Intracellular collagen fibrils in early gingivitis in the beagle dog

Mechanical signaling through the discoidin domain receptor 1 plays a central role in tissue fibrosis

The preservation of tissue and organ architecture and function depends on tightly regulated interactions of cells with the extracellular matrix (ECM). These interactions are maintained in a dynamic equilibrium that balances intracellular, myosin-generated tension with extracellular resistance conferred by the mechanical properties of the extracellular matrix. Disturbances of this equilibrium can lead to the development of fibrotic lesions that are associated with a wide repertoire of high prevalence diseases including obstructive cardiovascular diseases, muscular dystrophy and cancer. Mechanotransduction is the process by which mechanical cues are converted into biochemical signals. At the core of mechanotransduction are sensory systems, which are frequently located at sites of cell-ECM and cell-cell contacts. As integrins (cell-ECM junctions) and cadherins (cell-cell contacts) have been extensively studied, we focus here on the properties of the discoidin domain receptor 1 (DDR1), a tyrosine kinase that mediates cell adhesion to collagen. DDR1 expression is positively associated with fibrotic lesions of heart, kidney, liver, lung and perivascular tissues. As the most common end-point of all fibrotic disorders is dysregulated collagen remodeling, we consider here the mechanical signaling functions of DDR1 in processing of fibrillar collagen that lead to tissue fibrosis.

M2-like macrophages are responsible for collagen degradation through a mannose receptor-mediated pathway

Tissue remodeling processes critically depend on the timely removal and remodeling of preexisting collagen scaffolds. Nevertheless, many aspects related to the turnover of this abundant extracellular matrix component in vivo are still incompletely understood. We therefore took advantage of recent advances in optical imaging to develop an assay to visualize collagen turnover in situ and identify cell types and molecules involved in this process. Collagen introduced into the dermis of mice underwent cellular endocytosis in a partially matrix metalloproteinase-dependent manner and was subsequently routed to lysosomes for complete degradation. Collagen uptake was predominantly executed by a quantitatively minor population of M2-like macrophages, whereas more abundant Col1a1-expressing fibroblasts and Cx3cr1-expressing macrophages internalized collagen at lower levels. Genetic ablation of the collagen receptors mannose receptor (Mrc1) and urokinase plasminogen activator receptor-associated protein (Endo180 and Mrc2) impaired this intracellular collagen degradation pathway. This study demonstrates the importance of receptor-mediated cellular uptake to collagen turnover in vivo and identifies a key role of M2-like macrophages in this process.

Complementary roles of intracellular and pericellular collagen degradation pathways in vivo

Collagen degradation is essential for cell migration, proliferation, and differentiation. Two key turnover pathways have been described for collagen: intracellular cathepsin-mediated degradation and pericellular collagenase-mediated degradation. However, the functional relationship between these two pathways is unclear and even controversial. Here we show that intracellular and pericellular collagen turnover pathways have complementary roles in vivo. Individual deficits in intracellular collagen degradation (urokinase plasminogen activator receptor-associated protein/Endo180 ablation) or pericellular collagen degradation (membrane type 1-matrix metalloproteinase ablation) were compatible with development and survival. Their combined deficits, however, synergized to cause postnatal death by severely impairing bone formation. Interestingly, this was mechanistically linked to the proliferative failure and poor survival of cartilage- and bone-forming cells within their collagen-rich microenvironment. These findings have important implications for the use of pharmacological inhibitors of collagenase activity to prevent connective tissue destruction in a variety of diseases.

Intracellular collagen degradation mediated by uPARAP/Endo180 is a major pathway of extracellular matrix turnover during malignancy

We recently reported that uPARAP/Endo180 can mediate the cellular uptake and lysosomal degradation of collagen by cultured fibroblasts. Here, we show that uPARAP/Endo180 has a key role in the degradation of collagen during mammary carcinoma progression. In the normal murine mammary gland, uPARAP/Endo180 is widely expressed in periductal fibroblast-like mesenchymal cells that line mammary epithelial cells. This pattern of uPARAP/Endo180 expression is preserved during polyomavirus middle T-induced mammary carcinogenesis, with strong uPARAP/Endo180 expression by mesenchymal cells embedded within the collagenous stroma surrounding nests of uPARAP/Endo180-negative tumor cells. Genetic ablation of uPARAP/Endo180 impaired collagen turnover that is critical to tumor expansion, as evidenced by the abrogation of cellular collagen uptake, tumor fibrosis, and blunted tumor growth. These studies identify uPARAP/Endo180 as a key mediator of collagen turnover in a pathophysiological context.

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.

Ultrastructural morphometry of membrane-bound intracytoplasmic collagen fibrils in tendon fibroblasts exposed to He:Ne laser beam

Collagen fibrils are not found in fibroblast cytoplasm except in certain pathological conditions or in the presence of drugs and other agents that accelerate collagen turnover. Because low energy laser photostimulation is both a non-pathogenic and non-chemical accelerator of collagen synthesis, its effects were studied on four groups of calcaneal tendons from 18 rabbits (1) to test the hypothesis that vacuolar fibrils are not produced exclusively by diseases and chemical agents, and (2) to compare the morphometry of matrical and vacuolar fibrils. The right calcaneal tendons of nine rabbits were surgically tenotomized and repaired; six of these were transcutaneously irradiated with He:Ne laser everyday. The right calcaneal tendon of six of the remaining nine rabbits were similarly irradiated with laser, but without prior tenotomy and repair. 21 days later, all tendons were fixed in situ and processed for electron microscopy. Fibril-bearing vacuoles were found only in fibroblasts of tenotomized laser-irradiated tendons. Similar vacuoles were not seen in non-tenotomized laser-irradiated tendons nor in non-irradiated tendons whether tenotomized or not. Mann-Whitney U tests revealed no statistically significant differences in the cross-sectional areas or diameters of matrical and vacuolar fibrils. These findings suggest (a) that matrical and vacuolar fibrils have a common origin, and (b) that vacuolar fibrils can be induced by a non-pathologic, non-chemical accelerator of collagen synthesis.

Membrane-bound intracellular collagen fibrils in fibroblasts and myofibroblasts of regenerating rabbit calcaneal tendons

The ultrastructures of 33 rabbit calcaneal tendons were studied to determine (1) whether vacuolar fibrils are present in three regions of tendons undergoing normal healing after tenotomy and repair, and (2) to stimulate collagen synthesis via functional loading, and hence determine the effect of loading on the presence of vacuolar fibrils in healing tendons. In all the loaded tendons, electron microscopy revealed membrane-bound collagen fibril equivalents in sections of neotendon obtained from the site of tenotomy, and in sections of tendon segments proximal and distal to the site of surgery. Similar vacuolar fibrils were visualized in sections of the proximal and distal segments of the non-loaded regenerating tendons, and also in sections of neotendons formed at the site of tenotomy after 12 and 15 days of healing without functional loading. No such fibrils were visualized in the non-tenotomized normal control tendons. These findings indicate that chemical agents and disease are not necessary to induce the appearance of intracytoplasmic fibrils in vivo and that functional loading augments the presence of fibril-bearing vacuoles in regenerating tendons.

Activation of the vascular system: a main mediator of periodontal fiber remodeling in orthodontic tooth movement

The behavior and role of blood vessels and blood-borne cells in the process of the remodeling of the periodontal ligament (PDL) incident to experimental tooth movement was studied in rats. Particular interest was focused on areas of tension and of pressure with frontal bone resorption but without overt hyalinization. An increase of vascular activity occurred in the above mentioned situations. Extensive breakdown of collagen was observed in pressure areas with frontal resorption and in areas of tension concomitant with vascular invasion. Two patterns of fiber and bone remodeling were seen in areas of tension: intense vascular activity within the periodontal membrane and intense vascular activity inside the alveolar bone. Macrophages occurred consistently near blood vessels both in areas of tension and in areas of resorption. These are multipotent cells that obviously influence the remodeling process.

Bone resorption in osteogenic sarcoma--II. Resorption of the bone collagenous matrix by tumor cells, normal fibroblasts and macrophages

Three osteogenic sarcoma cell lines of human and canine origin were compared to normal fibroblastic cells and peptone-induced murine peritoneal macrophages in terms of bone collagenous matrix (BCM) resorption capacity. The dissolution of the BCM was measured in an in vitro system consisting of the tested cells and live or killed [3H]-proline-labeled fetal mouse long bones. Experiments with osteogenic sarcoma cells revealed a paradoxical phenomenon indicating an inverse relationship between the number of tumor cells and the rate of collagen resorption from live bones. On the other hand, collagen matrix of devitalized bones, particularly those denatured by exposure to heat, is strongly resorbed by osteogenic sarcoma cells even in the presence of serum. Contrary to osteogenic sarcoma cells, normal fibroblasts do not resorb collagen from either live or killed bones, regardless of the devitalization method and culture conditions utilized. Macrophages resorb collagenous matrix from live bones, but their collagen resorption activity from devitalized bones depends greatly on choice of the incubation condition. Results have shown that osteogenic sarcoma tumor cells, when acting alone, may not have the capacity to destroy healthy bone. We suggest, therefore, that bone destruction seen in osteogenic sarcoma patients depends on the metabolic condition of the affected bone, and the interaction between the tumor and normal host cells and tissues.

Ultrastructural changes of epithelium-connective tissue junction in experimental lingual tumors

In this study, plaque-like lesions, epithelial hyperplasias, and papillary invasive carcinomas were produced in the tongues of hamsters by the application of DMBA combined with trauma for 16 weeks. The junction between the neoplastic epithelial cells and the connective tissue was studied ultrastructurally. In plaque-like lesions, the basal lamina was partially decreased in both thickness and density or partially discontinuous. In epithelial hyperplasias, cytoplasmic projections were observed in the portion of the basal cell near the intercellular space, and the basal lamina was absent around these cytoplasmic projections. In papillary invasive carcinomas, however, numerous cytoplasmic projections extended from various portions of the basal surface of the basal cells into the underlying connective tissue. The basal lamina was also absent around these projections. In epithelial hyperplasias and papillary invasive carcinomas, some of the cytoplasmic projections appeared almost empty, some contained a small number of tonofilaments and ribosomes, and others were filled with a large number of ribosomes but did not contain the tonofilaments. The collagen fibrils were markedly reduced in number in the area around cytoplasmic projections. The cytoplasm of some fibroblasts in that area contained many intracellular collagen fibrils.

Ultrastructure of gastric myxofibroma with intracytoplasmic collagen

Electron microscopy of a small mass removed from the stomach of a 65-year-old women demonstrated a myxofibroma composed of a pure population of fibroblasts, many of which contained intracytoplasmic, membrane-bound collagen fibers similar to those found in a few other human tumors and in certain experimental conditions. This ultrastructural documentation supports the hypothesis that pure fibrous neoplasms of the stomach do occur. Electron microscopy of gastric mesenchymal tumors almost always allows for confident differentiation among fibrous, neural, and smooth muscle neoplasms.

Current concepts of the dentogingival junction: the epithelial and connective tissue attachments to the tooth

This review leads to a concept in which the tissues of the dentogingival junction are dynamic rather that static. Even when they are pathologic, they can be reconstituted by repair. Both their cellular and extracellular components exhibit a high rate of turnover. Some of the cells are specialized for specific functions, such as attachment formation, and do not generate additional cells, but generative pools are always nearby. The cells are capable of movement and of positional change. The junctional epithelium can advance and retract. The cuticle width is alterable. The entire tissue is capable of regeneration after wounding. This dynamic group of tissues is well adapted for the healing of direct injuries produced during mastication. The tissues do remarkably well, over long periods, in their response to periodontal disease, whether due to direct bacterial or toxic damage, or to indirect damage via the migration of inflammatory cells into the lesion. The tissues show a capacity for repair and regeneration following the elimination of plaque formation and the resultant resolution of the inflammatory infiltrate. The complete story is not yet developed. The past 60 years are replete with fine contributions by distinguished workers. Additional contributions continue to be made. The inheritance from our predecessors has been used well and our expanded knowledge in this area now serves as the conceptual framework for further study.

Collagenase in the human periodontal ligament

Collagenase activity was demonstrated by direct incubation to be present in human periodontal ligament. This activity was found in only one of two populations of ligament, i.e., those ligaments taken from teeth in which their attachment site was at least 2.5 mm apical to the cementoenamel junction. The collagenase was demonstrated to be of host origin because it degraded collagen into 3/4 and 1/4 alpha chain fragments characteristic of mammalian collagenases. The enzyme was shown to be inhibited in the presence of EDTA and to have a pH optimum of 7.5.

Ultrastructural observation of rickettsia-like bodies in erythema cronicum migrans

Electron microscopic examination of skin biopsies taken from the lesions in two cases of erythema chronicum migrans revealed Rickettsia-like bodies free in the cytoplasm or within phagolysosomes in macrophages. There was also phagocytosis of cellular debris and collagen, and elastic fibers within macrophages.

The possible role of the fibroblast in granuloma-induced bone resorption in the rat

Granulomas were produced over the calvaria of rats by injection of a mixture of turpentine and peanut oil. The granulomas induced localised areas of bone resorption and these resorptive lesions were examined at the fine structural level. A close association was found between osteoclasts and fibroblasts at resorbing surfaces. Furthermore, fibroblasts but not osteoclasts were observed to engulf and phagocytose bone collagen fibrils. It is suggested that the fibroblast either alone or in collaboration with the osteoclast is involved in the destruction of bone collagen during chronic inflammatory disorder of bone.

Microscopic assay for the phagocytotic-collagenolytic performance (PCP index) of human gingival fibroblasts in vitro

Using 16 human gingival fibroblasts cell lines from patients with periodontitis, Dilantin hyperplasia, and nonpathological gingiva, a microscopic assay was developed to quantitate the cells' ability to lyse collagen substrates. The method employs tissue culture chambers with one cover slip partially coated with a thin layer of undenatured fibrillar bovine collagen. The assay measures the relative numbers and sizes of holes in the collagen within defined regions of the cover slips effected by the phagocytotoc and collagenolytic performance (PCP) of the population of fibroblasts growing on the cover slip for 5 days. The effect on the PCP index by serum, heparin, prostaglandins, and endotoxin was evaluated.