The tridimensional structure of native collagenous fibrils, their proteinaceous filaments

Wavy nature of collagen fibrils deduced from the dispersion of their second-order nonlinear optical anisotropy parameters ρ

From P-SHG experiments, second-order nonlinear optical anisotropy parameters ρ = χ_ZZZ/χ_ZXX of collagen tissues are calculated assuming the same model of supercoiled collagen fibril characterized by a variable angle θ. Dispersion of experimental ρ values is converted into distribution of θ values based on the wavy nature of collagen fibrils deduced from EM studies. For tendon, the results show that the dispersion of experimental ρ values is mainly due to Poisson photonic shot noise assuming a slight fibrillar undulation with θ = 2.2^° ± 1.8^°. However for skin and vessels, the dispersion of experimental ρ values is mainly due to a stronger fibrillar undulation with θ = 16.2^° ± 1.3^°. The results highlight that this undulation is reduced during the development of liver fibrosis therefore, contributing to the rigidity of the tissue.

Determination of extracellular matrix collagen fibril architectures and pathological remodeling by polarization dependent second harmonic microscopy

Polarization dependence second harmonic generation (P-SHG) microscopy is gaining increase popularity for in situ quantification of fibrillar protein architectures. In this report, we combine P-SHG microscopy, new linear least square (LLS) fitting and modeling to determine and convert the complex second-order non-linear optical anisotropy parameter ρ of several collagen rich tissues into a simple geometric organization of collagen fibrils. Modeling integrates a priori knowledge of polyhelical organization of collagen molecule polymers forming fibrils and bundles of fibrils as well as Poisson photonic shot noise of the detection system. The results, which accurately predict the known sub-microscopic hierarchical organization of collagen fibrils in several tissues, suggest that they can be subdivided into three classes according to their microscopic and macroscopic hierarchical organization of collagen fibrils. They also show, for the first time to our knowledge, intrahepatic spatial discrimination between genuine fibrotic and non-fibrotic vessels. CCl₄-treated livers are characterized by an increase in the percentage of fibrotic vessels and their remodeling involves peri-portal compaction and alignment of collagen fibrils that should contribute to portal hypertension. This integrated P-SHG image analysis method is a powerful tool that should open new avenue for the determination of pathophysiological and chemo-mechanical cues impacting collagen fibrils organization.

Molecular tilting in dried elastoidin and its implications for the structures of other collagen fibrils

Wet elastoidin spicules (fish fin rays) yield low-angle meridional X-ray diffraction patterns which resemble those from tendons. However, when the spicule dries the meridian splits into the arms of a diagonal cross (sometimes only one arm appears). Of the possible explanations we reject shearing of the axial arrangement of molecules but confirm tilting. We suggest that, in three dimensions, the molecules are tilted at angles which vary from 0 degrees at the centre to some maximum value at the surface of the spicule, resembling torsion of the array of molecules. Molecular tilting probably occurs in other collagen fibrils.

Collagen fibers, reticular fibers and elastic fibers. A comprehensive understanding from a morphological viewpoint

Fibrous components of the extracellular matrix are light-microscopically classified into three types of fibers: collagen, reticular and elastic. The present study reviews the ultrastructure of these fibrous components as based on our previous studies by light, electron, and atomic force microscopy. Collagen fibers present a cord- or tape-shape 1-20 microm wide and run a wavy course in tissues. These fibers consist of closely packed thin collagen fibrils (30-100 nm thick in ordinary tissues of mammals), and exhibit splitting and joining in altering the number of the fibrils to form a three-dimensional network as a whole. Individual collagen fibrils (i.e., unit fibrils) in collagen fibers have a characteristic D-banding pattern whose length is ranges from 64 to 67 nm, depending on tissues and organs. During fibrogenesis, collagen fibrils are considered to be produced by fusing short and thin fibrils with tapered ends. Reticular fibers are usually observed as a delicate meshwork of fine fibrils stained black by the silver impregnation method. They usually underlie the epithelium and cover the surface of such cells of muscle cells, adipose cells and Schwann cells. Electronmicroscopically, reticular fibers are observed as individual collagen fibrils or a small bundle of the fibrils, although the diameter of the fibrils is thin (about 30 nm) and uniform. Reticular fibers are continuous with collagen fibers through the exchange of these collagen fibrils. In silver-impregnated specimens, individual fibrils in reticular fibers are densely coated with coarse metal particles, probably due to the high content of glycoproteins around the fibrils. Elastic fibers and laminae are composed of microfibrils and elastin components. Observations of the extracted elastin have revealed that elastin components are comprised of elastin fibrils about 0.1-0.2 microm thick. Elastic fibers and laminae are continuous with networks and/or bundles of microfibrils (or oxytalan fibers), and form an elastic network specific to individual tissues. The fibrous components of the extracellular matrix are thereby morphologically categorized into two systems: the collagen fibrillar system as a supporting framework of tissues and cells, and the microfibrilelastin system for uniformly distributing stress to maintain the resilience adapted to local tissue requirements.

Collagen structure and functional implications

The bio-mechanical requirements to which the connective tissue is subjected suggest that a causal correlation exist between the substructure and the collagen fibril function. We discuss the relationship between the inner structure of collagen fibrils, their diameter, their spatial layout and the functional requirements they have to withstand, and suggest that collagen fibrils may belong to two different forms indicated as "T-type" and "C-type". The first class, consisting of large, heterogeneous fibrils, parallely tightly packed, subjected to tensile stress along their axis is found in highly tensile structures such as tendons, ligaments and bone. The other class, consisting of small, homogeneous fibrils, helically arranged, resisting multidirectional stresses, is mostly present within highly compliant tissues such as blood vessel walls, skin and nerve sheaths. What causes these architectures to appear is discussed in detail in this review.

An energetic evaluation of a "Smith" collagen microfibril model

An energy minimized three-dimensional structure of a collagen microfibril template was constructed based on the five-stranded model of Smith (1968), using molecular modeling methods and Kollman force fields (Weiner and Kollman, 1981). For this model, individual molecules were constructed with three identical polypeptide chains [Gly-Pro-Pro)n, (Gly-Prop-Hyp)n, or (Gly-Ala-Ala)n, where n = 4, 12, and 16) coiled into a right-handed triple-helical structure. The axial distance between adjacent amino acid residues is about 0.29 nm per polypeptide chain, and the pitch of each chain is approximately 3.3 residues. The microfibril model consists of five parallel triple helices packed so that a left-handed superhelical twist exists. The structural characteristics of the computed microfibril are consistent with those obtained for collagen by X-ray diffraction and electron microscopy. The energy minimized Smith microfibril model for (Gly-Pro-Pro)12 has an axial length of about 10.2 nm (for a 36 amino acid residue chain), which gives an estimated D-spacing (234 amino acids per chain) of approximately 66.2 nm. Studies of the microfibril models (Gly-Pro-Pro)12, (Gly-Pro-Hyp)12, and (Gly-Ala-Ala)12 show that nonbonded van der Waals interactions are important for microfibril formation, while electrostatic interactions contribute to the stability of the microfibril structure and determine the specificity by which collagen molecules pack within the microfibril.

Different architectures of the collagen fibril: morphological aspects and functional implications

Several tissues known to contain collagen fibrils with a 'helical' arrangement were studied by t.e.m. and freeze-fracture. In all the tissues examined, the diameter of the collagen fibrils appeared to be tissue-specific and fairly constant within the same tissue. No statistical differences, on the contrary, were detectable in the coiling angle which appeared similar in all the tissues and independent of both diameter and age of the fibril. Rat tail tendon was also examined under the same technical conditions and showed collagen fibrils of large and very heterogeneous diameter and with a consistent 'straight' arrangement. These data seem to suggest that the 'helical' and 'straight' arrangements may actually identify different types of collagen fibrils. The authors discuss the possible functional significance of these arrangements and present two hypotheses on the three-dimensional structure of the 'helical' fibril.

Vascular degeneration in adenomatoid odontogenic tumour: an ultrastructural study

The blood vessels in 3 cases of adenomatoid odontogenic tumour (AOT) were investigated ultrastructurally. An estimated 70-90% of the blood vessels found in the stroma showed degenerative changes which affected both the endothelial lining and the perivascular connective tissue. These vessels showed multiplication of basal lamina and were also encircled by concentric lamellae consisting either of collagen or fine filaments measuring 5-15 nm in diameter. Degradation of the layered collagen into fine filaments similar to those forming the concentric layers was observed. The present results suggest that the fine filaments of the concentric lamellae probably result from degradation of the layered collagen surrounding these vessels.

In vitro formation of hybrid fibrils of type V collagen and type I collagen. Limited growth of type I collagen into thick fibrils by type V collagen

Type V collagen and type I collagen were obtained from human placentas by pepsin treatment, followed by salt fractionation. The precipitates formed at 37 degrees C from a mixed solution of type V collagen and type I collagen, reacted with antibodies to either type V collagen or type I collagen. The precipitates seen by electron microscopy were fine flexible fibrils, with a D-periodic banding pattern. The average diameter of hybrid fibrils was smaller than 50 nm, when the proportion of type V collagen exceeded that of type I collagen. Type V collagen directly interacts with type I collagen in forming hybrid fibrils, resulting in limitation of the growth of type I collagen fibrils into thicker fibrils. We propose that the fibrils with a predominant type V collagen content may occur in the pericellular environment of various tissues, as a basic structure in connecting basal laminae with interstitial collagen fibrils.

Polarized light microscopy in the study of the molecular structure of collagen and reticulin

Although collagen structure has been studied by polarized light microscopy since the early 19th century and continued since, modern studies and reviews failed to correlate the conclusions based on data obtained by the techniques with those of polarized light microscopy. Collagen I is intensely positively birefringent in respect to length of the fibres; the positive intrinsic birefringence indicates a quasi-crystalline alignment parallel to the fibre and molecule axis of the amino acid residues of the polypeptide chains. This would not have been compatible with a helical structure but has been achieved by similar tilt angles and opposite directions of the coiling and supercoiling. Birefringence characteristics of collagen are also affected by chemical treatments, extractions and staining procedures. Attachment of chemical groups to the anionic charges present on the surface of collagen molecules results in increased positive birefringence in the case of bipolar molecules attached to two or more anionic residues. Unipolar attachment to the same groups, or to the cationic groups of the associated proteoglycans, as well as sulfation or acetylation of hydroxyls of the protein and/or the carbohydrate, reduced or reversed the sign of birefringence. Increased birefringence caused by stretching cannot be due to intramolecular events and is caused by intermolecular changes. The same applies to changes in collagen during aging. Reticulin is a group of different substances which mostly contain collagen III. The pliability and deformability of this collagen is related to its weakly negative birefringence due to large side chains and presence of different and greater amounts of interstitial proteoglycans and other molecules. The so-called reticulin of healing wounds differs in its constitution from other reticulins but is also rich in intermolecular carbohydrate components.

Electron-microscopic study of the collagen fibrils of the rat tail tendon as revealed by freeze-fracture and freeze-etching techniques

The ultrastructure of the collagen of rat tail tendon was investigated by the freeze-fracture technique. Collagen fibers were pretreated with the digestive enzymes, alpha-amylase, elastase and collagenase to remove matrix substances. Some of the samples were etched for 20 min. Fibrils had an average diameter of 318 +/- 12 nm and a banded structure with a mean periodicity of 64.2 +/- 0.9 mm; the banding was most marked in alpha-amylase/elastase-treated specimens, although the periodicity was independent of pretreatment. Microfibrils were well-displayed following alpha-amylase/elastase and collagenase pretreatments. A difference in the diameters of microfibrils was, however, observed between etched specimens (8.3 +/- 0.3 nm) and those prepared by other experimental methods (11.4 +/- 0.5 nm). In replicas of collagenase-treated and etched specimens, the interconnecting filaments in the interfibrillar region formed a network that was continuous with the microfibrils of collagen fibrils. The diameter of the interconnecting filaments was the same as that of microfibrils. Microfibrillar bundles were observed in the interfibrillar region.

Arrangement of microfibrils in collagen fibrils of tendons in the rat tail. Ultrastructural and x-ray diffraction investigation

The microfibrillar arrangement in collagen fibrils of tendons in the tail of the rat was examined by electron microscopy and X-ray diffraction. Fresh and air-dried collagen fibers were examined in unstretched and stretched conditions. The results demonstrate that the microfibrils have a course parallel to the longitudinal axis of the collagen fibrils. The influence of stretching and hydration of the samples on the orientation of fibrils and microfibrils is also assessed.

Light microscopy, electron microscopy, and X-ray diffraction analysis of glycerinated collagen fibers

Light microscopy, transmission electron microscopy (freeze-fracture replicas and thin sections), and X-ray diffraction techniques were used to investigate the structure of rat tail tendon collagen fibers subjected to one of the following treatments: water, phosphate buffer, glutaraldehyde, glutaraldehyde followed by glycerol, glycerol, and glycerol followed by phosphate buffer. As seen by light microscopy, only treatment with glycerol induces a remarkable swelling of the native (untreated) collagen specimens. Replicas and thin sections show that this swelling is due to an expansion of the interfibrillar space, and to a dissociation of the collagen fibrils into microfibrils. X-Ray diffraction analysis reveals great disorder in the glycerol-swollen collagen fibers. However, this does not appreciably involve the microfibrillar and molecular structure. Light and electron microscopy as well as X-ray diffraction techniques show that the collagen fiber swelling induced by glycerol is an almost completely reversible process.

Structure of the myosin crossbridge lattice in insect flight muscle

Freeze fracture and deep-etching of quick-frozen insect flight muscles provides unusually clear views of thick filament projections in rigor and relaxed states. In rigor, these projections form crossbridges that are deployed helically. The tracks of these helices are left-handed, repeat every approximately 38 nm, tilt at approximately 42 degrees to the muscle axis, and, when viewed on edge, produce the unique "double chevron" pattern of crossbridges that characterizes this muscle type in thin sections (Reedy, 1968). These helical parameters substantiate Reedy's earlier deduction that rigor crossbridges form two-start helices in this muscle. On the other hand, deep-etchings of insect flight muscles relaxed with Mg-ATP before freezing do not fit with earlier results. Contrary to earlier thin section views and the expectations of X-ray diffraction, thick filaments in such relaxed muscles display no hint of a 14.5 nm axial periodicity; instead, their projections appear to be very disordered. This suggests that when crossbridges are detached, they are free to "wobble" by at least +/- 7 nm in the axial direction and thus obscure their points of origin from the thick filaments. With the images of detached crossbridges in mind, closer inspection of rigor thick filaments yields no indication of any "extra" projections between the helically deployed ones, i.e. there is no indication of any detached crossbridges in rigor muscles. Thus in this type of muscle, at least, the establishment of a rigor pattern may not involve a "selection" of suitably located myosin heads from a larger population, as is generally thought, but may instead involve a systematic redistribution of the whole population of heads until all of them became crossbridges.

Transformation of the structure of collagen. A time-resolved analysis of mechanochemical processes using synchrotron radiation

Mechanochemically induced molecular transformations of collagen fibres were analysed using time-resolved small-angle diffraction spectra and histomechanical measurements. In particular, the influence of aqueous and methanolic perchlorate solutions was examined. According to a transformation continuing from the periphery towards the centre, the macroscopic contraction that is completed less than five minutes after incubation with perchlorate is caused by peripherally transformed fibrils only, whereas the centrally situated fibrils first undergo an accordion-like folding, but after more than 20 minutes are transformed similarly. The triple-helical transformation is preceded by a structure-breaking effect on structural water that can be monitored in time-resolved diffraction spectra. The combined loss of meridional low-angle reflections and cross-striated fibrils in micrographs is irreversible. By dialysis of colloidally dissolved collagen against a solution of ATP, however, segment-long spacing aggregates are obtained. Under isometric conditions, an instantaneous transformation of intermittent regions leads to an increase in the long period of adjacent, still structured regions of the same fibril that is correlated with a delayed increase in tension in the fibre. Increase of tension under isometric conditions as well as the flow-properties of a fibre relaxed in perchlorate are interpreted in terms of the parallel sliding of subunits of varying lengths, which has been demonstrated by diffraction analysis.

Electron microscopy shows periodic structure in collagen fibril cross sections

X-ray diffraction was used to monitor the effects of electron microscope fixation, staining, and embedding procedures on the preservation of the three-dimensional crystalline order in collagen fibrils of rat tail tendon. A procedure is described in which the characteristic 3.8-nm lateral spacing is preserved, with increased contrast, in the diffraction pattern of the embedded fiber. This spacing is correlated with the separation between the tangentially oriented equally spaced lines of density observed in electron microscope ultrathin fibril cross sections of the same material. Optical diffraction of electron micrographs gives an objective measure of the periodicity and suggests that the fibril is composed of concentrically oriented crystalline domains. These observations, when combined with a recent interpretation of the native x-ray diffraction data [Hulmes, D. J. S. & Miller, A. (1979) Nature (London) 282, 878-880] suggest a tentative model for the three-dimensional structure of collagen fibrils.

The nature of the hyaline rings in chronic periostitis and other conditions: an ultrastructural study

The hyaline rings found in chronic periostitis and a radicular cyst were investigated ultrastructurally. These rings were composed of fine fibrillar material which was probably derived from degraded collagen fibrils, all of which eventually became degraded and unrecognizable. Foreign-body material in the form of circular bodies of probable plant origin were also found and should not be confused with the hyaline rings.

The limiting collagen microfibril. The minimum structure demonstrating native axial periodicity

Collagen fibers were grown from solutions of acid-soluble or neutral salt-soluble collagen in 0.5 M acetic acid by rapid dialysis. The collagen was obtained under conditions where protease inhibitors were present at every stage of extraction and purification. Under the conditions used, length-wise but not lateral filament growth proceeded rapidly and gel-like networks were formed, Water readily exuded from the networks. The networks were stretched to fibrous form during drying. Small-angle X-ray diffraction showed the stretched fibrils to be highly ordered, showing up to 20 orders of the 670 A meridional periodicity. Intermediate- and wide-angle photographs show equatorial reflections at a spacing corresponding to approximately 12.5 A which is related to the intermolecular distance but none related to a microfibrillar packing at the 35-40 A level. Electron microscopy of the gel networks before stretching shows the presence of thin filaments with diameters predominantly in the 35-40 A range. No cross-striated fibrils are seen in electron micrographs of either stretched fibers or unstretched fibers. Thus, intermolecular packing in accord with the 670 A axial periodicity can take place within approximately 40 A diameter thin filaments. These correspond to the structures previously postulated to be collagen 'microfibrils'.

A new variety of spondyloepiphyseal dysplasia characterized by punctate corneal dystrophy and abnormal dermal collagen fibrils

Several individuals from one family are described with a unique form of spondyloepiphyseal dysplasia. Characteristic features include short-trunked short stature, punctate corneal dystrophy and marked disorganization of dermal collagen fibrils when examined by transmission electron microscopy. Inheritance is compatible with either dominance and a variable expression or X-linkage. Although the basic defect has not been determined, the tissue distribution is consistent with a defect in a non-collagenous component that affects collagen fibril formation or stability.

The multicomposite structure of tendon

A revised morphological model for the crimp structure of tendon is presented. The 300-500 mu diameter tendons of the mature rat tail are comprised of from one to more than ten substructures, called fascicles, of 80-320 mu diameter. Fascicles each possess a "crimp structure" demonstrable in the polarizing microscope and neighboring fascicles within a tendon usually exhibit crimp registry. The fascicle itself is shown to be a cylindrical array of planar-zig-zag crimped 500-5000 A diameter collagen fibrils. The approximate cylindrical symmetry of the fascicle is domonstrated by SEM not equal to and polarizing optical microscopy. A method of replacing native water with other liquids of refractive index near to that of collagen is utilized to reduce or eliminate light diffusion and therby greatly improve OM observations. Small bunches of collagen fibrils removed from the tendon are shown to exhibit the simple planar zig-zag morphology described in previous literature. The planar crimping of collagen fibrils and their assemblage into cylindrically symmetric fascicles is verified by small angle X-ray diffraction.

The human cutaneous basement membrane--anchoring fibril complex: preparation and ultrastructure

The human cutaneous basement membrane (CBM) has been isolated together with the anchoring fibrils (AF) of the dermis. These structures are removed from the dermis as a single complex, the CBM-AF complex. The epidermis is first removed from the dermis by immersion of whole, defatted skin in cold 2 n NaSCN for 5 to 6 hr. Subsequently, removal of the CBM-AF complex is effected by focusing high-intensity ultrasonic energy against the dermal surface. Purification of the preparation is achieved by low-speed sedimentation. All stages of the process were monitored by electron microscopy, which demonstrated that the morphology of the CBM-AF complex was well preserved and that this complex represents the principal mass component in the system. In addition, our report describes an ultrasound-induced variation in collagen fibril morphology, which we have called 'disordered collagen.' This altered type of fibril lacks periodicity or recognizable banding and has a frayed appearance. Finally, the experiments provide the basis for an analysis of the relative strengths of certain bonds at the dermal--epidermal junction. The strongest of these appears to be the bond between basement membrane and anchoring fibril.

Collagen deposition on a preformed grid

Appearance of collagen fibrils in the cuticle was seen by electron microscopy to be preceded by formation of a finely filamentous matrix material. At first, the fine filaments of the matrix are unorganized. However, signs of orthogonal ordering soon appear in the most superficial portion of the cuticle, and subsequently appear more basally and closer to the underlying epidermis. Meanwhile, fibrils of different staining properties and identifiable as collagen begin to be deposited in the superficial portion of the cuticle, the same region which first showed organized fine filaments. Then, like the fine filaments before them, the collagen fibrils polymerize more basally. Collagen appears to polymerize on the preformed skeleton of fine filaments as though the fine filaments lagen fibrils seems to require direct cellular intervention but occur first in that portion of the cuticle which is furthest away from the underlying epidermis. The fine filaments may be self ordering, extracellular macromolecules which in turn determine the polymerization of collagen fibrils.

Axoplasm architecture and physical properties as seen in the Myxicola giant axon

A technique is described for extracting axoplasm from the giant axon of a marine worm, Myxicola infundibulum. The operation can be completed in 10 sec. 2. Axoplasm is pulled from the axon of a living worm as a long, clear cylinder, up to 35 cm long and 70 mg wet weight. The worm regenerates a new giant axon in about 4 months. 3. Myxicola axoplasm is a gel, 87% water, held together by protein neurofilaments. It contains small amounts of mitochondria and vesicles, but no detectable microtubules. 4. The internal structure of the gel is superficially similar to that of yarn. Closer inspection with light and electron microscopy, and X-ray diffraction, show it to be organized in a hierarchy of helical forms. Squid giant axons have a similar structure. 5. Initial estimates of the bulk physical properties of extracted Myxicola axoplasm give: breaking strength, 1400 g/cm2; specific gravity, 1-05 g/cm3; birefringence, 1-6 X 10(-4); index of refraction, 1-351; resistivity, 57 omega cm. These average values are shown to be compatible with the observed structure and composition. 6. Despite its mechanical strength, the axoplasm gel is so hydrated that Na+, K+ and homarine diffuse through it at rates approaching those in free solution. Fewer than about 5% of each of these ions are tightly bound to the gel. 7. It is argued that (a) the structure and physical properties of Myxicola axoplasm are representative of those in other axons, (b) the compound helix architecture results from twist of parallel, cross-linked fibrous proteins, and (c) this sturcture serves as a flexible internal skeleton for nerve cell processes.

Freeze-etch study of collagen. I. Native collagen from tendon and lung of rats

The freeze-fracture technique was used to study the fine structure of native collagen from rat tendon and lung. The collagen filaments were organized into a spiraled lamellar substructure within each fibril. Small particles, representing the broken ends of the collagen filaments, were found in nearly straight rows when the lamellae were broken tranversely. Filamentous connections were shown which span the interfibrillar matrix and unite all of the fibrils into a reticular network. The fibrils were coated with material that was greatly hydrated in vivo. Sublimation occurred more rapidly from some portions of the coating substance than from others, resulting in a banding pattern with a 610 A repeat distance.

A D-periodic narrow filament in collagen

Collagen may be reprecipitated as obliquely striated fibrils. The oblique striations are due to D -periodic subfibrils staggered axially by approxi­mately 9.0 nm with respect to their neighbours (Bruns, Trelstad & Gross 1973). The diameter of the subfibrils is variable. Optical diffraction analysis of the electron micrographs reveals instances of subfibrils with the D -repeat having diameter 3.7─4.0 nm. We argue that this structure is probably the five-stranded microfibril suggested by Smith (1968).