Fibrogenesis and biosynthesis of elastin in cartilage

Development of scaffold-free elastic cartilaginous constructs with structural similarities to auricular cartilage

External ear reconstruction with autologous cartilage still remains one of the most difficult problems in the fields of plastic and reconstructive surgery. As the absence of tissue vascularization limits the ability to stimulate new tissue growth, relatively few surgical approaches are currently available (alloplastic implants or sculpted autologous cartilage grafts) to repair or reconstruct the auricle (or pinna) as a result of traumatic loss or congenital absence (e.g., microtia). Alternatively, tissue engineering can offer the potential to grow autogenous cartilage suitable for implantation. While tissue-engineered auricle cartilage constructs can be created, a substantial number of cells are required to generate sufficient quantities of tissue for reconstruction. Similarly, as routine cell expansion can elicit negative effects on chondrocyte function, we have developed an approach to generate large-sized engineered auricle constructs (≥3 cm(2)) directly from a small population of donor cells (20,000-40,000 cells/construct). Using rabbit donor cells, the developed bioreactor-cultivated constructs adopted structural-like characteristics similar to native auricular cartilage, including the development of distinct cartilaginous and perichondrium-like regions. Both alterations in media composition and seeding density had profound effects on the formation of engineered elastic tissue constructs in terms of cellularity, extracellular matrix accumulation, and tissue structure. Higher seeding densities and media containing sodium bicarbonate produced tissue constructs that were closer to the native tissue in terms of structure and composition. Future studies will be aimed at improving the accumulation of specific tissue constituents and determining the clinical effectiveness of this approach using a reconstructive animal model.

Stromal elastosis in papillary thyroid carcinomas

Stromal elastosis, defined as dense aggregations of elastic fibers, is found in some neoplastic tissues especially in malignant tumors of the breast and lung. Although also found in thyroid tissue, stromal elastosis in thyroid neoplasms have received little attention. To clarify the histopathological significance of stromal elastosis in the thyroid, we examined neoplastic (n = 223) and hyperplastic (n = 82) thyroid tissues in conjunction with cancer tissues (n = 193) of various other organs. Stromal elastosis was observed as deposits of pale homogeneous material in hematoxylin and eosin stain, and distinctively highlighted by elastic-van Gieson's stain. On immunohistochemical examination, elastin and tropoelastin were confirmed in these deposits. Stromal elastosis was found in 66% of papillary thyroid carcinomas (PTCs), although it was not identified in other histological types of thyroid neoplasms. In PTCs, deposits of elastic fibers varied in size and shape, and were more frequently distributed in the periphery of the tumor tissue. The histological subtypes of PTC varied in prevalence of elastosis with the follicular variant's (9%) prevalence being significantly lower than that of the classical type (72%). The frequency of stromal elastosis in PTCs was very similar to the frequencies in breast and lung adenocarcinomas, and higher than the frequencies in carcinomas of other organs. In conclusion, our results suggest that stromal elastosis is a characteristic histological finding of PTCs, presumably associated with their growth pattern and/or histological architecture. It is, therefore, reasonable to propose that stromal elastosis is an ancillary feature in the histopathological diagnosis of PTCs.

Developmental expression of fibrillin genes suggests heterogeneity of extracellular microfibrils

Extracellular microfibrils, alone or in association with elastin, confer critical biomechanical properties on a variety of connective tissues. Little is known about the composition of the microfibrils or the factors responsible for their spatial organization into tissue-specific macroaggregates. Recent work has revealed the existence of two structurally related microfibrillar components, termed fibrillin-1 and fibrillin-2. The functional relationships between these glycoproteins and between them and other components of the microfibrils and elastic fibers are obscure. As a first step toward elucidating these important points, we compared the expression pattern of the fibrillin genes during mammalian embryogenesis. The results revealed that the two genes are differentially expressed, in terms of both developmental stages and tissue distribution. In the majority of cases, fibrillin-2 transcripts appear earlier and accumulate for a shorter period of time than fibrillin-1 transcripts. Synthesis of fibrillin-1 correlates with late morphogenesis and the appearance of well-defined organ structures; fibrillin-2 synthesis, on the other hand, coincides with early morphogenesis and, in particular, with the beginning of elastogenesis. The findings lend indirect support to our original hypothesis stating that fibrillins contribute to the compositional and functional heterogeneity of the microfibrils. The available evidence is also consistent with the notion that the fibrillins might have distinct, but related roles in microfibril physiology. Accordingly, we propose that fibrillin-1 provides mostly force-bearing structural support, whereas fibrillin-2 predominantly regulates the early process of elastic fiber assembly.

Polyclonal antibodies to tropoelastin and the specific detection and measurement of tropoelastin in vitro

Because tropoelastin is difficult to purify, most antibodies to elastin are raised against the insoluble form of the molecule. While these antibodies cross-react with tropoelastin, antigenic differences between insoluble and soluble elastin suggest that antibodies raised directly against tropoelastin might provide a more sensitive and specific reagent for evaluating tropoelastin production in elastin-producing systems. Using an improved method for purifying tropoelastin from tissue culture explants, we describe the generation and characterization of an antibody to bovine tropoelastin. This antibody was used to develop a sensitive, direct-binding immunoassay capable of quantifying small levels of tropoelastin in conditioned medium from cultured cells. This assay takes advantage of the propensity of tropoelastin to adsorb to vinyl microtiter plates, even in the presence of serum proteins. This property, in combination with the increased sensitivity obtained using antibodies to tropoelastin, provides for a direct-binding immunoassay that detects nanogram quantities of tropoelastin directly in cell culture medium, avoiding sample preparation steps that result in extensive loss of tropoelastin. In addition, this direct-binding assay is ten- to 30-fold more sensitive than the existing competitive ELISA assays.

Three-dimensional organization of extracellular matrix in elastic cartilage as viewed by quick freeze, deep etch electron microscopy

Freeze-etch electron microscopy was used to reexamine the ultrastructure of extracellular matrix in elastic cartilage. This revealed that extremely delicate, approximately 4 nm diameter fibrils join end-to-end and sometimes side-to-side to form a tightly woven mesh that extends continuously from the cell membrane throughout the intercellular space. Within this meshwork were found large, irregularly contoured and densely packed elastin fibers as well as long, thin (20 nm) fibers with the appearance of type II collagen. By comparison, type I collagen fibers found in the skin surrounding the cartilage appeared much thicker (30 nm) and displayed the usual periodic banding pattern. Freeze-etching the latter fibers displayed a helicoidal arrangement of subfibrils within. In both cartilage and type I collagen-rich extracellular matrix, some of the approximately 4 nm filaments in the matrix could be seen to contact collagen fibers orthogonally, apparently connecting adjacent fibers at regular intervals. The organization of these fine filaments and others composing the matrix has several features that suggest a different organization for cartilage than currently thought. Specifically, the distance between branch or contact points of fibrils in the matrix is seldom more than 35 nm, substantially less than the length of one extended proteoglycan monomer. This suggests that other proteoglycans, or other unidentified components of the matrix, bind along the proteoglycan core protein at intermediate binding sites in order to form a finely partitioned structure.

The occurrence of elastic system fibres in the matrix of normal articular cartilage

Normal articular cartilages from the weight-bearing areas of the femoral condyles of the knee joints of 11 patients (3-20 years old) and of 35 Schwarzkopf sheep (3 months to 2 years old) were studied using the electron microscope. The study has shown that the matrix of normal articular cartilage is not only composed of collagen fibrils and proteoglycans, but also contains two types of elastic system fibres. Small elastic fibres can be identified in the superficial and lower radiate zones of cartilage of man and sheep. Similar to elastic fibres in other tissues, they consist of a central amorphous core and are surrounded by aggregates of 10 nm microfibrils. Another type of elastic system fibres, oxytalan fibres, are found in the intermediate and upper radiate zones of the articular cartilage.

Elastin production in human skin fibroblast cultures and its decline with age

Recent studies have established that cultured human skin fibroblasts secrete the soluble precursor of elastin, tropoelastin (TE). The present studies evaluate, by an enzyme-linked immunosorbent assay, the stability of the TE phenotype and the effect of culture conditions and donor age on TE accumulation by human skin fibroblasts. Tropoelastin was maximally produced by 2 control fibroblast strains at early confluency (32-49 X 10(3) molecules/cell/h), and its serum-dependent accumulation in the medium was linear for at least 72 h. Inhibition of cross-linking had no effect on the rate of elastin production. Optimum serum concentrations for TE production differed for fibroblast cell strains derived from foreskin and trunk skin fibroblasts. Production of TE by human skin fibroblasts was stable through nearly 30 population doublings after which there was a greater than 2-fold decline in the rate of accumulation. In a cohort of donor strains, TE production appeared to decline at donor ages greater than or equal to 70 years. Under standard culture conditions, cell strains from normal donors of various ages produced TE at rates ranging from 25-69 X 10(3) molecules/cell/h. Rates of TE accumulation in medium were not significantly altered by degradation of TE, as a variety of cell strains tested exhibited minimal cell-associated elastolytic activity. Based on the demonstration of a stable elastin phenotype, skin fibroblast cultures provide a new system for studying regulation of elastin biosynthesis and evaluating potential defects in elastin metabolism associated with certain connective tissue disorders.

Alteration of the extracellular matrix of smooth muscle cells by ascorbate treatment

The protein composition in the extracellular matrix of cultured neonatal rat aortic smooth muscle cells has been monitored over time in culture. The influence of ascorbate on insoluble elastin and collagen has been described. In the absence of ascorbate, the cells accumulate an insoluble elastin component which can account for as much as 50% of the total protein in the extracellular matrix. In the presence of ascorbate, the amount of insoluble collagen increases, while the insoluble elastin content is significantly less. When ascorbate conditions are varied at different times during the culture, the extracellular matrices are altered with respect to collagen and elastin ratios. The decrease in elastin accumulation in the presence of ascorbate may be explained by an overhydroxylation of tropoelastin. Approximately 1/3 of the prolyl residues in the soluble elastin fractions isolated from cultures grown in the presence of ascorbate are hydroxylated. Since the insoluble elastin accumulated in these cultures contain the unique lysine-derived cross-links in amounts comparable to aortic tissue, this culture system proves ideal for studying the influence of extracellular matrix elastin on cell growth and metabolism.

Elastin production by cultured calf pulmonary artery endothelial cells

Calf pulmonary artery (CPA) endothelial cells synthesize and secrete soluble elastin when incubated in medium conditioned by arterial smooth muscle cells. Endothelial cell tropoelastin cross-reacts with antiserum to bovine ligamentum nuchae elastin and comigrates on SDS-PAGE with tropoelastins from fetal bovine ligamentum nuchae fibroblasts, aortic smooth muscle cells, and ear chondroblasts at an apparent molecular weight of 70,000. Endothelial cells synthesize only one-third as much elastin as these other cell types, however. Approximately 80% of the elastin synthesized by endothelial cells in confluent culture is released into the culture medium. The remaining 20% remains associated with the cell layer and is readily extractable with dilute acetic acid as un-cross-linked, 70,000-dalton tropoelastin. The addition of beta-aminopropionitrile to culture medium did not alter the ratio of tropoelastin in the medium and cell layer, suggesting that cross-linking of tropoelastin does not occur in culture. Immunofluorescent staining of confluent endothelial cell cultures with antielastin serum demonstrated elastin occurring as a web-like network of fine filaments extending throughout the extracellular space. The fibrous elastin was different in organization and distribution from fibers stained with antifibronectin serum, which were localized primarily beneath the cell layer and in regions of cell-cell contact. Extracellular matrix remaining after solubilization of cellular material with Triton X-100 stained positive for fibronectin, but not for elastin.

Effects of ascorbate on insoluble elastin accumulation and cross-link formation in rabbit pulmonary artery smooth muscle cultures

Cultured pulmonary artery smooth muscle cells derived from the medial vessel layer of weanling rabbits were grown in the presence or absence of sodium ascorbate. The connective tissue elements insoluble elastin and collagen were identified and quantified. Formation and accumulation of alpha-aminoadipic acid gamma-semialdehyde (allysine) and the intermolecular cross-links desmosine (Des), isodesmosine (Ides), and aldol condensation product (Aldol) were evaluated from [14C]lysine pulse-chase experiments. [14C]Des, [14C]Ides, peptide-bound [14C]lysine, [14C]allysine, and [14C]Aldol were determined from amino acid analysis. The latter two components were determined after reduction with NaBH4. [14C]Proline conversion to hydroxy[14C]proline and collagenase susceptibility were used to identify and quantify collagen synthesis. Ascorbate dramatically affects insoluble elastin synthesis, accumulation, and cross-link formation. Cells grown in the presence of ascorbate synthesize and accumulate significantly less insoluble elastin than non-ascorbate cultures. Those elastin molecules which do become incorporated into the extracellular matrix in the presence of ascorbate contain a slightly elevated content of hydroxyproline and lysine and, most importantly, are turned over more rapidly.

Elastin in diseases

Previous morphologic observations have suggested abnormalities in the elastic fibers in a number of both inherited and acquired diseases. Recent progress made in understanding of the normal biology of elastin has allowed us to examine these diseases by biochemical means. In this review we are discussing the current status of the research on the elastin diseases with particular emphasis on clinical conditions affecting skin, as for example, cutis laxa, pseudoxanthoma elasticum, and the Buschke-Ollendorff syndrome. In addition, we present new data which appears to be the first demonstration of an elastin abnormality in the Marfan syndrome.

Review of the repair of defects in articular cartilage: part I *

A review of the literature was undertaken to determine the extent and nature of the repair of articular cartilage. While repair is normally limited, under appropriate conditions the repair process appears to be of clinical significance. J Orthop Sports Phys Ther 1982;3(4):186-192.

Review of Repairs of Defects in Articular Cartilage: Part II *

A review of the literature was undertaken to determine the extent and nature of the repair of articular cartilage. While repair is normally limited, under appropriate conditions the repair process appears to be of clinical significance. J Orthop Sports Phys Ther 1982;4(1):6-15.

Current concepts in the diagnosis of human soft tissue sarcomas

The surgical pathologist is often called upon to make a rapid decision about the malignant potential of a soft tissue tumor. These tumors have specific stromas that support growth and endow the tumor with unique qualities, as well as being manifestations of the phenotypic expression of tumor cells. Collagen, the major protein of the stroma of tumors, is now recognized as a family of proteins, each a different gene product. Immunofluorescence microscopy studies using antibodies directed against purified stromal components, particularly the different type specific collagens, may soon be available in large quantities. These are potentially useful tools for the surgical pathologist not only in making that important decision, but also for developing a classification scheme for human sarcomas. This scheme will inevitably have a more sound biological basis than those currently used.

Ultrastructure of elastic cartilage in the rat external ear

The structure of elastic cartilage in the external ear of the rat was investigated by transmission and scanning electron microscopy. The narrow subperichondrial, boundary zone contains predominantly ovoid cells rich in cell organelles: mitochondria, Golgi complex, granular endoplasmic reticulum and small (40--100 nm) vesicles. Scarce glycogen granules and bundles of 6--7 nm cytoplasmic filaments are also present. Deeper in the boundary zone, one or more cytoplasmic lipid droplets appear and cytofilaments become more abundant. Fully differentiated chondrocytes in the central zone of the cartilage plate resemble white adipose cells. They are globular and contain a single, large cytoplasmic lipid droplet. The cytoplasm is reduced to a thin peripheral rim; it contains a flattened nucleus, few cytoplasmic organelles and abundant, densely packed, cytoplasmic filaments. The intercellular matrix is very sparse. The pericellular ring consists of collagen fibrils about 20 nm in diameter and a proteoglycan cartilage matrix in the form of a "stellate reticulum". The complex of these two structures appears in the scanning electron micrographs as a a network of randomly oriented, ca 100 nm thick fibrils. Spaces between pericellular rings of matrix also contain thick elastic fibers or plates, apparently devoid of microfibrils. In scanning electron micrographs elastic fibers could be detected only in a few areas, in which they were not obscured by other constituents of the matrix. Immature forms of elastic fibers, oxytalan (pre-elastic) and elaunin fibers, were found in the perichondrial and boundary zones.

Elastin biosynthesis and cross-link formation in rabbit aortic smooth muscle cell cultures

Rabbit aortic smooth muscle cells in culture produce insoluble elastin which can be purified by treatment with hot alkali. These cells, when maintained in the same flask for long periods of time, continue to accumulate elastin. Both desmosine and isodesmosine, cross-links unique to insoluble elastin, have also been found to increase with time in culture. The results from pulse-chase studies with radiolabeled proline and lysine confirm these observations. All the data indicate that the appearance of the desmosines in the elastin in these cell cultures is a relatively slow process, while the lysine-derived aldehydes appear quite rapidly.

Desmosine radioimmunoassay as a means of studying elastogenesis in cell culture

Elastin is synthesized by fibroblasts and chondroblasts in cell culture shortly before the cells become confluent. Fibroblasts secrete elastin into the medium as soluble tropoelastin molecules, which form desmosine crosslinks and become constituents of the cell layer only after three weeks in culture. Even then only a small fraction of the available tropoelastin molecules from crosslinks. Conversely, the chondrocytes secrete an elastin which never reaches the media as soluble elastin in significant quantities. Crosslinking occurs immediately in the chondroblast cell layer forming stable, insoluble elastic fibers. Both cells in culture produce lysyl oxidase at approximately the same levels. The reason for the marked differences between these cells in the mode of conversion of soluble elastin to insoluble elastin is not known. The suggestion of Mecham that the extracellular matrix may play a major role in the development of elastogenesis may provide an answer.

Formation of elastic fibers and elastin in rabbit aortic smooth muscle cell cultures

Cultured rabbit aortic smooth muscle cells produce elastic fibers and elastin in their extracellular matrix. Morphological analyses of elastic fibers indicate that they consist of two distinct components which play a major role in elastic early fiber formation namely, the amorphous component and the "microfibrillar" component. During elastogenesis, the early fiber consists of small bundles of filaments. Later, clearly distinguishable small conglomerates of amorphous material are found distributed among the bundles of filaments. The mature elastic fibers have large conglomerates of amorphous material with the filaments present within the interstices. Long term cultures of these cells in the second passage continue to accumulate elastin. The crosslinking amino acid isomers desmosine and isodesmosine, which are unique to insoluble elastin, increase with time in culture. Twenty-four hour pulses with 14C-proline followed by measurements of 14C-hydroxyproline in the collagenase resistant protein of the cell layer show that the synthesis of insoluble elastin is constant up to the 14 week period studied from the time the amorphous material becomes evident ultrastructurally.

Isolation of elastic tissue microfibrils derived from cultured cells of calf ligamentum nuchae

Cells cultured from calf ligamentum nuchae produce extracellular microfibrils identical to those of intact elastic tissue as determined by ultrastructural appearance, degradative enzyme susceptibilities and amino acid composition. A method to isolate large amounts of the microfibrillar component using fluorescein mercuric acetate is described. The cultures do not appear to synthesize the amorphous component, elastin, in that radioactive desmosine and isodesmosine were not detected after incubating cultures with labeled lysine nor was elastin seen ultrastructurally.

Biosynthesis of tropoelastin by elastic cartilage

Elastic ear cartilage and aortae from nine-day-old hamsters were incubated in Krebs-Ringer medium containing ascorbate, beta-aminopropionitrile and either [14C]proline or [3H]valine. The [14C]proline-labeled proteins were separated by chromatography on Agarose A-5 m in sodium dodecyl sulfate and their [14C]-hydroxyproline content measured. A fraction having an approximate molecular weight of 70,000 measured by polyacrylamide gel electrophoresis had a [14C]hydroxyproline content of 9.1% in the cartilage and 11.8% in the aorta. This fraction was also relatively heavily labeled with [3H]valine. The 70,000 dalton, [3H]valine-labeled protein of both the aorta and ear cartilage was precipitated by elastin-specific antibody prepared against hamster insoluble aorta elastin, but no higher molecular weight protein was immunoprecipitated. Based on these results we identify the 70,000 dalton protein from the elastic cartilage as tropoelastin.