows the number of collagen molecules to be calculated for every transverse section of the fibril along its entire length. All fibrils show a near-linear AMD in their tip regions. Only fibrils formed in tissue environments, however, show a characteristic abrupt change in mass slope along their tips. It appears that this tip growth characteristic is common to fibrils from evolutionarily diverse systems including vertebrate tendon and the mutable tissues of the echinoderms. Computer models of collagen fibril assembly have now been developed based on interpretation of the STEM data. Two alternative models have so far been generated for fibril growth by accretion; one is based on diffusion limited aggregation (DLA) and the other based on an interface-limited growth mechanism. Inter-fibrillar fusion can also contribute to the growth of fibrils in vertebrate tissues and STEM data indicates the presence of a tight regulation in this process. These models are fundamental for the hypotheses regarding how cells synthesise and spatially organise an extracellular matrix (ECM), rich in collagen fibrils.

Axial structure of the heterotypic collagen fibrils of vitreous humour and cartilage

We have compared the axial structures of negatively stained heterotypic, type II collagen-containing fibrils with computer-generated staining patterns. Theoretical negative-staining patterns were created based upon the "bulkiness" of the individual amino acid side-chains in the primary sequence and the D-staggered arrangement of the triple-helices. The theoretical staining pattern of type II collagen was compared and cross-correlated with the experimental staining pattern of both reconstituted type II collagen fibrils, and fibrils isolated from adult and foetal cartilage and vitreous humour. The isolated fibrils differ markedly in both diameter and composition. Correlations were significantly improved when a degree of theoretical hydroxylysine glycosylation was applied, showing for the first time that this type of glycosylation influences the negative-staining pattern of collagen fibrils. Increased correlations were obtained when contributions from types V/XI and IX collagen were included in the simulation model. The N-propeptide of collagen type V/XI and the NC2 domain of type IX collagen both contribute to prominent stain-excluding peaks in the gap region. With decreasing fibril diameter, an increase of these two peaks was observed. Simulations of the fibril-derived staining patterns with theoretical patterns composed of proportions of types II, V/XI and IX collagen confirmed that the thinnest fibrils (i.e. vitreous humour collagen fibrils) have the highest minor collagen content. Comparison of the staining patterns showed that the organisation of collagen molecules within vitreous humour and cartilage fibrils is identical. The simulation model for vitreous humour, however, did not account for all stain-excluding mass observed in the staining pattern; this additional mass may be accounted for by collagen-associated macromolecules.

Tip-mediated fusion involving unipolar collagen fibrils accounts for rapid fibril elongation, the occurrence of fibrillar branched networks in skin and the paucity of collagen fibril ends in vertebrates

Collagen fibrils are the principal source of mechanical strength of connective tissues such as tendon, skin, cornea, cartilage and bone. The ability of these tissues to withstand tensile forces is directly attributable to the length and diameter of the fibrils, and to interactions between individual fibrils. Although electron microscopy studies have provided information on fibril diameters, little is known about the length of fibrils in tissue and how fibrils interact with each other. The question of fibril length has been difficult to address because fibril ends are rarely observed in cross-sections of tissue. The paucity of fibril ends, or tips, has led to controversy about how long individual fibrils might be and how the fibrils grow in length and diameter. This review describes recent discoveries that are relevant to these questions. We now know that vertebrate collagen fibrils are synthesised as short (1-3 microm) early fibrils that fuse end-to-end in young tissues to generate very long fibrils. The diameter of the final fibril is determined by the diameter of the collagen early fibrils. During a late stage of tissue assembly fibril tips fuse to fibril shafts to generate branched networks. Of direct relevance to fibril fusion is the fact that collagen fibrils can be unipolar or bipolar, depending on the orientation of collagen molecules in the fibril. Fusion relies on: (1) specific molecular interactions at the carboxyl terminal ends of unipolar collagen fibrils; and (2) the insulator function of small proteoglycans to shield the surfaces of fibrils from inappropriate fusion reactions. The fusion of tips to shafts to produce branched networks of collagen fibrils is an elegant mechanism to increase the mechanical strength of tissues and provides an explanation for the paucity of fibril tips in older tissue.

Echinoderm collagen fibrils grow by surface-nucleation-and-propagation from both centers and ends

Collagen fibrils from sea cucumber (class Holothuroidea) dermis were previously found to grow by coordinated monomer addition at both centers and ends. This analysis of sea urchin (class Echinoidea) collagen fibrils was undertaken to compare the growth characteristics of fibrils from two classes of echinoderms, and to determine whether a single growth model could account for the main features of fibrils from these two taxa. Native collagen fibrils (37-431 micrometer long) from the spine ligaments of the sea urchin Eucidaris tribuloides were studied by scanning transmission electron microscopy and image analysis. The analyses revealed the mass per unit length, and hence the number of molecules in cross-section, along the entire length of each fibril. The fibrils were symmetrically spindle shaped. The maximum mass per unit length occurred in the center of each fibril, where the fibril contains anti-parallel molecules in equal numbers. The two pointed tips of each fibril showed similar linear axial mass distributions, indicating that the two tips retain shape and size similarity throughout growth. The linear axial mass distributions showed that the tips were paraboloidal, similar to those of vertebrate and sea cucumber fibrils. The computed maximum diameters of the fibrils increased linearly with fibril length. The overall shapes of the fibrils showed that they retain geometric similarity throughout growth. Computer modeling showed that the simplest self-assembly mechanism that can account for the features of these fibrils, and of the sea cucumber fibrils that have been described, is one in which the fibril tips produce independent axial growth, while lateral growth takes place through a surface nucleation and propagation mechanism. This mechanism produces coordinated growth in length and diameter as well as geometric similarity, characteristic features of echinoderm collagen fibrils.

Identification of collagen fibril fusion during vertebrate tendon morphogenesis. The process relies on unipolar fibrils and is regulated by collagen-proteoglycan interaction

The synthesis of an extracellular matrix containing long (approximately mm in length) collagen fibrils is fundamental to the normal morphogenesis of animal tissues. In this study we have direct evidence that fibroblasts synthesise transient early fibril intermediates (approximately 1 micrometer in length) that interact by tip-to-tip fusion to generate long fibrils seen in older tissues. Examination of early collagen fibrils from tendon showed that two types of early fibrils occur: unipolar fibrils (with carboxyl (C) and amino (N) ends) and bipolar fibrils (with two N-ends). End-to-end fusion requires the C-end of a unipolar fibril. Proteoglycans coated the shafts of the fibrils but not the tips. In the absence of proteoglycans the fibrils aggregated by side-to-side interactions. Therefore, proteoglycans promote tip-to-tip fusion and inhibit side-to-side fusion. This distribution of proteoglycan along the fibril required co-assembly of collagen and proteoglycan prior to fibril assembly. The study showed that collagen fibrillogenesis is a hierarchical process that depends on the unique structure of unipolar fibrils and a novel function of proteoglycans.

Growth of sea cucumber collagen fibrils occurs at the tips and centers in a coordinated manner

Collagen fibrils are the principle source of mechanical strength in the mutable dermis of the sea cucumber Cucumaria frondosa. To obtain information about the mechanism by which collagen molecules self-assemble into fibrils, we have isolated single intact fibrils with lengths in the range 14-444 microm. These fibrils have been studied by scanning transmission electron microscopy, yielding data that show how cross-sectional mass, and hence the number of molecules in the cross-section, depend on axial location. In an individual fibril, the two ends always display similar mass distributions. The two tips of each fibril must therefore maintain identity in shape and size throughout growth. The linear relationship between cross-sectional mass and distance from the adjacent end shows that a growing tip is (like the tip of a vertebrate collagen fibril) paraboloidal in shape. Comparison of data from many different fibrils, over a wide range of lengths, however, revealed that the paraboloidal tip becomes blunter as the fibril grows in length. In contrast to vertebrate fibrils, those from C. frondosa do not have a central shaft region of constant cross-sectional mass. Rather, the cross-sectional mass increases to a maximum in the center of each fibril. The maximum cross-sectional mass of the fibrils increases exponentially with increasing fibril length. The centrosymmetry, the paraboloidal shape of the tips, and the hyperbolic increase in maximum cross-sectional mass with fibril length, is evidence for a co-ordinated regulation of length and diameter, which differs from the kind of regulation that gives rise to collagen fibrils in vertebrates (chickens and mice).

Collagen fibrils forming in developing tendon show an early and abrupt limitation in diameter at the growing tips

The formation of very long and near-uniform diameter collagen fibrils is fundamental to the assembly of the extracellular matrix of animals. However, how growth in length and diameter is regulated, and how fibrils increase in diameter during development, are poorly understood. The approach in this study was to examine the tips and central shaft regions of fibrils from 12 and 18-day embryonic chick metatarsal tendon using quantitative mass mapping electron microscopy. We found that the fibrils had smoothly tapered C and N-terminal tips, which had linear axial mass distributions and were consequently parabolic in shape. An invariant feature of all tips (N and C) was an abrupt stop in lateral growth leading to a local plateau in diameter. The distance from the end of the fibril to the abrupt stop occurred at multiples of five D-periods (where D=67 nm). This implies that D-periods at the ends of fibrils are not equivalent sites for accretion, and that diameter regulation relies on surface structural features, which repeat every 5D. Mass mapping of entire fibrils at day 12 showed that, on average, the coarseness of the fibril tips was independent of fibril length, consistent with individual fibrils growing at constant tip shape. Comparison of diameters in the plateau (close to the tips) and shaft regions of the fibril showed that fibrils in day 12 tendons grow in length at constant diameter. Analysis of tendons from day 18 embryos showed that the increase in diameter at this stage of development was the result of both increases in the coarseness of the tips and continued lateral accretion of mass onto the central shafts at distances away from the growing tips. Regulated tip growth provides an attractive explanation for how cells are able to synthesise very long fibrils during the organisation of the extracellular matrix.

Surface located procollagen N-propeptides on dermatosparactic collagen fibrils are not cleaved by procollagen N-proteinase and do not inhibit binding of decorin to the fibril surface

Dermatosparaxis is a recessive disorder of animals (including man) which is caused by mutations in the gene for the enzyme procollagen N-proteinase and is characterised by extreme skin fragility. Partial loss of enzyme activity results in accumulation of pNcollagen (collagen with N-propeptides) and abnormal collagen fibrils in the fragile skin. How the N-propeptides persist in the tissue and how abnormal fibril morphology results in fragile skin is poorly understood. Using biochemical and quantitative mass mapping electron microscopy we showed that the collagen fibrils in the skin of a dermatosparactic calf contained 57% type I pNcollagen and 43% type I collagen and the fibrils were irregularly arranged in bundles and hieroglyphic in cross-section. Image analysis of the fibril cross-sections suggested that the deviation from circularity of dermatosparactic fibrils was caused by N-propeptides of pNcollagen being located at the fibril surface. Comparison of experimental and theoretical axial mass distributions of the fibrils showed that the N-propeptides were located to the overlap zone of the fibril D-period (where D=67 nm, the characteristic axial periodicity of collagen fibrils). Treatment of the dermatosparactic fibrils with N-proteinase did not remove the N-propeptides from the fibrils, although the N-propeptides were efficiently removed by trypsin and chymotrypsin. However, the N-propeptides were efficiently cleaved by the N-proteinase when the pNcollagen molecules were extracted from the fibrils. These results are consistent with close packing of N-propeptides at the fibril surface which prevented cleavage by the N-proteinase. Long-range axial mass determination along the fibril length showed gross non-uniformity with multiple mass bulges. Of note is the skin fragility in dermatosparaxis, and also the appearance of mass bulges along the fibril long axis symptomatic of the fragile skin of mice which lack decorin. Western blot analysis showed that the dermatosparactic fibrils bound elevated levels of the proteoglycan, compared with normal skin fibrils. The results showed that N-propeptides can distort the morphology of fibrils, that they do not inhibit binding of gap-associated macromolecules (such as decorin) and that the normal mechanical properties of skin are strongly dependent on the close association of near-cylindrical fibrils, thereby enabling maximal fibril-fibril interactions.

Scanning transmission electron microscopy mass analysis of fibrillin-containing microfibrils from foetal elastic tissues

We have applied scanning transmission electron microscopy to intact native fibrillin-containing microfibrils isolated from foetal bovine elastic tissues in order to derive new insights into microfibril organisation. This technique provides quantitative data on the mass per unit length and axial mass distribution of unstained, unshadowed macromolecules. Scanning transmission electron microscopy of microfibrils from aorta, skin and nuchal ligament revealed that the beads corresponded to peaks of mass and the interbead regions to troughs of mass. These major features of axial mass distribution were characteristic of all microfibrils examined. Tissue-specific and age-dependent variations in mass were identified in microfibrils that were structurally comparable by rotary shadowing electron microscopy. Increased microfibril mass correlated with increasing gestational age. The additional mass was associated predominantly at, or close to, the bead. Some microfibril populations exhibited pronounced assymetry in their axial mass distribution. These data indicate that intact native microfibrillar assemblies from developing elastic tissues are heterogeneous in composition. Loss of mass following chondroitinase ABC or AC lyase treatment confirmed the presence of chondroitin sulphate in nuchal ligament microfibrillar assemblies.

Targeted disruption of decorin leads to abnormal collagen fibril morphology and skin fragility

Decorin is a member of the expanding group of widely distributed small leucine-rich proteoglycans that are expected to play important functions in tissue assembly. We report that mice harboring a targeted disruption of the decorin gene are viable but have fragile skin with markedly reduced tensile strength. Ultrastructural analysis revealed abnormal collagen morphology in skin and tendon, with coarser and irregular fiber outlines. Quantitative scanning transmission EM of individual collagen fibrils showed abrupt increases and decreases in mass along their axes. thereby accounting for the irregular outlines and size variability observed in cross-sections. The data indicate uncontrolled lateral fusion of collagen fibrils in the decorindeficient mice and provide an explanation for the reduced tensile strength of the skin. These findings demonstrate a fundamental role for decorin in regulating collagen fiber formation in vivo.

Enzymic control of collagen fibril shape

The shape of collagen fibrils growing in vitro in a cell-free enzyme/substrate system is shown to be dependent on the enzyme/substrate (E/S) ratio. Long fibrils with tapered ends were generated by exposing pCcollagen (procollagen from which the N-propeptides had been removed) to procollagen C-proteinase (which acts by cleaving the C-propeptides from the pCcollagen, converting it to insoluble fibril-forming collagen). Tip shape profiles, established quantitatively by scanning transmission electron microscopy, depended critically on the C-proteinase/pCcollagen ratio. The finest tips occurred at low ratios, the coarsest at high ratios. All fibrils had molecules oriented with amino termini closest to the pointed ends, i.e. N,N-bipolar fibrils in which molecules change orientation abruptly at one location along the fibril. Fibrils had maximal diameter at this molecular switch region. Shape asymmetric fibrils occurred at low E/S ratios, near-shape symmetric fibrils occurred at high ratios. Fibrils generated at low E/S ratios bore the closest resemblance to those formed in vivo except that the central shaft regions of fibrils formed in vitro showed no tendency to be limited to a uniform diameter.

Assembly in vitro of thin and thick fibrils of collagen II from recombinant procollagen II. The monomers in the tips of thick fibrils have the opposite orientation from monomers in the growing tips of collagen I fibrils

Human type II procollagen was prepared in a recombinant system and cleaved to pC-collagen II by procollagen N-proteinase. The pC-collagen II was then used as a substrate to generate collagen II fibrils by cleavage with procollagen C-proteinase at 37 degrees C. Electron microscopy of the fibrils demonstrated that, at the early stages of fibril assembly, very thin fibrils were formed. As the system approached equilibrium over 7-12 h, however, the thin fibrils were largely but not completely replaced by thick fibrils that had diameters of about 240 nm and a distinct D-period banding pattern. One typical fibril was photographed and analyzed in its entirety. The fibril was 776 D-periods (52 microM) long. It had a central shaft with a uniform diameter that was about 516 D-periods long and two tips of about 100 D-periods each. Most of the central shaft had a symmetrical banding pattern flanked by two transition regions of about 30 D-periods each. Measurements by scanning transmission electron microscopy demonstrated that the mass per unit length from the tips to the shafts increased linearly over approximately 100 D-periods from the fibril end. The linear increase in mass per unit length was consistent with previous observations for collagen I fibrils and established that the tips of collagen II also had a near paraboloidal shape. However, the orientation of monomers in the tips differed from the tips of collagen I fibrils in that the C termini instead of the N termini were directed toward the tips. The thin fibrils that were present at early stages of assembly and at equilibrium were comparable to the collagen II fibrils seen in embryonic tissues and probably represented intermediates on the pathway of thick fibrils formation. The results indicated that the molecular events in the self-assembly of collagen II fibrils are apparently similar to those in self-assembly of collagen I fibrils, but that there are also important differences in the structural information contained in collagen I and collagen II monomers.

Collagen fibril formation

Collagen is most abundant in animal tissues as very long fibrils with a characteristic axial periodic structure. The fibrils provide the major biomechanical scaffold for cell attachment and anchorage of macromolecules, allowing the shape and form of tissues to be defined and maintained. How the fibrils are formed from their monomeric precursors is the primary concern of this review. Collagen fibril formation is basically a self-assembly process (i.e. one which is to a large extent determined by the intrinsic properties of the collagen molecules themselves) but it is also sensitive to cell-mediated regulation, particularly in young or healing tissues. Recent attention has been focused on "early fibrils' or "fibril segments' of approximately 10 microns in length which appear to be intermediates in the formation of mature fibrils that can grow to be hundreds of micrometers in length. Data from several laboratories indicate that these early fibrils can be unipolar (with all molecules pointing in the same direction) or bipolar (in which the orientation of collagen molecules reverses at a single location along the fibril). The occurrence of such early fibrils has major implications for tissue morphogenesis and repair. In this article we review the current understanding of the origin of unipolar and bipolar fibrils, and how mature fibrils are assembled from early fibrils. We include preliminary evidence from invertebrates which suggests that the principles for bipolar fibril assembly were established at least 500 million years ago.

Implications of the EC Manual Handling of Loads Directive for nurse managers

The European Community has introduced new regulations designed to stop people being injured when moving loads at work. These rules came into effect on 1 January 1993. So far, however, many managers have done nothing about them. If nurses are injured, their employers may be liable to pay large sums in compensation. Managers must start to take these issues seriously.

Vertebrate (chick) collagen fibrils formed in vivo can exhibit a reversal in molecular polarity

A reversal in molecular polarity can occur in vertebrate collagen fibrils. This has been demonstrated using a method for isolating, from chick embryo tendon, entire collagen fibrils 2 to 14 microns in length and suitable for electron-optical examination. A polarity reversal is present in some, but not all, of these fibrils. Such fibrils have two N-ends. The transition region, occupying several D-periods in which the reversal occurs, is not restricted to a central location in a fibril. Analysis of the fibril banding pattern through the transition region shows that the relative axial alignment of antiparallel molecules brings oppositely-directed C-telopeptides into axial register. This could allow antiparallel molecules to be covalently linked via polymeric cross-links involving these C-telopeptides.

Ehlers-Danlos syndrome type VIIB. Morphology of type I collagen fibrils formed in vivo and in vitro is determined by the conformation of the retained N-propeptide

Previously we showed that fibrils generated from collagen and pNcollagen-ex6 from fibroblasts of an individual with Ehlers-Danlos syndrome (EDS) type VIIB were hieroglyphic in cross-section and all N-propeptides were located at the fibril surface. Hieroglyphs were resolved to near-cylindrical fibrils (that were similar in appearance to the fibrils seen in the tissues of individuals with EDS type VIIB) by treatment with N-proteinase which cleaved the pN alpha 1(I) chains but not the pN alpha 2(I)-ex6 chains (Watson, R. B., Wallis, G. A., Holmes, D. F., Viljoen, D., Byers, P. H., and Kadler, K. E. (1992) J. Biol. Chem. 267, 9093-9100). Here, quantitative scanning transmission electron microscopy (STEM) showed that N-propeptides in hieroglyphs were in a "bent-back" conformation and thus located exclusively in the overlap zone of the fibril D-period (D = 67 nm). In contrast, STEM of fibrils from the dermis of an individual with EDS type VIIB showed that partially cleaved N-propeptides (in which cleaved pN alpha 1(I) remained in noncovalent association with pN alpha 2(I)-ex6 chains) were distributed equally between the gap and overlap zones of the fibrils. Comparison of experimental data with theoretical mass distributions of the fibril based on amino acid sequence data gave a consistent value of 33 nm for the total axial extent for the N-propeptides in hieroglyphic and tissue fibrils irrespective of the location of N-propeptides to the gap or overlap zone. These data exclude the possibility that N-propeptides adopt a random configuration, but rather, that they locate to specific sites in the gap and overlap zones. The results demonstrated that cleavage of pN alpha 1(I) chains in vivo releases the N-propeptides from the constraints of the bent-back conformation. Co-distribution of partially cleaved N-propeptides between gap and overlap zones allows a higher surface packing density of N-propeptides and explains how circularity of large diameter fibrils can be achieved despite the retention of N-propeptides in tissues of individuals with EDS type VIIB.

Type I procollagens containing substitutions of aspartate, arginine, and cysteine for glycine in the pro alpha 1 (I) chain are cleaved slowly by N-proteinase, but only the cysteine substitution introduces a kink in the molecule

Type I procollagen was purified from the medium of dermal fibroblasts cultured from four individuals with osteogenesis imperfecta (OI) type II who had mutations in the COL1A1 gene of type I procollagen. The procollagens were mixtures of normal molecules and molecules that contained substitutions of aspartate for glycine 97, arginine for glycine 550, cysteine for glycine 718, and aspartate for glycine 883 in one or both of the alpha 1 (I) chains of the molecule. The procollagens were cleaved more slowly than control type I procollagen by procollagen N-proteinase. Double-reciprocal plots of initial relative velocities and initial substrate concentrations indicated that the OI procollagens were all cleaved slowly by N-proteinase because of decreased Vmax, rather than increased Km. This suggested that slow cleavage of the OI procollagens by N-proteinase was the result of slow conversion of the N-proteinase-procollagen complex. Further experiments showed that the vertebrate collagenase A fragment of the aspartate for glycine alpha 1(I) 883 OI procollagen that contained the N-proteinase cleavage site but not the site of the substitution was also cleaved more slowly by N-proteinase than the normal vertebrate collagenase A fragments in the samples. These data show, for the first time, that an altered triple-helical structure is propagated from the site of a substitution of a bulky residue for glycine to the amino-terminal end of the procollagen molecule and disrupts the conformation of the N-proteinase cleavage site. Rotary shadowing electron microscopy of molecules in the preparation of cysteine for glycine alpha 1(I)-718 showed the presence of a kink in approximately 5% of a population of molecules in which 60% were abnormal and 20% contained a disulfide bond. In contrast, procollagens containing aspartate and arginine for glycine were indistinguishable by rotary shadowing electron microscopy from those in control samples. The results here confirm previous suggestions that substitution of cysteine for glycine in the alpha 1(I) chain of type I collagen can introduce a kink near the site of the substitution. However, the presence of a kink is not a prerequisite for delayed cleavage of abnormal procollagens by N-proteinase.

Growing tips of type I collagen fibrils formed in vitro are near-paraboloidal in shape, implying a reciprocal relationship between accretion and diameter

Collagen fibrils generated in vitro at 37 degrees C by enzymic removal of C-terminal propeptides from type I pC-collagen (an intermediate in the normal processing of type I procollagen to collagen containing the C-terminal propeptides but not the N-terminal propeptides) display shape polarity, with one tip fine tapered and the other coarse tapered. Mass measurements by scanning transmission electron microscopy show that the mass per unit length along both kinds of tip increases roughly linearly over distances of approximately 100 D periods from the fibril end [D (axial periodicity) = 67 nm]. The fine tips of fibrils of widely differing lengths exhibit near-identical mass distributions, the mass in all cases increasing at the rate of approximately 17 molecules per D period, irrespective of fibril length. Coarse tips display less regular behavior. These results show that (i) the shape of a fine tip is not conical but resembles more closely a paraboloid of revolution, and (ii) for this shape to be maintained throughout growth, accretion (rate of mass uptake per unit area) cannot everywhere be the same on the surface of the tip but must decrease as the diameter increases. To a first approximation, accretion alpha (diameter)-1.

Ehlers Danlos syndrome type VIIB. Incomplete cleavage of abnormal type I procollagen by N-proteinase in vitro results in the formation of copolymers of collagen and partially cleaved pNcollagen that are near circular in cross-section

We have shown that a child with Ehlers Danlos syndrome (EDS) type VII has a G to A transition at the first nucleotide of intron 6 in one of her COL1A2 alleles. Half of the cDNA clones prepared from the proband's pro alpha 2(I) mRNA lacked exon 6. The type I procollagen secreted by the proband's dermal fibroblasts in culture was purified, and collagen fibrils were generated in vitro by cleavage of the procollagen with the procollagen N- and C-proteinases. Incubation of the procollagen with N-proteinase resulted in a 1:1 mixture of pCcollagen and uncleaved procollagen. Incubation of this mixture with C-proteinase generated collagen and abnormal pNcollagen (pNcollagen-ex6) that readily copolymerized into fibrils. By electron microscopy these fibrils resembled the hieroglyphic fibrils seen in the N-proteinase-deficient skin of dermatosparactic animals and humans and were distinct from the near circular cross-section fibrils seen in the tissues of individuals with EDS type VII. Further incubation of the hieroglyphic fibrils with N-proteinase resulted in partial cleavage of the pNcollagen-ex6 in which the abnormal pN alpha 2(I) chains remained intact. These fibrils were not hieroglyphic but were near circular in cross-section. Fibrils formed from collagen and pNcollagen-ex6 that had been partially cleaved with elevated amounts of N-proteinase prior to fibril formation were also near circular in cross-section. The results are consistent with a model of collagen fibril formation in which the intact N-propeptides are located exclusively at the surface of the hieroglyphic fibrils. Partial cleavage of the pNcollagen-ex6 by N-proteinase allows the N-propeptides to be incorporated within the body of the fibrils. The model provides an explanation for the morphology and molecular composition of collagen fibrils in the tissues of patients with EDS type VII.

Morphology of sheet-like assemblies of pN-collagen, pC-collagen and procollagen studied by scanning transmission electron microscopy mass measurements

At high concentrations, type I pN-collagen, pC-collagen and procollagen (the first 2 generated from procollagen by enzymic cleavage of C-propeptides and N-propeptides, respectively) can all be made to assemble in vitro into thin D-periodic sheets or tapes. Scanning transmission electron microscopy mass measurements show that the pN-collagen sheets and procollagen tapes have a mass per unit area corresponding to that of approximately 6.8 monolayers of close-packed molecules. pN-collagen sheets are extensive and remarkably uniform in mass thickness (fractional S.D. 0.035); procollagen tapes are neither as extensive nor as uniform in thickness. The mean thickness of pC-collagen tapes is less and the variability is greater. In pN-collagen sheets, the overlap: gap mass contrast in a D-period is increased from 5:4 (the ratio in a native collagen fibril) to 6:4, showing that the N-propeptides do not project into the gap but are folded back over the overlap zone. Assuming all N-propeptides to be constrained to the two surfaces of a sheet, their surface density can be found from the mass thickness of the sheet. In a lateral direction (i.e. normal to the axial direction where the spacing is D-periodic), the N-propeptide domains are calculated to be spaced, centre to centre, by 2.23 (+/- 0.1) nm on both surfaces. This value (approx. 1.5 x the triple-helix diameter) implies close-packing laterally with adjacent domains in contact. Sheet formation and the "surface-seeking" behaviour of propeptides can be understood in terms of the dual character of the molecules, evident from solubility data, with propeptides possessing interaction properties very different from those displayed by the rest of the molecule. The form and stability of sheets (and of first-formed fibrils assembling in vivo) could, it is suggested, depend on the partially fluid-like nature of lateral contacts between collagen molecules.

D-periodic assemblies of type I procollagen

The solubility limit of purified chick type I procollagen, incubated at 37 degrees C in phosphate-buffered saline, was found to be in the range 1 to 1.5 mg/ml. At higher concentrations large aggregates formed. These comprised: (1) D-periodic assemblies; (2) narrow filaments with no apparent periodicity; and (3) segment-long-spacing-like aggregates. The D-periodic assemblies, which predominated at high concentrations, were separated from the other types of aggregate and found to be ribbon-like. Ribbons were uniform in thickness (approximately 8 nm) and up to 1 micron wide. Staining patterns showed features similar to those in native-type collagen fibrils. Immunolabelling indicated that the carboxyl-terminal propeptide domains were close to the carboxyl-terminal gap-overlap junction, and that the amino-terminal propeptide domains were folded over into the amino-terminal side of the overlap zone. Both propeptide domains appeared to be located on the surface of the assemblies. These observations show that intact propeptide domains hinder, but do not prevent, the formation of D-periodic assemblies. The presence of the propeptide domains on the surface of a growing assembly could restrict its lateral growth and limit its final thickness.

Pleomorphism in type I collagen fibrils produced by persistence of the procollagen N-propeptide

The assembly of type I collagen and type I pN-collagen was studied in vitro using a system for generating these molecules enzymatically from their immediate biosynthetic precursors. Collagen generated by C-proteinase digestion of pC-collagen formed D-periodically banded fibrils that were essentially cylindrical (i.e. circular in cross-section). In contrast, pN-collagen generated by C-proteinase digestion of procollagen formed thin, sheet-like structures that were axially D-periodic in longitudinal section, of varying lateral widths (up to several microns) and uniform in thickness (approximately 8 nm). Mixtures of collagen and pN-collagen assembled to form a variety of pleomorphic fibrils. With increasing pN-collagen content, fibril cross-sections were progressively distorted from circular to lobulated to thin and branched structures. Some of these structures were similar to fibrils observed in certain heritable disorders of connective tissue where N-terminal procollagen processing is defective. The observations are considered in terms of the hypothesis that the N-propeptides are preferentially located on the surface of a growing assembly. The implications for normal diameter control of collagen fibrils in vivo are discussed.

Quantitation of mucus glycoproteins blotted onto nitrocellulose membranes

A sensitive assay for mucus glycoproteins (mucins) and fragments thereof is presented. The macromolecules are blotted onto nitrocellulose membranes and visualized using a periodate-Schiff (PAS) reaction and the color yield quantitated with an image analysis system used as a reflectance densitometer. At least 50 ng of the macromolecules was detected. "Whole" mucins and subunits were assayed on 0.2-micron pore size nitrocellulose membranes whereas immobilization of the high-molecular-weight mucin glycopeptides (Mr 300-500,000) required pretreatment of membranes with poly-L-lysine. Binding of the glycopeptides to the polylysine-treated membranes was found to decrease with increasing salt concentration suggesting an electrostatic interaction. The data obtained with this method and a solution PAS assay are in good agreement but the former is more sensitive and can be performed on samples dissolved in chaotropic solvents.

Human diploid cell rabies vaccine purified by zonal centrifugation: a controlled study of antibody response and side effects following primary and booster pre-exposure immunizations

Systemic allergic reactions following booster immunizations have complicated rabies pre-exposure prophylaxis with the human diploid cell rabies vaccine licensed in the US (conventional HDCV). We conducted two studies comparing an HDCV purified by zonal centrifugation to conventional HDCV. In a study of primary pre-exposure immunization, volunteers received one of four regimens: three 1.0-ml intramuscular (i.m.) or 0.1-ml intradermal (i.d.) doses of conventional or purified HDCV over 28 days. Although volunteers vaccinated i.m. had significantly greater rabies neutralizing antibody titres (VNA) 49 days, 91 days and 26 months after immunization began than volunteers vaccinated i.d. (p less than 0.005-p less than 0.05), there were no significant differences between vaccines. In a study of booster immunizations, 77 volunteers immunized with conventional HDCV 2 years earlier received a 0.1-ml i.d. booster with either conventional or purified HDCV. VNA was significantly greater with the conventional HDCV on days 7 and 28 after booster, but not on day 365. A moderate or severe reaction was reported by 5 (13%) of the 40 persons who received boosters with conventional HDCV, versus none of 37 who received the purified HDCV (p = 0.03). Purified HDCV appears to be preferable to conventional HDCV for booster vaccination.

Body temperature and behaviour of mares during the last two weeks of pregnancy

Average daily core body temperature and behavioural patterns of pregnant mares were studied, in search of definitive signs of parturition within 24 h of the event. Nineteen pony mares were sampled twice daily for core body temperature. A significant temperature drop, averaging 0.1 degrees C (0.2 degrees F) was observed during the day prior to parturition. Between 18.00 h and 06.00 h, during the two weeks before parturition, Thoroughbred and Standardbred mares (n = 52) spent an average 66.8 per cent of their time standing, 27.0 per cent eating, 4.9 per cent lying in sternal recumbency, 1.0 per cent lying in lateral recumbency, and 0.3 per cent walking. On the night before parturition, mares spent significantly less time lying in sternal recumbency than on previous nights and on the night of parturition all behaviour patterns except eating were significantly different from the nights of the two weeks before parturition. There was an increase in walking (5.3 per cent), lying in sternal recumbency (8 per cent) and lying in lateral recumbency (5.3 per cent) whereas standing (53.3 per cent) was decreased. In 58 observed pregnancies, 54 mares (97 per cent) foaled in a recumbent position and 50 mares (86 per cent) foaled between 18.00 h and 06.00 h.

Rabies preexposure prophylaxis with human diploid cell rabies vaccine: a dose-response study

Recent studies suggest that human diploid cell rabies vaccine (HDCV) may not always produce acceptable titers after intradermal (id) preexposure prophylaxis. To stimulate accidental deviation from the recommended route of administration and to determine the immunogenicity of smaller-than-recommended doses of HDCV, we injected each of 154 persons either intramuscularly (im) with 100%, 10%, or 3% of the standard im dose of vaccine or id with 10%, 3% or 1% of the standard im dose. Seroconversion (titers of antibody greater than or equal to 1:11) was found in all subjects at 49 and 90 days after vaccination. Titers were higher for subjects receiving 100% of the recommended dose im than for those receiving 10% of this dose id (P less than .01); these titers in turn were higher than those from persons receiving smaller doses (P less than .05). Persons receiving 10% or 3% of the standard im dose had lower titers on day 49 than did those receiving the same dose id (P less than .05).

Response of dairy calves to vaccinia viruses that express foreign genes

Repeated intradermal inoculations of calves with wild-type vaccinia virus and recombinant vaccinia viruses expressing human hepatitis B virus surface antigen and herpes simplex virus, type 1, glycoprotein D produced characteristic pox lesions at each site of injection. In some instances, calves were inoculated as many as five times at intervals from 4 to 7 weeks. The lesions invariably were more severe after the second inoculation. Subsequent inoculations produced a less severe area of redness, swelling, necrosis, and scab formation. No other signs of illness, such as an elevation in temperature, were noted in the calves. Vaccinia virus was isolated in low titers from scabs taken at various times after inoculation. No lesions were formed at the sites injected with tissue culture fluid and cellular debris at the same time that virus inoculations were made. Calf contact controls remained normal through the 8-week exposure in isolation units with calves inoculated twice with vaccinia virus. No neutralizing antibody to vaccinia virus was detected in the contact controls. In contrast, the virus-inoculated calves developed neutralizing antibody to vaccinia virus and to herpes simplex virus glycoprotein D in serum. In all cattle, a second inoculation significantly enhanced the neutralizing antibody response within 1 week, suggesting that an anamnestic response had occurred. No antibody to hepatitis B virus surface antigen was elicited in calves after repeated inoculations with vaccinia recombinants that express hepatitis B virus surface antigen and are known to elicit in rabbits antibodies reactive with hepatitis B virus surface antigen.

Crystalline regions in collagen fibrils

A new image processing technique, content-dependent anisotropic spatial frequency filtering, has been developed to visualize the location and orientation of crystalline regions in collagen fibril cross-sections. The results show that most crystalline regions are oriented with their approximately 4 nm periodicity directed radially from the fibril centre. This periodicity corresponds to the separation between rows of molecular ends in the quasi-hexagonal molecular packing scheme. The extent of crystallinity increases with radius and frequently the lattice is either continuously distorted or interrupted by sharp discontinuities.

Type X collagen, a product of hypertrophic chondrocytes

The synthesis of collagen types IX and X by explants of chick-embryo cartilages was investigated. When sternal cartilage labelled for 24h with [3H]proline was extracted with 4M-guanidinium chloride, up to 20% of the 3H-labelled collagen laid down in the tissue could be accounted for by the low-Mr collagenous polypeptides (H and J chains) of type IX collagen; but no type X collagen could be detected. Explants of tibiotarsal and femoral cartilages were found to synthesize type IX collagen mainly in zones 1 and 2 of chondrocyte proliferation and elongation, whereas type X collagen was shown to be a product of the hypertrophic chondrocytes in zone 3. Pulse-chase experiments with tibiotarsal (zone-3) explants demonstrated a time-dependent conversion of type X procollagen into a smaller species whose polypeptides were of Mr 49 000. The processed chains [alpha 1(X) chains] were shown by peptide mapping techniques to share a common identity with the pro alpha 1(X) chains of Mr 59 000. No evidence for processing of type IX collagen was obtained in analogous pulse-chase experiments with sternal tissue. When chondrocytes from tibiotarsal cartilage (zone 3) were cultured on plastic under standard conditions for 4-10 weeks they released large amounts of type X procollagen into the medium. However, 2M-MgCl2 extracts of the cell layer were found to contain mainly the processed collagen comprising alpha 1(X) chains. The native type X procollagen purified from culture medium was shown by rotary shadowing to occur as a short rod-like molecule 148 nm in length with a terminal globular extension, whereas the processed species comprising alpha 1(X) chains of Mr 49 000 was detected by electron microscopy as the linear 148 nm segment.

Mica sandwich technique for preparing macromolecules for rotary shadowing

The sandwich technique, in which a drop of sample solution is spread into a thin layer between two pieces of freshly cleaved mica, is a simple-to-use alternative to spraying for depositing macromolecules onto mica. Test specimens of collagen molecules and actin filaments were found to suffer less shear-induced damage, they were more uniformly distributed, and only very small sample volumes were needed. Either drying from a glycerol solution (40-70% v/v) or freeze-drying can be employed. Glycerol-drying is simpler, but freeze-drying may offer better preservation of supra-molecular assemblies.

Coonhound paralysis. Further clinical studies and electron microscopic observations

Prior study of coonhound paralysis (CHP) revealed an acute polyradiculoneuritis in raccoon-hunting dogs with clinical and pathologic features resembling those of Guillain-Barré syndrome (GBS). In the present series of five cases, the clinical features were investigated with emphasis on electrodiagnostic and CSF findings, and pathologic changes were evaluated with both the light and electron microscope. The demonstration of motor nerve conduction delay and CSF albuminocytologic dissociation in affected dogs further supported the clinical similarity of CHP and GBS. As in GBS, affected roots and nerves contained mononuclear cell infiltrates, segmental myelin changes and axon degeneration. Despite these general pathologic similarities, the present study suggested that axon damage was a more consistent finding in CHP than in GBS. In contrast to ultrastructural findings in GBS, the demyelinating process in CHP did not appear dependent upon macrophages for its initiation. Swelling, separation and vesiculation of myelin occurred around axons of reduced diameter often in the absence of proximate macrophages. Macrophages, rather than initiating demyelination, appeared to be superimposed on existing damage. In this regard, the observed changes resembled those reported in galactocerebroside-induced EAN and sera-mediated in vivo demyelination.

Live temperature-sensitive equine influenza virus vaccine: generation of the virus and efficacy in hamsters

Temperature-sensitive (ts) reassortants of an equine influenza virus, subtype A-1, were produced by mating a human influenza ts donor virus with an equine influenza A/Cornell/16/74 wild-type virus and by isolating a ts reassortant virus possessing the equine hemagglutinin and neuraminidase surface antigens. Two equine its reassortant clones, 8B1 and 71A1, were produced which had an in vitro shutoff temperature for plaque formation of 38 and 37 C, respectively. The human ts donor virus had ts mutation(s) on the polymerase 3 (P3) and nucleoprotein genes so that a ts equine reassortant virus could have either or both of these ts genes. It was found by complementation analysis that reassortant clone 8B1 had a ts lesion on the P3 gene and clone 71A1 had ts lesions on the nucleoprotein and P3 genes. An analysis of the parental origin of the genes in each ts equine reassortant virus indicated that clone 8B1 received 6 of its 8 genes and clone 71A1, 3 of its 8 from the equine parent virus, the remainder genes being from the human ts donor virus. The growth of both clones was restricted in the lungs of hamsters, but similar to that of the equine wild-type virus in the nasal turbinates. Each virus isolate obtained from the hamster's lungs or nasal turbinates retained the ts phenotype. These findings form the basis for further evaluation of the equine ts reassortant viruses for their level of attenuation and immunogenicity in horses.

Lyophilized hyperimmune equine serum as a source of antibodies for neonatal foals

In a study with 15 neonatal foals (5 per treatment group), foals were fed within 4 hours of birth as follows: 250 ml of colostrum, 250 ml of lyophilized serum reconstituted at 5 times the original concentration, or 250 ml of a mixture (1:1) of colostrum and lyophilized serum. Foal serum samples were tested for immunoglobulin (Ig)G concentration and titrated for anti-equine rhinovirus 1 and anti-equine influenza A1 and A2 antibodies at 0 and 24 hours after foals were born. Except in a foal which had suckled the dam before treatment, there was no evidence of IgG or specific viral antibodies in the samples taken at birth. There were no significant differences found in the serum IgG concentrations and antibody titers among the 3 treatment groups. Seemingly, IgG was absorbed efficiently from both serum and colostrum, so that the use of reconstituted lyophilized serum as a prophylactic measure of conferring passive immunity to a newborn foal deserves serious consideration.

Interference problems in a coherent aberration sensor

Fast IR wave-front aberration sensors are essential to the improved performance of HEL systems. However, one must not transfer sensor designs which function well (in principle and in practice) for the incoherent blackbody realm over into the coherent environment without first considering possible interference problems. A case history of one such sensor which failed this precept is considered.

Experimental coonhound paralysis: animal model of Guillain-Barré syndrome

Coonhound paralysis (CHP), a polyradiculoneuritis of dogs that resembles the human Guillain-Barré syndrome, was experimentally reproduced by inoculating a dog with raccoon saliva. The test animal was a coonhound that had previously sustained two naturally occurring attacks of CHP. Success in inducing the disease strengthened the notion that raccoon saliva contains the etiologic factor for CHP and that only specifically susceptible dogs are at risk of developing CHP when exposed to this factor.

Serologic classification of feline caliciviruses by plaque-reduction neutralization and immunodiffusion

Serologic classification of 14 isolated of feline caliciviruses was carried out, using plaque-reduction neutralization tests. Sixty to 200 plaque-forming units of virus were employed against 20 antibody units of hyperimmune goat antiserums prepared to individual isolates. The results established that these viruses were related in various degrees, but no 2 were identical. Arrangements in order of the greatest intergroup reactions yielded essentially 1 serotype with the exception of 1 virus showing 1-way neutralization reactions with heterologous isolates. The geographic origin, the site of viral isolation, or the plaque size had no relationship with the neutralization pattern. The F-9 isolate that showed the greatest intergroup reactions was proposed as the possible reference virus. The viruses were indistinguishable by using gel-diffusion analysis against cat antiserums.