A stereologic analysis of collagen phagocytosis by fibroblasts in three soft connective tissues with differing rates of collagen turnover

Antifibrotic effects of sulforaphane treatment on gingival elasticity reduces orthodontic relapse after rotational tooth movement in beagle dogs

Objective

Increased gingival elasticity has been implicated as the cause of relapse following orthodontic rotational tooth movement and approaches to reduce relapse are limited. This study aimed to investigate the effects of sulforaphane (SFN), an inhibitor of osteoclastogenesis, on gene expression in gingival fibroblasts and relapse after rotational tooth movement in beagle dogs.

Methods

The lower lateral incisors of five beagle dogs were rotated. SFN or dimethylsulfoxide (DMSO) were injected into the supra-alveolar gingiva of the experimental and control group, respectively, and the effect of SFN on relapse tendency was evaluated. Changes in mRNA expression of extracellular matrix components associated with gingival elasticity in beagles were investigated by real-time polymerase chain reaction. Morphology and arrangement of collagen fibers were observed on Masson's trichrome staining of buccal gingival tissues of experimental and control teeth.

Results

SFN reduced the amount and percentage of relapse of orthodontic rotation. It also decreased the gene expression of lysyl oxidase and increased the gene expression of matrix metalloproteinase (MMP) 1 and MMP 12, compared with DMSO control subjects. Histologically, collagen fiber bundles were arranged irregularly and were not well connected in the SFN-treated group, whereas the fibers extended in parallel and perpendicular directions toward the gingiva and alveolar bone in a more regular and well-ordered arrangement in the DMSO-treated group.

Conclusions

Our findings demonstrated that SFN treatment may be a promising pharmacologic approach to prevent orthodontic rotational relapse caused by increased gingival elasticity of rotated teeth in beagle dogs.

In vitro phagocytosis of collagens by immortalised human retinal Müller cells

PURPOSE

This study is a first step to investigate phagocytosis of collagens by human retinal Müller cells, since Müller cells could be involved in remodelling of the vitreous and vitreoretinal interface in the human eye.

METHODS

Müller cells in culture were exposed to 2.0 microm fluorescent latex beads coated with BSA and human types I, II, and IV collagen and to non-coated beads for 2, 12, 24, and 48 h. To influence phagocytosis, cytochalasin B and anti-integrin subunits (alpha1, alpha2, and beta1) were added to the cells. Phagocytosis was evaluated by flow cytometry, transmission electron microscopy (TEM) and confocal microscopy.

RESULTS

Müller cells preferred to phagocytose beads coated with type II collagen compared with type IV collagen-, BSA- and non-coated beads. Phagocytosis of type I collagen-coated beads was intermediate. TEM and confocal microscopic evaluation confirmed phagocytosis of the beads. No significant differences were observed in phagocytosis of type II collagen-coated beads in the case of addition of cytochalasin B and anti-integrin subunits. Immunohistochemical analyses revealed that Müller cells were positive, under all tested circumstances, for vimentin and CRALBP. Less than 5% of the cells tested were GFAP positive.

CONCLUSIONS

Our observations demonstrate that human Müller cells in culture prefer to phagocytose type II collagen. In contrast, the phagocytosis of type IV collagen is comparable with the control coatings. We speculate that the relatively limited collagen phagocytosis by Müller cells supports a possible role for Müller cells in the slow process of vitreoretinal remodelling in adult human eyes.

Distinctive localization and function for lumican, fibromodulin and decorin to regulate collagen fibril organization in periodontal tissues

BACKGROUND

Small leucine-rich proteoglycans (SLRPs) decorin, biglycan, fibromodulin and lumican are secreted extracellular matrix molecules that associate with fibrillar collagens and regulate collagen fibrillogenesis. Collagens are the major extracellular matrix components of periodontal connective tissues where they provide mechanical attachment of the tooth to the bone and gingiva and mediate signals that regulate cell functions, including remodeling of the periodontal ligament and bone. Structural organization of collagen may also be important for the defense against periodontal disease, because in certain conditions abnormal collagen fibrils associate with increased susceptibility to periodontal disease.

OBJECTIVES

The purpose of this study was to find out the role of SLRPs to regulate collagen fibril and fibril bundle formation in periodontal tissues.

METHODS

The localization of SLRPs in human and mouse periodontal tissues was studied using immunohistochemical methods. To assess the function of SLRPs we studied periodontal tissues of mice harboring targeted deletions of decorin, fibromodulin or lumican genes and lumican and fibromodulin double knockout mice using histological and electronmicroscopical methods.

RESULTS

The SLRPs were coexpressed in human and mouse gingival and periodontal ligament connective tissues where they colocalized with collagen fibril bundles. Teeth in the knockout animals were fully erupted and showed normal gross morphology. Targeted deletion of decorin, fibromodulin, lumican or both lumican and fibromodulin resulted in abnormal collagen fibril and fibril bundle morphology that was most evident in the periodontal ligament. Each of the gene deletions resulted in a unique fibril and fibril bundle phenotype.

CONCLUSIONS

These findings indicate that decorin, fibromodulin and lumican coordinately regulate the fibrillar and suprafibrillar organization of collagen in the periodontal ligament.

Collagen Phagocytosis by Fibroblasts Is Regulated by Decorin

Decorin is a small, leucine-rich proteoglycan that binds to collagen and regulates fibrillogenesis. We hypothesized that decorin binding to collagen inhibits phagocytosis of collagen fibrils. To determine the effects of decorin on collagen degradation, we analyzed phagocytosis of collagen and collagen/decorin-coated fluorescent beads by Rat-2 and gingival fibroblasts. Collagen beads bound to gingival cells by alpha2beta1 integrins. Binding and internalization of decorin/collagen-coated beads decreased dose-dependently with increasing decorin concentration (p < 0.001). Inhibition of binding was sustained over 5 h (p < 0.001) and was attributed to interactions between decorin and collagen and not to decorin-collagen receptor interactions. Both the non-glycosylated decorin core protein and the thermally denatured decorin significantly inhibited collagen bead binding (approximately 50 and 89%, respectively; p < 0.05). Mimetic peptides corresponding to leucine-rich repeats 1-3, encompassed by a collagen-binding approximately 11-kDa cyanogen bromide fragment of decorin and leucine-rich repeats 4 and 5, previously shown to bind to collagen, were tested for their ability to inhibit collagen bead binding. Although the synthetic peptide 3 alone exhibited saturable binding to collagen, neither peptides 3 nor 1 and 2 markedly inhibited phagocytosis. Leucine-rich repeat 3 bound to a triple helical peptide containing the alpha2 integrin-binding site of collagen. When collagen beads were co-incubated with peptides 3 and 4, inhibition of collagen phagocytosis (55%) was equivalent to intact native/recombinant core protein. Thus a novel collagen binding domain in decorin acts cooperatively with leucine-rich repeat 4 to mask the alpha2beta1 integrin-binding site on collagen, an important sequence for the phagocytosis of collagen fibrils.

Packages of vitreous collagen (type II) in the human retina: an indication of postnatal collagen turnover?

The purpose of this study was to evaluate the vitreoretinal border in the (pre-)equatorial area in nonpathologic human donor eyes, because the majority of retinal defects induced by posterior vitreous detachment (PVD) are located there. Nine eyes (24-80 years) were fixed and embedded in Technovit 8100. After evaluation by light microscope, areas of interest were selected for immunotransmission electron microscope. Anti-type II collagen antibody was used to stain vitreous fibrils and lamellae; anti-type IV collagen antibody was used to identify the internal limiting lamina (ILL); anti-vimentin and anti-CD-68 antibodies stained retinal Muller cells and macrophages, respectively. Observations included fusing of lamellae with the ILL, an intravitreal course of the ILL, and clear focal interruptions in the ILL. In addition, an obvious finding was the presence of intraretinal packages of type II collagen. Interestingly these collagen packages were closely related to Muller cells and, in several eyes, also to macrophages, cell debris and interruptions in the ILL. In our opinion, the collagen packages can reflect the net result of a process of interactive remodelling, in which both breakdown and synthesis of vitreous and ILL collagens take place. Connections between vitreous and intraretinal collagen networks can make the (pre-)equatorial area more vulnerable to tearing and retinal detachment in the case of liquefaction and PVD.

Integrin clustering drives phagocytosis coupled to collagenase 1 induction through RhoA GTPase and superoxide production

Integrin-mediated phagocytosis in fibroblasts is associated to collagenase 1 induction when the particles are coated with high-affinity binding ligands. This study shows that the high density of ligand coating on the particle elicits RhoA-dependent particle uptake coupled to signal transduction. Integrin clustering induced by anti-integrin antibodies or cell surface-binding lectins is sufficient to trigger the pathway. The GTPase RhoA is recruited in response to integrin aggregation at the plasma membrane when uptake is inhibited at 4 degrees C and is necessary for particle engulfment, as function interference with the dominant negative mutant RhoAN19, but not with RacN17, abrogates particle ingestion. Phagocytosis driven by clustering is associated with signal transduction through a transient rise in cellular hydrogen peroxide production to induce a proinflammatory cascade leading to collagenase 1 induction.

Identification and characterization of the endocytic transmembrane glycoprotein Endo180 as a novel collagen receptor

Endo180, a member of the mannose receptor family, is constitutively recycled between clathrin-coated pits on the cell surface and intracellular endosomes. Its large extracellular domain contains an N-terminal cysteine-rich domain, a single fibronectin type II domain and eight C-type lectin-like domains. The second of these lectin-like domains has been shown to mediate Ca2+-dependent mannose binding. In addition, cross-linking studies have identified Endo180 as a urokinase plasminogen activator receptor-associated protein and this interaction can be blocked by collagen V. Here we demonstrate directly using in vitro assays, cell-based studies and tissue immunohistochemistry that Endo180 binds both to native and denatured collagens and provide evidence that this is mediated by the fibronectin type II domain. In cell culture systems, expression of Endo180 results in the rapid uptake of soluble collagens for delivery to lysosomal degradative compartments. Together with the observed restricted expression of Endo180 in both embryonic and adult tissue, we propose that Endo180 plays a physiological role in mediating collagen matrix remodelling during tissue development and homeostasis and that the observed receptor upregulation in pathological conditions may contribute to disease progression.

Phagocytosis of fibronectin and collagens type I, III, and V by human gingival and periodontal ligament fibroblasts in vitro

BACKGROUND

Electron microscopic studies have suggested that the volume density of collagen-containing vacuoles in fibroblasts is higher in the periodontal ligament (PDL) than in the gingiva. Whether this difference reflects intrinsic differences in phagocytic capacity among the cells in these tissues is not known.

METHODS

PDL and gingival fibroblasts were isolated from subjects and cultured under identical conditions in the presence of fluorescent beads coated with collagen type I, III, or V or fibronectin. Control beads were coated with bovine serum albumin or an enamel matrix protein mixture that does not constitute part of the extracellular matrix of PDL and gingiva. After various time intervals (1 to 24 hours), the percentage of cells that had internalized beads was assessed by flow cytometry. Since alkaline phosphatase activity has been suggested to play a role in collagen phagocytosis, the activity of this enzyme was determined for all cell populations.

RESULTS

The results demonstrated the following order in the percentage of cells internalizing protein-coated beads: fibronectin > collagen type I > III > V. Internalization of collagen type I-coated beads exceeded that of beads coated with bovine serum albumin or enamel matrix proteins by 6 and 3 times, respectively. No differences were observed in collagen phagocytic activity between PDL and gingival fibroblasts, and no relationship could be demonstrated between collagen phagocytosis and alkaline phosphatase activity.

CONCLUSIONS

We conclude that differences in collagen phagocytosis between PDL and gingiva, as observed in vivo, are not likely to be explained in terms of intrinsic phagocytic capacities of these cells.

Involvement of actin filaments and integrins in the binding step in collagen phagocytosis by human fibroblasts

In physiological conditions, collagen degradation by fibroblasts occurs primarily via phagocytosis, an intracellular pathway that is thought to require collagen receptors and actin assembly for fibril internalization and degradation. Currently it is unclear which specific steps of collagen phagocytosis in fibroblasts involve actin filament assembly. As studies of phagocytosis in fibroblasts are complicated by the relatively slow rate of particle internalization compared to professional phagocytes, we have examined the role of collagen receptors and actin only in the initial collagen binding step. Prior to the binding of collagen-coated fluorescent beads by human gingival fibroblasts, a cell type that is avidly phagocytic in vitro, cells were treated with cytochalasin D (actin filament barbed-end capping) or swinholide A (actin dimer sequestering and severing) or latrunculin B (actin monomer sequestering). Bead binding and immunostaining of (alpha)(2)(beta)(1) and (alpha)(3)(beta)(1) integrin collagen receptors were measured by flow cytometry. After 1–3 hours of coincubation with beads, cytochalasin D or swinholide A eliminated actin filaments stained by rhodamine-phalloidin and inhibited collagen bead binding (reductions of 25% and 50%, respectively), possibly because of cell rounding and restricted interactions with beads. In contrast, latrunculin enhanced binding dose-dependently over controls (twofold at 1 microM) and induced the formation of brightly staining aggregates of actin and the retention of long cytoplasmic extensions. Latrunculin also reduced surface (beta)(1), (alpha)(2) and (alpha)(3) integrin staining up to 40% in bead-free and bead-loaded cells, indicating that latrunculin enhanced collagen receptor internalization. As determined by fluorescence recovery after photobleaching, latrunculin increased the mobility of surface-bound (beta)(1) integrin. The stimulatory effect of latrunculin on collagen bead binding was reduced to control levels by treatment with a (beta)(1) integrin inactivating antibody while a (beta)(1) integrin blocking antibody abrogated both bead binding and the latrunculin-induced stimulation. Immunoblotting of bead-associated proteins showed that latrunculin completely eliminated binding of (beta)-actin to collagen beads but did not affect (beta)(1) integrin binding. These data indicate that latrunculin-induced sequestration of actin monomers facilitates the disengagement of actin from (beta)(1) integrin receptors, increases collagen bead binding and enhances collagen receptor mobility. We suggest that these alterations increase the probability of adhesive bead-to-cell interactions.

Decreased capacity of asthmatic bronchial fibroblasts to degrade collagen

The mechanisms of fibrillar collagen accumulation in asthmatic bronchi remain unclear, an imbalance between synthesis and degradation of collagen may be implicated in this process. The aim of this study was to compare the capacities of normal (BNF) and asthmatic (BAF) bronchial fibroblasts to degrade collagen. Metalloproteinases and their inhibitors were measured by ELISA, types I and III procollagen synthesis was determined by liquid RIA and, finally, zymography was used to assess the presence of active and latent forms of MMPs. The capacity of fibroblasts to degrade collagen coated onto latex beads was evaluated by flow cytometry. Our results showed that MMP-2 secretion was significantly higher in BNF when compared to BAF and this was confirmed by gelatin zymography. In BNF culture, TIMP-1 and MMP-1 secretions positively correlated with types I and III procollagen synthesis. However, in BAF, this correlation was negative. This suggests that a balance exists between collagen synthesis and degradation in BNF and that this balance is compromised in BAF. On the other hand, BAF did show significantly reduced capacity to degrade collagen when compared to that of BNF. This reduced phagocytic activity was not associated with a decrease in collagen receptor expression. This study establishes for the first time that a relationship exists between metalloproteinases enzyme dysregulation and the reduced capacity of asthmatic bronchial fibroblast to degrade collagen. These events may shed light on why accumulation of collagen can be observed in asthmatic airways.

Remodeling of a collagenous tissue at fixed lengths

Mature tissues can often adapt to changes in their chemical, mechanical, or thermal environment. For example, in response to sustained increases or decreases in mechanical loads, some tissues grow and remodel so as to restore the stress or strain to its homeostatic state. Whereas most previous work addresses gross descriptors of tissue growth, this paper introduces a possible cell-mediated mechanism by which remodeling may occur in a soft connective tissue--that the kinetics of collagen deposition and degradation is similar regardless of the configuration of the body at which it occurs. The proposed theoretical framework applies to three-dimensional settings, but it is illustrated by focusing on the remodeling of a uniaxial collagenous tissue that is maintained at a fixed length for an extended period. It is shown that qualitative features expected of such remodeling (e.g., an increased compliance and increased stress-free length when remodeling occurs at an extended length) are easily realized. Growth and remodeling are complex phenomena, however, and are likely accomplished via multiple complementary mechanisms. There is a need, therefore, to identify other candidate mechanisms and, of course, to collect experimental data suitable for testing and refining the possible theories.

Collagen-phagocytosing ability of periodontal osteoblasts at the bone surface

The collagen-phagocytosing activity of osteoblasts at the alveolar bone-ligament interface of rat mandibular first molars was investigated both histologically and histochemically. Alveolar bones of male Wistar rats (6 months old) were used in this study. Collagen-containing phagosomes appeared in cuboidal osteoblasts aligned on the bone surface. The 5.7% of the osteoblasts exhibiting alkaline phosphatase activity revealed collagen-containing phagosomes, and the collagen fibrils within the phagosomes were at various stages of degradation. In addition, acid phosphatase activity and the immunocytochemical distribution of cathepsin B were found in these collagen-containing phagosomes at similar locations. The presence of both enzymes in the phagosomes suggests that an intracellular degradation of collagen occurs. Therefore, in addition to the osteoblastic functions of synthesizing and secreting bone matrices, osteoblasts are also capable of phagocytosis and the intracellular disintegration of collagen. Our findings suggest that osteoblasts at the alveolar bone-periodontal ligament interface have a collagen-phagocytosing ability and play an important role in the physiological remodeling and metabolic breakdown of collagen fibrils of periodontal ligament without osteoclastic bone remodeling.

Deficiency in collagen and fibronectin phagocytosis by human buccal mucosa fibroblasts in vitro as a possible mechanism for oral submucous fibrosis

Oral submucous fibrosis (OSF), a chronic oral mucosal condition commonly found in south Asians, is a disorder characterized by a quantitative as well as a qualitative alteration of collagen deposition within the subepithelial layer of the oral mucosa. Since degradation of collagen by fibroblast phagocytosis is an important pathway for physiological remodelling of soft connective tissues, we have investigated phagocytosis of collagen- and fibronectin-coated latex beads by fibroblast cultures with an in vitro model system. Coated fluorescent latex beads were incubated with human oral mucosa fibroblasts and the fluorescence associated with internalized beads was measured by flow cytometry. Cells from normal tissues that had been incubated with beads for 16 h contained a mean of 75% collagen phagocytic cells and 70% fibronectin phagocytic cells; however, about 15% and 10% of phagocytic cells individually contained more than twice the mean number of beads per cell. In contrast, cells from OSF tissues exhibited a 40% reduction of the proportions of collagen phagocytic cells (mean=35%) and a 48% decrease of the proportions of fibronectin phagocytic cells (mean=22%), none of the cells having a high number of beads as compared to normal fibroblasts. OSF lesions appear to contain fibroblasts with marked deficiencies in collagen and fibronectin phagocytosis. To investigate if inhibition of phagocytosis could be demonstrated in vitro, normal fibroblast cultures were incubated with areca nut alkaloids (arecoline, arecaidine). The cultures had a dose-dependent reduction in the proportions of phagocytic cells. On the other hand, corticosteroid used in the treatment of OSF exhibited a dose-dependent enhancement in the proportion of phagocytic cells. Therefore, our hypothesis for OSF, although oversimplified, is that betel nut alkaloids (arecoline, arecaidine) inhibit fibroblast phagocytosis and this provides a mechanism for the development of OSF. The benefit of a local intralesional injection of corticosteroid is also possibly, at least in part, through an enhancement of fibroblast collagen phagocytosis.

Participation of intracellular cysteine proteinases, in particular cathepsin B, in degradation of collagen in periosteal tissue explants

The involvement of cysteine proteinases in the degradation of soft connective tissue collagen was studied in cultured periosteal explants. Using cysteine proteinase inhibitors that were active intracellularly or extracellularly (Ep453 and Ep475, respectively), it was shown that over-all collagen degradation, as measured by the release of hydroxyproline, decreased significantly on inhibition of the intracellular pool of cysteine proteinases by Ep453. This inhibitor also induced an accumulation of intracellular fibrillar collagen in fibroblasts, indicating a decreased degradation of phagocytosed collagen. The extracellular inhibitor, Ep475, had minor or no effects. Histochemical analysis using a substrate for the cysteine proteinases cathepsins B and L revealed a high level of enzyme activity, which was completely blocked in explants preincubated with a selective intracellular inhibitor of cathepsin B, Ca074-Me. Moreover, the cathepsin B inhibitor strongly affected collagen degradation, decreasing the release of hydroxyproline and increasing the accumulation of phagocytosed collagen. These effects were comparable or slightly stronger than those found with the general intracellular inhibitor (Ep453). Taken together, these data strongly suggest that intracellular cysteine proteinases, in particular cathepsin B, play an important role in the digestion of soft connective tissue collagen.

Deregulation of collagen phagocytosis in aging human fibroblasts: effects of integrin expression and cell cycle

Intracellular degradation of collagen by phagocytosis in fibroblasts is essential for physiological remodeling of the extracellular matrix in a wide variety of connective tissues but imbalances between degradation and synthesis can lead to loss of tissue collagen. As aging is associated with loss of dermal and periodontal collagen and with increased lysomomal enzyme content in fibroblasts, we examined the regulation of collagen phagocytosis by integrin expression and the cell cycle in an in vitro fibroblast aging model. Two different fibroblast lines (CL1; CL2) at the fourth subculture were passaged up to replicative senescence to model aging processes in vitro. Cells were incubated with collagen-coated or BSA-coated green fluorescent beads for 3 h to assess alpha 2 beta 1-integrin-mediated or nonspecific phagocytosis, respectively. Single-cell suspensions were stained with DAPI and sulforhodamine 101 to separate cycling G1 and noncycling G0 cells. Staining for alpha 2-integrin, bead internalization, and bivariate analyses of DNA/protein content were measured by three-color flow cytometry. Serum deprivation was used to induce increases in the proportion of G0 cells. For G1 cells, the proportion of collagen phagocytic cells was > 50% for all passages and collagen beads were internalized > 5-fold more frequently than BSA beads. In contrast, G0 cells with diploid DNA content but low protein content exhibited greatly reduced phagocytic capacity (< 10% of cells internalized collagen or BSA beads), the number of beads per cells was 4-fold less, and alpha 2 integrin expression was very low compared to G1 cells. The proportion of collagen phagocytic cells and the proportion of alpha 2-integrin-positive cells increased with transit through the cell cycle. At higher passage numbers mean cell volume and cytoplasmic granularity were reduced approximately 30% but at replicative senescence cells with large surface area and subdiploid DNA predominated. The proportion of collagen and BSA phagocytic G1 cells increased 1.5- and 5-fold, respectively, and the number of beads per cell increased < 3-fold. However, surface alpha 2-integrin staining remained unchanged. These data indicate that the collagen and nonspecific internalization pathways were greatly unregulated, independent of cell cycle phase, and that cellular aging in vitro strongly influences the specificity and rate of phagocytic processes in fibroblasts. We suggest that age-related loss of collagen in connective tissues undergoing turnover may be a manifestation of a deregulated increase of collagen phagocytosis in which the net loss of degraded collagen exceeds new synthesis.

Collagen phagocytosis and apoptosis are induced by high level alkaline phosphatase expression in rat fibroblasts

Study of fibroblast origins and lineages is complicated by the lack of unambiguous markers that could be used to identify discrete subpopulations on the basis of functional attributes. We have studied the role of the membrane-anchored hydrolytic enzyme tissue-nonspecific alkaline phosphatase (TN-AP) and the placental alkaline phosphatase (PL-AP) in collagen phagocytosis and in the deletion of cells by apoptosis. Rat-2 cells, which do not constitutively express AP, were transfected with full-length rat TN-AP or PL-AP cDNAs to determine the impact of the TN-AP collagen-binding domain on cell function. Various levels of expression were driven by early (strong) or late (weak) SV40 promoters in the plasmid construct. Controls were transfected with plasmids that did not contain AP cDNA. AP expression in transfected cells was confirmed by Northern blotting, histochemical analysis, and SDS-PAGE analysis of membrane-anchored enzyme released by phosphatidyl inositol phospholipase C. Low levels of TN-AP expression increased cell spreading slightly, nearly doubled the percentage of collagen phagocytic cells (up to 80%), and increased the number of internalized collagen-coated fluorescence beads per cell. In cells transfected with PL-AP (i.e., no collagen-binding domain), collagen phagocytosis was not affected. Internalization of BSA beads was also not affected by either AP isozyme, indicating that AP was selective for integrin-mediated phagocytosis. In single cells, histochemically demonstrable TN-AP activity on cell membranes was colocalized with the binding of collagen beads, but this colocalization was not detected in cells transfected with PL-AP. Phagocytosis was inhibited by antibodies to the alpha 2 integrin and to AP but not by levamisole, an inhibitor of AP phosphohydrolytic activity. High-level TN-AP expression caused a fivefold reduction of cell proliferation and was associated with the development of cells with sub-G1 DNA content, nuclear condensation, and nuclear budding. In AP-positive cultures, there was a greatly increased number of floating cells; nick-labeling of DNA by terminal transferase and biotinylated dUTP showed a 15-fold increase of stained cells. These data indicate that low-level TN-AP expression enhances collagen phagocytosis, presumably through the TN-AP collagen-binding domain. High-level AP expression promotes cell deletion by apoptosis. We suggest that the expression of AP by fibroblasts indicates a novel role for this enzyme in collagen degradation by phagocytosis.

Role of integrins in regulation of collagen phagocytosis by human fibroblasts

Phagocytosis of collagen fibrils by fibroblasts is an important pathway for degradation of extracellular matrix in mature connective tissues. To study regulatory mechanisms in phagocytosis, 2-microns fluorescent beads coated with either collagen (COL) or bovine serum albumin (BSA) were incubated with human gingival fibroblasts in vitro. For these studies single cell suspensions were prepared by trypsinization, and bead internalization and collagen receptor expression were assessed by flow cytometry. After 3-h incubations, up to 8-fold more cells internalized COL beads than BSA-coated beads. Increased collagen coating concentration was associated with elevated proportions of cells that internalized COL beads, and was observed also in the presence of competing fibronectin-coated beads. The number of beads per cell and the percent of phagocytic cells increased proportionally with higher bead loadings. At > 4 beads per cell a maximum of approximately 80% of cells were phagocytic. Cells reacted with mAbs against the alpha 1, alpha 2, and alpha 3 integrin subunits were, respectively, 5%, 98% and 93% positively stained above background controls. All cells that internalized COL beads exhibited alpha 2 staining but there were large proportions of phagocytic cells that were not stained for alpha 1. In unfixed cells, bead internalization caused an immediate reduction of surface staining of membrane-bound alpha 2 by approximately 55% which returned to control levels within 3 h, indicating that cell-surface alpha 2 was internalized by phagocytosis. Preincubation of cells with up to 8 COL beads per cell reduced the proportion of phagocytic cells and the number of internalized beads after a second COL bead incubation 4 h later. To assess the relationship between the percent of phagocytic cells and alpha 2 integrin levels, serum starvation and cycloheximide experiments were conducted. Compared to controls, serum starvation for 24 h induced a 3.2-fold increase of cells internalizing COL beads but did not alter alpha 2 staining levels. In contrast, 3 h cycloheximide treatment reduced alpha 2 staining to 60% of control levels and this treatment also inhibited COL bead internalization. GRGDTP peptide as well as mAbs against the alpha 1 and alpha 2 subunits significantly reduced internalization of COL beads by 1.8 to 2.6-fold, whereas GRGESP peptide and alpha 3 mAb exerted no effect. Internalization of BSA beads was not affected by any of these treatments. Collectively, these data indicate that the alpha 2 integrin, along with other, as yet unidentified components, is likely involved in COL bead internalization. The alpha 2 integrin subunit is rapidly recycled or synthesized following a phagocytic load. In contrast, the alpha 1 integrin is not directly required for phagocytosis but may regulate the internalization step.

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

Long-spacing collagen and proteoglycans in pathologic tissues

The presence of proteoglycans in fibrous long-spacing collagen (FLSC) was assessed in various pathologic tissues using the highly selective proteoglycan stains cuprolinic blue and polyethyleneimine. Two types of FLSC could be distinguished: one that contained proteoglycans and one that did not. The conditions in which these types occurred suggested a completely different physiologic significance. Also, their morphologies were different: The FLSC containing proteoglycans constituted compact, often fusiform structures long known in diagnostic electron microscopy as Luse bodies. In accordance with the literature, this compact type of FLSC was found especially in schwannomas and other neurogenic tumors. Enzymatic digestion experiments indicated that the proteoglycan present was dermatan sulfate proteoglycan. The average periodicities measured ranged from 101 to 147 nm. The type of FLSC lacking proteoglycans, on the other hand, formed dispersed aggregates. This dispersed FLSC had periodicities ranging from 79 to 103 nm (ie, just below those of the compact type). It was found only under circumstances in which there was high collagen breakdown and/or turnover. That dispersed FLSC is a marker for collagen degradation was further supported by its presence inside fibroblasts engaged in collagen phagocytosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Mid-term results with cultured epidermal autografts, allogenic skin transplants and cyclosporin A medication

Transplantation of allogenic split thickness skin grafts (STSG) and immunosuppression with cyclosporin A enable early and definitive skin replacement of extensive, deep partial and full thickness burns. Covering burn defects with definitely engrafted, allogenic dermis and cultivated epithelial autografts (CEA) permits the subsequent withdrawal of cyclosporin A medication. Light-microscopy examination of biopsies, taken 12 and 24 months postgrafting, and electron microscopy of biopsies taken 12 months postgrafting, demonstrates a re-established, but somewhat reduced anchoring of the CEA as compared with a normal epidermal-dermal junction. Clinical inspection, 20 months postgrafting, confirms the histological observations that epifascial transplantation is qualitatively inferior to placing the allogenic STSG on the subcutaneous tissue. In the first situation, the dermis is inelastic and collagen deposition is excessive, whereas in the second case collagen deposition is comparatively reduced and the dermis shows clinically some elasticity.

Deficiencies in collagen phagocytosis by human fibroblasts in vitro: a mechanism for fibrosis?

Degradation of collagen by fibroblast phagocytosis is an important pathway for physiological remodelling of soft connective tissues. Perturbations of this pathway may provide a mechanism for the development of fibrotic lesions. As collagen phagocytosis may be regulated by either a change of the proportions or the activity of phagocytic cells, we quantified phagocytosis with an in vitro model system. Collagen-coated fluorescent latex beads were incubated with human gingival fibroblasts and the fluorescence associated with internalized beads was measured by flow cytometry. Cells from normal tissues that had been incubated with beads for 3 hours contained a mean of 64% phagocytic cells; however, a small subpopulation (10% of phagocytic cells) contained more than threefold higher numbers of beads per cell than the mean. In contrast, cells from fibrotic lesions exhibited a large reduction of the proportions of phagocytic cells (mean = 13.8%) and there were no cells with high numbers of beads. On the basis of 3H-Tdr labeling, cells from fibrotic lesions that had internalized beads failed to proliferate, in contrast to phagocytic cells from normal tissues, which underwent repeated cell divisions. This result was not due to variations of cell cycle phase as there was no preferential internalization of beads during different phases of the cell cycle. The low phagocytic rate of cells from fibrotic lesions was also not due to asymmetric partitioning of phagosomes at mitosis as videocinemicrography of bead-labeled phagosomes in single, pre-mitotic cells demonstrated that > 90% of phagocytic cells equally partitioned beads to daughter cells. To investigate if inhibition of phagocytosis could be replicated in vitro, cells were incubated with the fibrosis-inducing drugs nifedipine or dilantin. These cultures exhibited marked (15-75%), dose-dependent reductions in the proportions of phagocytic cells, but there was no reduction in bead number per cell. Fibrotic lesions appear to contain fibroblasts with marked deficiencies in phagocytosis and the reduced phagocytic activity of these cells may contribute to unbalanced degradation and fibrosis.

Enhanced collagen phagocytosis by rat molar periodontal fibroblasts after topical application of lipopolysaccharide--ultrastructural observations and morphometric analysis

To investigate the effect of lipopolysaccharide (LPS) on phagocytic activity of collagen fibrils by periodontal fibroblasts, we studied rat molar gingival connective tissue and periodontal ligament under light and electron microscopy after topical application of LPS (5 mg/ml in physiological salt solution (PS)) on the gingival sulcus. Phagocytic activity of collagen fibrils by fibroblasts was evaluated by counting the number of collagen-containing vacuoles inside fibroblasts that were present within a defined area (1200 microns2). Values obtained from fibroblasts in the subepithelial connective tissue, the region near the alveolar crest, and the middle region of periodontal tissue were compared. Periodontal ligament fibroblasts showed increased phagocytosis of the collagen fibrils from 3 hours to 1 day after topical LPS application, but no differences were observed in the gingival tissue. The intracytoplasmic vacuoles containing collagen fibrils were of various sizes and shapes, showing positive for acid phosphatase and/or alkaline phosphatase reaction. Collagen phagocytic activity of the fibroblasts in the middle region of the periodontal ligament also increased after PS treatment. However, this was significantly less than that observed in LPS-treated animals (p less than 0.01). This study indicates that LPS may enhance the degradation of collagen by stimulating the phagocytic activity of the periodontal ligament fibroblasts.

Cellular roles in physiological root resorption of deciduous teeth in the cat

This study has attempted to assess the importance of mesenchymal cells, fibroblasts, cementoblasts, and mononuclear phagocytes (i.e., macrophages) in physiological root resorption of feline deciduous teeth. Deciduous incisors of three- to six-month-old kittens undergoing root resorption were investigated by means of electron microscopy. In an early phase of root resorption, the resorption organ consisted of many fibroblasts and relatively few macrophages and odontoclasts, the last with a wide, clear zone and narrow, immature, ruffled border. In the active phase of root resorption, the resorption organ contained many odontoclasts with a well-developed ruffled border and a reduced clear zone, cementoblasts, fibroblasts, macrophages, neutrophils, and many blood vessels. Cementoblasts were present usually on the resorbing dentin surface adjacent to odontoclasts and, in many cases, these cells communicated with each other via gap junctions. Cementoblasts frequently extended broad cell processes with secretion granules and with phagosomes containing collagen fibrils into the dentinal tubules exposed to resorption lacunae. Some macrophages exhibiting a clear zone-like structure also appeared on resorbing dentin surfaces. In the resting phase of root resorption, the dentin surface was covered mostly with cementoblasts resembling bone lining cells. There was an occasional macrophage, but no odontoclasts were observed during this phase. During removal of the periodontal ligament concomitant with root resorption, many fibroblasts phagocytosed mature collagen fibrils, as well as amorphous fluffy material. These results suggest that these mesenchymal cells, as well as odontoclasts, are essential for the cellular removal of dental hard and soft tissues during shedding of feline deciduous teeth.

Distribution of 35S-sulfate within the transseptal ligament of the mouse

The distribution of 35S-sulfate-labeled macromolecules was examined within three regions of the transseptal ligament: the 1) mesial, 2) middle and 3) distal thirds. Swiss mice, 6 weeks of age, were injected with 35S-sulfate and killed after 1, 6, and 12 hours and 1, 2, 3, 4, 5, and 7 days. Silver grains and cell nuclei were counted on autoradiographs which had been counterstained by the Van Gieson method, and mean counts were analyzed statistically. Analysis of variance revealed no significant differences in mean number of cell nuclei between regions throughout the course of the experiment. 35S-sulfate was rapidly incorporated into the transseptal ligament macromolecules. Grain counts were highest 6 hours after injections: counts were highest over the middle and lowest over the mesial thirds of the ligament. The rate of grain removal was significantly higher in the middle third compared to the mesial or distal thirds (P less than 0.001) and was significantly lower in the mesial third compared to the middle or distal thirds (P less than 0.001). The half-life of labeled macromolecules was significantly greater in the mesial and distal thirds than in the middle third (P less than 0.005). The data demonstrate significantly higher rates of turnover of 35S-sulfate-labeled macromolecules in the middle region of the transseptal ligament. Since cellular density was similar throughout the transseptal ligament, higher turnover rates of 35S-sulfate-labeled macromolecules probably indicate higher rates of cellular activity in this region, possibly a result of tissue remodeling coincident to stresses generated by occlusal forces and physiologic drift of the adjacent teeth.

Collagen degradation in the rabbit skin during short-term tissue culture

Full thickness rabbit skin explants were cultured on plastic dish for 1 week and the sequential morphological changes were examined daily by light and electron microscopy. During the cultured period, bundles of dermal collagen fibres gradually loosened and were removed from the upper dermis and from the cut margin of the explant, which was covered by a sheet of migrating epidermal cells. In these areas, cells containing phagocytosed collagen fibrils were observed from the 3rd day to the end of the culture period. These cells containing phagocytosed collagen fibrils included dermal fibroblasts and macrophages, epidermal keratinocytes and endothelial cells lining blood vessels. The presence of acid phosphatase activity in vacuoles containing the collagen fibrils suggested that intracellular degradation of collagen was occurring. In addition, extracellular collagen degradation was recognized around fibroblasts and beneath the migrating epidermis by the high collagenolytic activity at these sites. These findings suggest that both intra- and extracellular collagen degradation may participate in collagen removal from dermal connective tissue in cultured skin explants.

New attachment through periodontal treatment and orthodontic intrusion

The present study was performed to investigate the tissue reaction related to orthodontic intrusion of teeth with a reduced periodontium and further to evaluate the influence of oral hygiene on this reaction. In each of five Macaca fascicularis monkeys, periodontal tissue breakdown was induced around the premolars and the upper incisors by placing orthodontic elastic ligatures around the teeth. The breakdown procedure was continued until a minimum of four pockets could be measured on probing. Following removal of the elastics, a flap operation was performed. The pocket epithelium and granulation tissue were excised. During the surgical procedure, a notch was placed just above the bone. The teeth were divided postoperatively into four groups according to treatment: group 1 = flap operation, no oral hygiene program; group 2 = flap operation plus oral hygiene program three times per week; group 3 = flap operation plus intrusion, no oral hygiene program; and group 4 = flap operation plus intrusion plus oral hygiene. Groups 3 and 4 were subdivided into two observation periods. A total of 60 teeth corresponding to 120 approximate surfaces were studied. The animals were killed with perfusion; histologic sections were produced and stained alternatively with hematoxylin and eosin, and van Gieson's solution. The histologic analysis showed that new cementum formation and new collagen attachment were observed following the surgical procedure if the oral hygiene was maintained, but also demonstrated that the intrusion improved the quantity of new attachment if carried out under healthy conditions. New attachment was a consistent finding in group 4, but varied from 0.7 to 2.3 mm. In case of intrusion without oral hygiene, the results varied from moderate new attachment to an aggravation of the periodontal bone loss. On the basis of the results presented here, the combination of periodontal treatment and orthodontic intrusion seems to be a method by which improvement of the periodontal condition can be obtained, provided that both the biomechanical force system and the oral hygiene are kept under control.

Collagen remodelling in rat periodontal tissues: compensation for precursor reutilization confirms rapid turnover of collagen

Measurement of collagen turnover is complicated by the reutilization of isotopic precursors used to label the collagen. In an earlier study a novel approach was used to circumvent the problems of precursor recycling and unusually short half-lives were determined for collagen in adult rat periodontal tissues (Sodek, 1977). To verify these results we have used an alternate procedure devised by Poole (1971) in which the decay profile for the radiolabelled protein is corrected in accordance with the decay of the radiolabelled precursor. In this manner real half-lives for mature, neutral salt-insoluble collagen were determined as 3 days in the molar periodontal ligament, 6 days in the continuously erupting incisor ligament and approximately 10 days in the lamina propria of the gingiva, compared to apparent half-lives for these tissues of 6, 12 and approximately 20 days, respectively. The values calculated for actual half-lives are, therefore, approximately two-fold faster than values determined without compensating for reutilization, a difference that is in agreement with other protein turnover studies in which the effects of precursor reutilization have been measured. Although the real half-lives determined in this study indicate turnover rates for the periodontal tissues that are slightly slower than reported previously, the relative differences between the tissues in the rates of collagen turnover are similar. Moreover, the study confirms the existence of a remarkably high rate of collagen remodelling in these tissues.

Collagen phagocytosis by fibroblasts in the periodontal ligament of the mouse molar during the initial phase of hypofunction

This study was undertaken in order to determine whether hypofunction of teeth is associated with changes in collagen phagocytosis by fibroblasts of the periodontal ligament. In mice, the lower right molars were extracted and the animals killed one, two, three, four, or seven days later. The maxillary first molars with their surrounding periodontium were processed for electron microscopy and their periodontal ligament subjected to morphometric analysis. It was observed that, whereas the volume density of extracellular collagen in the ligament of the hypofunctional molars decreased from 50% to 30% during the course of the experiment, the fraction of fibrillar collagen ingested by the cells increased over two-fold. This increase was already manifest very shortly after the onset of the experiment and offers an explanation for the net loss of collagen fibrils from the extracellular space.

The influence of the estrous cycle on the volume density and appearance of collagen containing vacuoles in fibroblasts of the rat uterus

The influence of the estrous cycle on the number of intracellular collagen-containing vacuoles (CCVs) has been studied in the uterus of the virgin Wistar rat. CCVs seem to be involved in two processes. Translucent CCVs dark CCVs and dark residual bodies appear to be involved in collagen uptake and degradation, whilst the so-called filamentous bodies (FBs), which are large structures filled with amorphous filaments may be concerned with collagen synthesis. The volume density of these FBs is influenced by the estrous cycle and peak values are accompanied by the highest values of extracellular collagen.

Electron-microscopic study on the fibrillar network and fibrocyte--collagen interactions in the vitreous cortex at the ora serrata of human eyes with special regard to the role of disintegrating cells

The diameter of the fibrils in the normal vitreous cortex at the ora serrata of human eyes was found to range from 10.8 to 12.4 nm. There is no significant increase in thickness with age. Periodic cross-striation typical for collagen could be demonstrated by microdensitometry. In senile eyes, secondary lateral aggregations of the fibrils are a common factor, usually in proximity to disintegrating cells. Disintegrating cells in the vitreous cortex were already observable in childhood. They were more often seen with increasing age. In a pathological case, acid phosphatase could be demonstrated both within the vitreous fibrocytes and in the extracellular space, released from disintegrating cells. Fibrils and segments of banded material were observed within pits and invaginations, and in intracytoplasmic channels and vacuoles of the aged vitreous fibrocytes. Linear aggregations of ruthenium red-positive material were seen at the surface and within invaginations of the same cells. The findings are interpreted as an indication for phagocytosis and--or secretion of collagen by the vitreous fibrocytes even in senile eyes.

The effect of tension on collagen remodelling by fibroblasts: a stereological ultrastructural study

Peak levels of collagen phagocytic activity by fibroblasts during scar maturation have been shown to coincide with a reorientation of major fibre bundles in a process analogous to Wolff's Law. This activity has been shown to increase when greater tension exists across the scar. In this study, the influences of reduced tension on collagen remodeling was studied. In two groups of five rats the right achilles tendon was either: transected at its muscle insertion or was exposed in a similar fashion but left intact. The tendon samples were retrieved five days following initial surgery with a third group of five rats serving as a source of control tendon. Relative volume of collagen phagocytosis was measured at an ultrastructural level using stereological methods. No significant influence of reduced tension on collagen phagocytosis was observed. The average relative volume of fibroblast cytoplasm was significantly increased in the transected tendon. This may represent an inductive influence of as yet undetermined nature.

Multicentric reticulohistiocytosis: a report of a case and a review of the pathology

A case of multicentric reticulohistiocytosis (MR) in a 24-yr-old woman is presented. MR is a rare disorder characterized by progressive polyarthropathy and a papulo-nodular skin rash. The diagnosis was established by histological examination of biopsies of erythematous nodules on the fingers which showed circumscribed collections of large mononuclear cells and multinucleate giant cells in the reticular dermis. These were embedded in a fine network of mature fibrous tissue with a scanty lymphocytic infiltrate. Histochemical, immunopathological and ultrastructural investigations confirmed that the large mononuclear cells had the properties of macrophages. The histopathological features of MR are reviewed in the light of current knowledge of macrophage physiology, and evidence for lymphocyte-histiocyte interactions in the pathogenesis of this bizarre granulomatous disorder is presented.

A quantitative comparison of collagen phagocytosis in periodontal ligament and transseptal ligament of the rat periodontium

The phagocytosis of collagen within soft connective tissues can be used as an index of collagen turnover and remodeling. The objective of this study was to compare the levels of collagen phagocytosis in periodontal and transseptal ligaments in the rat in order to provide an indication of the relative collagen-turnover activities within these two tissues. Periodontal and transseptal ligaments from two rats were examined by the techniques of electron microscope stereology. Parameters quantitated included cytoplasm (CC), extracellular collagen (ECC), and intracellular or phagocytosed collagen (ICC). The results indicated that the cells of the transseptal ligament have significantly more ICC than those of periodontal ligament. This suggests that the transseptal ligament in the rat may have a high rate of collagen turnover. There seems to be an increasing body of evidence, including the present results, that the transseptal ligament region of gingiva has a high level of collagen turnover. It seems likely, therefore, that, unless there are marked species differences in collagen turnover, slow remodeling of the transseptal ligament is not responsible for orthodontic relapse. Why relapse of rotated teeth can be reduced by surgical severing or removal of the transseptal tissue remains to be determined.

Fibronectin and wound healing

I have tried to briefly review the evidence (summarized in Table II) indicating that fibronectin is important in cutaneous wound healing. Fibronectin appears to be an important factor throughout this process. It promotes the spreading of platelets at the site of injury, the adhesion and migration of neutrophils, monocytes, fibroblasts, and endothelial cells into the wound region, and the migration of epidermal cells through the granulation tissue. At the level of matrix synthesis, fibronectin appears to be involved both in the organization of the granulation tissue and basement membrane. In terms of tissue remodeling, fibronectin functions as a nonimmune opsonin for phagocytosis of debris by fibroblasts, keratinocytes, and under some circumstances, macrophages. Fibronectin also enhances the phagocytosis of immune-opsonized particles by monocytes, but whether this includes phagocytosis of bacteria remains to be determined. In general, phagocytosis of bacteria has not appeared to involve fibronectin. On the contrary, the presence of fibronectin in the wound bed may promote bacterial attachment and infection. Because of the ease of experimental manipulations, wound healing experiments have been carried out on skin more frequently than other tissues. As a result, the possible role of fibronectin has not been investigated thoroughly in the repair of internal organs and tissues. Nevertheless, it seems reasonable to speculate that fibronectin plays a central role in all wound healing situations. Finally, the wound healing problems of patients with severe factor XIII deficiencies may occur because of their inability to incorporate fibronectin into blood clots.

Fibronectin-mediated binding and phagocytosis of polystyrene latex beads by baby hamster kidney cells

The binding and phagocytosis of fibronectin (pFN)-coated latex beads by baby hamster kidney (BHK) cells was studied as a function of fibronectin concentration and bead diameter. Cells were incubated with radioactive pFN-coated beads, and total bead binding (cell surface or ingested) was measured as total radioactivity associated with the cells. Of the bound beads, those that also were phagocytosed were distinguished by their insensitivity to release from the cells by trypsin treatment. In continuous incubations, binding of pFN-coated beads to cells occurred at 4 degrees C or 37 degrees C, but phagocytosis was observed only at 37 degrees C. In addition, degradation of 3H-pFN from ingested beads occurred at 37 degrees C, as shown by the release of trichloroacetic acid-soluble radioactivity into the incubation medium. When the fibronectin density on the beads was varied, binding at 4 degrees C and ingestion at 37 degrees C were found to have the same dose-response dependencies, which indicated that pFN densities that permitted bead binding were sufficient for phagocytosis to occur. The fibronectin density for maximal binding of ingestion was approximately 250 ng pFN/cm2. When various sized beads (0.085-1.091 micron), coated with similar densities of pFN, were incubated with cells at 4 degrees C, no variation in binding as a function of bead size was observed. Under these conditions, the absolute amount of pFN ranged from less than 100 molecules on the 0.085-micron beads to greater than 15,000 molecules on the 1.091-micron beads. Based upon these results it can be concluded that the critical parameter controlling fibronectin-mediated binding of latex beads by BHK cells is the spacing of the pFN molecules on the beads. Correspondingly, it can be suggested that the spacing between pFN receptors on the cell surface that is optimal for multivalent interactions to occur is approximately 18 nM. When phagocytosis of various sized beads was compared, it was found that the largest beads were phagocytosed slightly better (two fold) than the smallest beads. This occurred both in continuous incubations of cells with beads and when the beads were prebound to the cells. Finally, the kinetic constants for the binding of 0.085 microM pFN-coated beads to the cells were analyzed. There appeared to be approximately 62,000 binding sites and the KD was 4.03 X 10(-9) M. Assuming a bivalent interaction, it was calculated that BHK cells have approximately 120,000 pFN receptors/cell and the binding affinity between pFN and its receptor is approximately 6 X 10(-5) M.

The collagen fibrils in the collapsed and the chronically stretched intestinal wall

A partial and progressive obstruction of the ileum (stenosis) was produced by surgery in adult rats and guinea pigs. Oral to the stenosis the accumulation of ingesta imposed a condition of chronic stretch on the intestinal wall: the lumen was much distended and the wall increased in thickness, mainly by hypertrophy of the mucosa and muscle coat. The submucosa too increased in volume and its collagen fibrils showed marked ultrastructural changes: in the rat the collagen fibrils (which were of rather uniform diameter in the control submucosa, mean 87 nm) varied greatly in size and formed two distinct populations: large fibrils (similar to those of controls) and small fibrils (4-5 nm in diameter). The latter were probably newly formed fibrils and their number increased in the more advanced stages of hypertrophy. The small fibrils were usually gathered into groups of 15-60 but were found throughout the entire thickness of the collagen bundles. Aboral to the stenosis the intestine was collapsed. In the submucosa there was a widening of the range of fibril sizes, a small increase in the average size and the occurrence of very large and irregular fibrils. Similar changes occurred in the guinea pig; however, the size of the control collagen fibrils (57 nm) was smaller than in the rat, and in the condition of chronic stretch small and large fibrils could not be clearly separated into distinct populations.

A role for collagen phagocytosis by fibroblasts in scar remodeling: an ultrastructural stereologic study

A role for collagen phagocytosis and intracellular degradation by fibroblasts during remodeling activity has been suggested by studies on several connective tissues characterized by high rates of collagen turnover and remodeling. The possible importance of such activity in the normal remodeling of scar tissue has been studied by a quantitative ultrastructural stereologic measure of collagen phagocytosis by fibroblasts at various post-wounding intervals in mouse skin scars. The results demonstrate a correlation between the peak periods of such phagocytic activity and the interval during which collagen fiber reorientation across the scar appears to take place.

Cell density and cell generation in the periodontal ligament of mice

The number of cell nuclei per mm2 and the volume density of cell nuclei and blood vessels in the periodontal ligament at different levels of the mesial root of the first mandibular molar of the adult mouse and in different areas of the ligament at each level have been examined in the light microscope. Significantly higher numbers of cell nuclei per mm2 were observed adjacent to bone, cementum, and blood vessels than in the avascular body of the ligament at all levels and on all aspects of the root. This distribution of number of cell nuclei per mm2 was constant over 4 1/2 months of aging and a doubling of body weight. The volume density of cell nuclei was significantly higher in cells adjacent to bone and cementum and in gingival connective tissue than in both the vicinity of blood vessels and the body of the ligament. The blood vessels, which were present predominantly in the bone-related half of the ligament, were absent from the zone immediately adjacent to cementum. The labeling indexes of periodontal ligament cells were determined from autoradiographs of the mesial root of the first mandibular molar of the mouse after pulse-labeling with 3H-Tdr. Labeling indexes were highest in zones adjacent to blood vessels, and the labeling index was significantly higher in the middle of the ligament than in zones adjacent to bone and cementum, and consequently was inversely related to cell density.

Collagen fibril diameter and its relation to collagen turnover in three soft connective tissues in the rat

Collagen fibril diameters were measured in electron micrographs of rat skin, gingiva and periodontal ligament. Gingiva was divided into two zones, termed elastin-containing gingiva and attached gingiva, depending on the presence or absence of elastic fibrils. The results revealed that skin had the largest fibrils, followed by elastin-containing gingiva, attached gingiva and periodontal ligament respectively. These differences in fibril diameter were highly significant. The observed trend in fibril diameter was the inverse of that documented for collagen turnover and collagen phagocytosis in the same tissues. A link between fibril diameter and collagen turnover is discussed.

A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.

A stereologic analysis of collagen phagocytosis by periodontal ligament fibroblasts during occlusal hypofunction in the rat

Maxillary molars were extracted unilaterally from 4 young male rats so as to leave the agonist mandibular molars in hypofunction. The mandibular molars on the opposite side of the mouth acted as controls. Three days later, the animals were killed by perfusion and all mandibular molars removed en bloc. After demineralization, each block was trimmed, prepared for electron microscopy and embedded in Epon. The Epon blocks were trimmed so as to comprise the middle-third of the buccal periodontal ligament of the mesio-buccal root of the first molar together with some cementum and alveolar bone as landmarks. Two light-gold sections were cut from each trimmed block and systematically photographed in the electron microscope. The photographs were analysed stereologically so as to quantitate phagocytosed collagen (ICC), cytoplasm (CC) and extracellular fibrillar collagen (ECC). The results for the hypofunctional and contralateral control ligaments were compared with each other and with earlier results from similar but untreated (baseline) animals. The results were expressed as mean values for ICC, CC and ECC, and as the ratios ICC/ECC (the fraction of extracellular collagen phagocytosed), ICC/CC (collagen phagocytic activity per unit cytoplasm) and CC/ECC (cell density per unit extracellular collagen fibril). Significant differences were found for all measurements except ICC/ECC which remained the same in the three states of the ligament. The hypofunctional ligaments had the lowest collagen phagocytic activity and the highest cell density, whereas the ligaments from the untreated animals had the highest collagen phagocytic activity and the lowest cell density. The values for the contralateral control ligaments were intermediate between those for the other two states of the ligament. There was also a significant loss of recognizable, extracellular fibrillar collagen in the hypofunctional ligament. It is suggested that this loss of ECC and its replacement by less organized, pre-fibrillar forms of collagen may be an important mechanism in tooth eruption.