Supramolecular structure of polymorphic collagen fibrils.

Reconstituted cartilage collagen fibrils with an oblique banding pattern or with two types of symmetrical patterns, and reconstituted rattail tendon fibrils with a third type of symmetrical pattern were examined by electron microscopy and found to consist of narrow subfibrils having native-type cross-striations. Analysis of the four types of patterns by a graphic method of specific band matching revealed the orientation and axial relation of individual subfibrils and their component molecules. In fibrils with an oblique pattern, subfibrils have the same orientation and a regular 100A axial displacement. Observations on staining characteristics, folded fibrils, and transverse sections of embedded fibrils suggest that the obliquely banded fibrils are ribbonlike or layered structures. In the three types of fibrils with a symmetrical pattern, adjacent subfibrils are oppositely oriented and aligned within a 119-A segment of the 670-A major period. Considered together, the observations suggest that interaction sites on the surface of subfibrils (and perhaps on the surface of native collagen fibrils) occur in various patterns that are manifested accouding to the nature of the environment during fibril formation, and that such patterns can be mapped on the surface of subfibrils by noting the arrangement of subfibrils in polymorphic forms.

A symmetrical, extracellular fibril.

Symmetrical, extracellular fibrils, which are related to the "special fibrils" of the dermis described by Palade and Farquhar, have been found along the outer surface of the basement membrane covering the notochord in the tail of Rana catesbeiana (bullfrog) tadpoles. The fibrils are approximately 7,500 A long and occur singly or in clusters. The single fibrils are characterized by a symmetrical transverse band pattern and by attachment at both ends to the basement membrane. The clusters are various complex configurations which seemingly represent symmetrical fibrils in different states of aggregation. Symmetrical fibrils also occur in the skin of the tadpole tail and in the skin of the toad, Bufo marinus. It is proposed that a narrow, symmetrical fibril is the fundamental "special fibril."

Structural modulations of plasmalemmal vesicles.

Structural modulations affecting a small fraction of the population of plasmalemmal vesicles of vascular endothelia are described. They include forms which are apparently produced by the fusion of the vesicular membrane with the plasmalemma and by the successive elimination of the layers of the two fused membranes. Such modulations are assumed to represent stages in the discharge process of vesicular contents. Other forms, characterized by their flask shape and elongated neck, are assumed to represent stages in the formation and loading of membrane invaginations, followed by their being pinched off to form isolated vesicles. Stages in a membrane-fusion process leading to the formation of apertured fenestrae and channels are also described in fenestrated endothelia. The visualization of these structural details is greatly facilitated by staining tissue specimens with uranyl acetate before dehydration.

Studies on blood capillaries. I. General organization of blood capillaries in muscle.

THE WALL OF THE BLOOD CAPILLARIES OF SKELETAL MUSCLES (DIAPHRAGM, TONGUE, HIND LEGS) AND MYOCARDIUM OF THE RAT, GUINEA PIG, AND HAMSTER CONSISTS OF THREE CONSECUTIVE LAYERS OR TUNICS: the endothelium (inner layer), the basement membrane with its associated pericytes (middle layer), and the adventitia (outer layer). The flattened cells of the endothelium have a characteristic, large population of cytoplasmic vesicles which, within the attenuated periphery of the cells, may attain a maximum frequency of 120/micro(2) of cell front and occupy approximately 18% of the cytoplasmic volume; these values decrease as the cells thicken toward the perikaryon. The vesicles are 650-750 A in over-all diameter and are bounded by typical unit membranes. They occur as single units or are fused to form short chains of two to three vesicles. Each configuration may lie entirely within the cytoplasm or open onto the cell surface. In the latter case, the unit membrane of the vesicle is continuous, layer by layer, with the plasmalemma. Chains of vesicles opening simultaneously on both the blood and tissue fronts of the endothelial tunic have not been observed either in sections or in a tridimensional reconstruction of a sector of endothelial cell cytoplasm. Adjacent endothelial cells are closely apposed to one another and appear to be joined over a large part of their margins, possibly over their entire perimeter, by narrow belts of membrane fusion (zonulae occludentes). Except for tongue capillaries, patent intercellular gaps are rare or absent. The middle layer is formed by a continuous basement membrane ( approximately 500 A thick) and by pericytes which lie in between leaflets of this membrane. The tips of the pericyte pseudopodia penetrate through the inner leaflet of the basement membrane and join the endothelium in maculae occludentes. The adventitia is a discontinuous layer comprising cellular (macrophages, fibroblasts, mast cells) and extracellular (fibrils, amorphous matrix) elements. The same general type of construction appears to be used along the entire length of the capillary.

Studies on blood capillaries. II. Transport of ferritin molecules across the wall of muscle capillaries.

The pathway by which intravenously injected ferritin molecules move from the blood plasma across the capillary wall has been investigated in the muscle of the rat diaphragm. At 2 min after administration, the ferritin molecules are evenly distributed in high concentration in the blood plasma of capillaries and occur within vesicles along the blood front of the endothelium. At the 10-min time point, a small number of molecules appear in the adventitia, and by 60 min they are relatively numerous in the adventitia and in phagocytic vesicles and vacuoles of adventitial macrophages. Thereafter, the amount of ferritin in the adventitia and pericapillary regions gradually increases so that at 1 day the concentration in the extracellular spaces approaches that in the blood plasma. Macrophages and, to a lesser extent, fibroblasts contain large amounts of ferritin. 4 days after administration, ferritin appears to be cleared from the blood and from the capillary walls, but it still persists in the adventitial macrophages and fibroblasts. At all time points examined, ferritin molecules within the endothelial tunic were restricted to vesicles or to occasional multivesicular or dense bodies; they were not found in intercellular junctions or within the cytoplasmic matrix. Ferritin molecules did not accumulate within or against the basement membranes. Over the time period studied, the concentration of ferritin in the blood decreased, first rapidly, then slowly, in two apparently exponential phases. Liver and spleen removed large amounts of ferritin from the blood. Diaphragms fixed at time points from 10 min to 1 day, stained for iron by the Prussian Blue method, and prepared as cleared whole mounts, showed a progressive and even accumulation of ferritin in adventitial macrophages along the entire capillary network. These findings indicate: (1) that endothelial cell vesicles are the structural equivalent of the large pore system postulated in the pore theory of capillary permeability; (2) that the basement membrane is not a structural restraint in the movement of ferritin molecules across the capillary wall; (3) that transport of ferritin occurs uniformly along the entire length of the capillary; and (4) that the adventitial macrophages monitor the capillary filtrate and partially clear it of the tracer.

Biosynthesis of sulphated macromolecules by rabbit lens epithelium. II. Relationship to basement membrane formation.

Rabbit lens epithelial cells display a similar "cobblestone" morphology and produce the same complement of sulphated macromolecules (also see Heathcote, J.G., and R.W. Orkin, 1984, J. Cell Biol., 99:852-860) whether grown on plastic or glass, dried films of gelatin or type IV collagen with laminin, or on gels of type I collagen. There was no evidence of basement membrane formation by these cells when they were grown on plastic, glass, or dried films. In contrast, cultures that had been grown on gels deposited a discrete basement membrane that followed the contours of the basal surfaces of the cells and in addition, they secreted amorphous basement membrane-like material that diffused into the interstices of the gel and associated with the collagen fibrils of the gel. A significant proportion (approximately 70%) of the heparan sulphate proteoglycan fraction that was secreted into the culture medium (fraction MI) when the cells were grown on plastic became associated with the cell-gel layer in the gel cultures. Further, when basement membrane was isolated by detergent extraction, greater than 90% of the 35S-labeled material present was in this heparan sulphate proteoglycan.

Type VI collagen in extracellular, 100-nm periodic filaments and fibrils: identification by immunoelectron microscopy.

Filaments and fibrils that exhibit a 100-nm axial periodicity and occur in the medium and in the deposited extracellular matrix of chicken embryo and human fibroblast cultures have been tentatively identified with type VI collagen on the basis of their similar structural characteristics (Bruns, R. R., 1984, J. Ultrastruct. Res., 89:136-145). Using indirect immunoelectron microscopy and specific monoclonal and polyclonal antibodies, we now report their positive identification with collagen VI and their distribution in fibroblast cultures and in tendon. Primary human foreskin fibroblast cultures, labeled with anti-type VI antibody and studied by fluorescence microscopy, showed a progressive increase in labeling and changes in distribution with time up to 8 d in culture. With immunoelectron microscopy and monoclonal antibodies to human type VI collagen followed by goat anti-mouse IgG coupled to colloidal gold, they showed in thin sections specific 100-nm periodic labeling on extracellular filaments and fibrils: one monoclonal antibody (3C4) attached to the band region and another (4B10) to the interband region of the filaments and fibrils. Rabbit antiserum to type VI collagen also localized on the band region, but the staining was less well defined. Control experiments with antibodies to fibronectin and to procollagen types I and III labeled other filaments and fibrils, but not those with a 100-nm period. Heavy metal-stained fibrils with the same periodic and structural characteristics also have been found in both adult rat tail tendon and embryonic chicken tendon subjected to prolonged incubation in culture medium or treatment with adenosine 5'-triphosphate at pH 4.6. We conclude that the 100-nm periodic filaments and fibrils represent the native aggregate form of type VI collagen. It is likely that banded fibrils of the same periodicity and appearance, reported by many observers over the years in a wide range of normal and pathological tissues, are at least in part, type VI collagen.

Cartilage collagen: a staggered substructure in reconstituted fibrils.

In contrast to the typical transverse banding pattern of native and reconstituted skin collagen fibrils, reconstituted fibrils of cartilage collagen have an oblique banding pattern that results from a regular axial shift (89 angstroms) of component "subfibrils." The 89-angstrom shift may be related to the major helix of the collagen molecule.

A large-scale, orthogonal network of microfibril bundles in the corneal stroma.

Orthogonal and parallel arrays of microfibril bundles are described in the corneal stroma of embryonic and adult chickens. The arrays lie parallel to the corneal surface and are distributed in the extracellular matrix between Bowman's layer and Descemet's membrane. The individual microfibril bundles measure 0.1-0.25 micron in diameter and consist of 20-30 "tubular" rods, of approximately 15 nm diameter, and an apparently structureless matrix. The arrays have a spacing of approximately 3 microns. We speculate that the microfibril bundles serve as a scaffolding for the corneal stroma or as a light-diffracting element. With some variations in distribution and organization, the arrays of microfibril bundles also occur in calf, rabbit, rat, newborn mice, toad, and goldfish, but not in adult human corneas.

Beaded filaments and long-spacing fibrils: relation to type VI collagen.

"Beaded filaments" have been found in fibroblast cultures prepared from chicken embryo leg tendons and cornea and from the dermis of human skin. With negative staining, they appear as single, unbranched, flexible strands approximately 3 nm in width and up to at least 2 microns in length. Pairs of "beads" are distributed on the filament at regular intervals of 110 nm. The beaded filaments appear to be resistant to the action of trypsin and bacterial collagenase. The filaments also occur in bundles with beads laterally aligned to form long-spacing-type fibrils, which appear to be identical with many fibrous-long-spacing-type fibrils described, but not identified, by others in a variety of normal and pathological tissues. The long-spacing fibrils are numerous at several sites of active collagen fibrillogenesis. Comparison of the beaded filament structure with molecular models for various collagens described in the literature suggests that they are a filamentous form of type VI collagen.

On the state of aggregation of newly secreted procollagen.

Procollagen and partially processed procollagen from cultures of primary chicken embryo tendon cells appeared as segment-long-spacing (SLS)-like aggregates when drops of medium were negatively stained and examined by electron microscopy. Similar aggregates were obtained after negative staining of medium partially purified by gel filtration and also after staining thin sections of fixed, dehydrated, and embedded pellets formed by prolonged ultracentrifugation of whole culture medium. In contrast to results from electron microscopy, analysis by velocity density gradient sedimentation or sedimentation equilibrium indicated the exclusive presence of procollagen or partially processed procollagen monomers in solution. These contradictory data can be reconciled if procollagen exists in monomeric form when greatly diluted (as in culture medium), and in specific aggregated form (SLS) at high concentration. We believe that cells in vivo secrete procollagen in high, local concentration packaged in the SLS form. We propose that such zero-D arrayed packages are the precursors of native collagen fibrils.

Type XII and XIV collagens mediate interactions between banded collagen fibers in vitro and may modulate extracellular matrix deformability.

Type XII and XIV collagens are very large molecules containing three extended globular domains derived from the amino terminus of each alpha chain and an interrupted triple helix. Both collagens are genetically and immunologically unique and have distinct distributions in many tissues. These collagens localize near the surface of banded collagen fibrils. The function of the molecules is unknown. We have prepared a mixture of native type XII and XIV collagens that is free of contaminating proteins by electrophoretic criteria. In addition, we have purified the collagenase-resistant globular domains of type XII or XIV collagens (XII-NC-3 or XIV-NC-3). In this study, we have investigated the effect of intact type XII and XIV and XII-NC-3 or XIV-NC-3 on the interactions between fibroblasts and type I collagen fibrils. We find that both type XII and XIV collagens promote collagen gel contraction mediated by fibroblasts, even in the absence of serum. The activity is present in the NC-3 domains. The effect is dose-dependent and is inhibited by denaturation. The effect of type XII NC-3 is inhibited by the addition of anti-XII antiserum. To elucidate the mechanism underlying this phenomenon, we examined the effect of XII-NC-3 or XIV-NC-3 on deformability of collagen gels by centrifugal force. XII-NC-3 or XIV-NC-3 markedly promotes gel compression after centrifugation. The effect is also inhibited by denaturation, and the activity of type XII-NC3 is inhibited by the addition of anti-XII antiserum. The results indicate that the effect of XII-NC-3 or XIV-NC-3 on collagen gel contraction by fibroblasts is not due to activation of cellular events but rather results from the increase in mobility of hydrated collagen fibrils within the gel. These studies suggest that collagen types XII and XIV may modulate the biomechanical properties of tissues.

Procollagen segment-long-spacing crystallites: their role in collagen fibrillogenesis.

Naturally occurring segment-long-spacing crystallites of procollagen collagen have been found in the culture medium of fibroblasts from chick embryo tendon, human skin, and dermatosparaxic calf skin; in whole-mount preparations of cultured human skin fibroblasts; in homogenates of lathyritic chick embryo tendon, cartilage, and cornea; and in a partially purified preparation of procollagen. Bundles of similar aggregates occurred within secretory vacuoles of collagen-synthesizing fibroblasts and chondrocytes. These observations suggest that fibroblasts and chondrocytes secrete procollagen assemblies that are stable in the extracellular environment. We propose that subsequent enzymatic processing is accompanied by direct incorporation of such structures into the assembling fibril, which then may be considered as an n X 67 nm staggered array of segment-long-spacing crystallites.

Type VI collagen: immunohistochemical identification as a filamentous component of the extracellular matrix of the developing avian corneal stroma.

Selected stages of the developing chicken cornea have been examined for type VI collagen, employing monoclonal antibodies specific for this molecule. By immunofluorescence, the molecule is not detectable in 5 1/2 day corneas, a time at which the epithelial-derived, acellular primary stroma is the only corneal matrix present. One day later, the presumptive stromal fibroblasts have invaded this stroma and have initiated synthesis of the secondary (mature) stroma. By that time, a strong fluorescent signal for the type VI collagen molecule is detectable throughout the stroma. It is present in all subsequent ages examined. The molecule is not restricted to the cornea, and is present in most stromal matrices examined, including those of the sclera, eyelid, and nictitating membrane. Immunoelectron microscopy was also performed, utilizing a colloidal gold-labeled secondary antibody. These data show that the type VI collagen is not a component of the striated collagen fibrils, but instead is assembled in the form of thin filaments. The monoclonal antibody bound to the filaments at periodic intervals of about 100 nm.

Molecular structure of short-chain (SC) cartilage collagen by electron microscopy.

We have recently observed that aged and/or hypertrophying chondrocytes in culture synthesize a small collagen molecule termed short-chain (SC) collagen. Our previous biochemical studies have suggested that this molecule is slightly less than half the length of "typical" interstitial collagens and should have both a helical, collagenous domain and a nonhelical, globular one. In the present study we have examined the structure of this molecule by electron microscopy of rotary-shadowed preparations and segment-long-spacing crystallites. Rotary-shadowed SC collagen molecules appear as rods with a length of 132 nm and a knob at one end. Preparations of native molecules that have been treated by limited pepsin digestion show only the rod-like domain. These results are consistent with the rod-like domain having the molecular structure of a collagen helix, which is refractory to pepsin digestion, and the knob representing a globular, nonhelical domain. Segment-long-spacing crystallites of pepsin-digested molecules confirm the length of the helical domain to be 132 nm. Positively stained crystallites show a banding pattern different from other collagens.

Inhibition of tumor growth, vascularization, and collagenolysis in the rabbit cornea by medroxyprogesterone.

Medroxyprogesterone, dexamethasone, or cortisone, locally applied in sustained release polymer to rabbit V2 carcinoma implanted in the rabbit cornea, blocked neovascularization and three-dimensional growth of the tumor. These hormones similarly prevented the vascular proliferative response to implants in the rabbit cornea of mouse B-16 melanoma and also the response to implants of polymer containing tumor extract with angiogenesis activity. The inhibitory responses were accompanied by considerable reduction in collagenolytic activity released into culture medium by explants of the two tumors and of the corneal region containing angiogenic hepatoma extract. Morphologic studies revealed extensive three-dimensional disruption of the compact laminated collagenous structure of the cornea by untreated V2 carcinoma. In the presence of hormone the tumor grew slowly as a noninvasive two-dimensional plaque limited to the narrow region of the insertion pocket in the cornea, with no obvious disturbance of structure elsewhere. Cortisone was much les effective than medroxyprogesterone or dexamethasone. Testosterone and estradiol had no effect on the three measured properties. The data suggest that local hormonal interference with neovascularization, collagenase production, and tumor growth can prevent neoplastic invasion and destruction of a dense collagenous connective tissue.

Quantitative removal of hepatitis B viral antigens from serum by a monoclonal IgM coupled to a biocompatible solid-phase support.

We covalently linked to regenerated cellulose filters a high-affinity monoclonal IgM produced against epitopes that reside on hepatitis B viral surface antigen (HBsAg). Conditions were established whereby as much as 250 micrograms of anti-HBsAg IgM could be linked to 2-4 mg of regenerated cellulose acetate by using cyanogen bromide and trichloro-s-triazine coupling agents. The immunoreactivity of the monoclonal anti-HBsAg IgM was preserved, and quantitative binding studies with HBsAg suggests that more than one functional binding site on the IgM molecule was operative. The specificity of the monoclonal anti-HBsAg IgM was established by demonstrating that a nonspecific monoclonal IgM (against influenza hemagglutinin), when coupled to the filters under identical conditions, had no effect on removal of HBsAg from serum. Most importantly, the monoclonal anti-HBsAg IgM-coupled filters quantitatively removed low levels of HBsAg from serum; after the third pass through the filter, HBsAg was undetectable in the perfusate. Further, the stability of the covalent bond between the anti-HBsAg IgM and regenerated cellulose acetate was shown by the lack of detectable murine monoclonal anti-HBsAg IgM in filtered serum despite 50 passages through the filter. Thus, we have demonstrated that monoclonal IgM antibodies with predefined specificity, when coupled to a biocompatible solid-phase support, may serve as a high-affinity and specific immunoabsorbant for quantitative removal and recovery of viral antigens from human serum. By using this approach, specific removal and recovery of many other substances from serum or plasma would seem possible.

Monoclonal IgM radioimmunoassay for hepatitis B surface antigen: high binding activity in serum that is unreactive with conventional antibodies.

Using a monoclonal IgM antibody (anti-HBs) to hepatitis B surface antigen (HBsAg) in a radioimmunoassay for hepatitis B, we have detected high binding activity in human serum that was unreactive in assays employing conventional anti-HBs reagents. The binding material was isolated from serum by affinity chromatography on monoclonal IgM anti-HBs, and comparison of the material with HBsAg (by sodium dodecyl sulfate/polyacrylamide gel electrophoresis) demonstrated that the two shared several similar polypeptides. Furthermore, comparison of the binding properties of HBsAg and concentrated monoclonal immunoreactive material with conventional and monoclonal anti-HBs reagents demonstrated some antigenic crossreactivity. The molecular weight of the monoclonal immunoreactive material was approximately 2 X 10(6). Immunoprecipitation of the material with monoclonal IgM antibodies and examination by electron microscopy revealed clumped and "spiculated" particles that resembled 22-nm hepatitis B particles coated with the same antibody. Thus, this study suggests that the high-binding-activity material, detected in serum only by the monoclonal radioimmunoassay, is not identical with HBsAg, but it shares some common properties.