Structural variations of collagen in normal and pathological tissues: role of electron microscopy

An Automated Approach for Analyzing D-Periods in Collagen Fibril Images

This paper considers an approach for analyzing fibrillar collagen structures based on fundamental concepts of pattern recognition. It focuses on the quantitative comparison between collagen structural data (electron-optical data) and chemical data. Theoretical models in the form of sequence-generated histograms are used as reference for extracting and analyzing the structural unit in images from collagen fibrils. In this respect, collagen provides a valuable model system for studying the chemical basis of ultrastructure, as well as detecting the alterations in collagen fibril structure produced by a disorder. Application examples are presented and the results are compared with biochemical studies.

Perineurioma: a clinicopathological study of eight cases

AIMS

Perineuriomas (PN) are uncommon, benign neoplasms that mimic a number of benign and malignant soft tissue lesions. There are two main forms: a rare intraneural PN (IPN), and a relatively more common extraneural soft tissue PN (STPN) including a conventional form (STPNc), sclerosing (SPN), reticular and lipomatous variants. Their diagnosis requires immunohistochemical (IHC) and/or ultrastructural (US) confirmation of perineurial cell differentiation. This study aims to review the clinicopathological features of eight PN we encountered recently, to raise awareness of PN as an entity and to highlight the differential diagnoses which include potentially aggressive lesions.

METHODS

Clinical, histological, IHC and US features of seven STPN and one IPN were studied.

RESULTS

The eight PN arose in the limbs of six females and two males aged 30-58 years. Five STPN occurred in subcutis, one intramuscularly and one intradermally. The STPN were well-circumscribed, multinodular growths. STPNc contained bland spindle cells with long cytoplasmic processes arranged in lamellae, storiform patterns and whorls. Three SPN were acrally located and additionally contained small epithelioid cells in cords and clusters in a myxohyaline stroma with extensive sclerosis. One SPN had giant collagen rosettes of spiral collagen. The IPN showed 'pseudo-onion bulbs' of perineurial cells. All PN were at least focally EMA positive, six of eight were Claudin-1 positive and all were S100 protein negative. Common US features were organelle-poor cell processes, many pinocytotic vesicles, sparse intermediate filaments, and tight junctions and patchy external lamina. There were no recurrences (follow-up 1-49 months).

CONCLUSION

PN has a variable morphology and can mimic many benign, borderline and malignant lesions, the differential diagnoses of which are discussed. When confronted with a subcutaneous (in particular) spindle and/or epithelioid cell lesion, EMA/Claudin-1 stains and/or US are essential to identify PN and thereby avoid inappropriately aggressive therapy.

Resolution and reproducibility of measures of the diameter of small collagen fibrils by transmission electron microscopy--application to the rabbit corneal stroma

AIM

To assess the impact of measurements of different numbers of small collagen fibrils at different final magnification values on the resultant mean values for fibril diameter (FD).

METHODS

Very high magnification (33,000x) electron micrographs were taken of the posterior-central zone of the corneal stroma from young adult rabbits (2 kg), printed at 46,000 or 50,000x, scanned at 400 d.p.i. and 35 mm slides prepared. These were projected to give final magnifications between 150,000x and 450,000x. An overlay of fibril outlines was prepared from the projected images and the fibril diameters (FD's) measured to within 0.5 mm.

RESULTS

On the overlays, at different projection magnifications, the average FD's ranged from 4 to 13.5 mm to allow measures to be made at a real resolution of between 3.5 and 1 nm. Using a fixed sized region of interest (ROI) on the overlays, the average values of FD's ranged between 24.9 and 31.7 nm, and could vary (for any particular micrograph) by up to 3.6 nm according to the number of FD's measured (n=34-384/ROI). Using a fixed number of FD measures (n=100) at different projection magnifications yielded average FD values from different corneas of between 25.1 and 35.2 nm, which could vary by up to 4.3 nm depending on the magnification used.

CONCLUSIONS

The results indicate that different average values for measures of fibril diameter of small collagen fibrils can easily be obtained according to the final magnification used and the number of fibrils measured, and that the overall averages can depend on whether the data sets are averaged or pooled. These aspects of the morphometry of small collagen fibrils therefore need to be carefully specified.

Ultrastructure of internal limiting membrane in myopic foveoschisis

PURPOSE

To reveal the pathogenesis of myopic foveoschisis (MF).

DESIGN

Clinicopathological report.

METHODS

Internal limiting membranes (ILMs) were collected from ten patients with MF and five patients with idiopathic macular hole (IMH) as a control. Samples were subjected to transmission electron microscopic study. Characteristics of the ILM were compared between the two groups.

RESULTS

Collagen fiber and cell debris were identified on the inner surface of ILM in seven eyes (70%) with MF, significantly more (P < .05) than found in IMH subjects (0%). More fibrous glial cells were likely to be found on the inner surface of ILM. No significant difference in fibroblast-like cell adhesion was observed.

CONCLUSIONS

Collagen fiber and cellular component are suggested to play an important role in developing MF. ILM peeling may be essential for vitrectomy for MF.

The ultrastructural architecture of the adult Schistosoma japonicum tegument

The tegument of the adult blood fluke Schistosoma japonicum is in direct contact with the host blood and immune systems. A comprehensive understanding of the ultrastructure of the tegument is crucial to the understanding of how the parasite maintains itself within the mammalian host. Important functions such as nutritional uptake and immune evasion are suspected functions of the tegument and this review discusses these aspects and presents some insights into some of these crucial functions. Transmission electron microscopy has allowed the identification of ultrastructural features of the adult S. japonicum, some of which differ from the reported features of other schistosome species. Morphological differences within the tegument of the adult S. japonicum are noted between sexes, among different regions of the worms and between aspects along the length of the parasite. Differences included variations in the ultrastructure, size and number of tegumental bodies and mitochondria within the matrix, and differences in the relative area of the apical surface of the tegument. Functions of the various components of the tegument matrix and specialised functions of different regions of the male and female parasites are discussed based on ultrastructural findings and previously reported biochemical and molecular data.

Potential modifier role of the R618Q variant of proalpha2(I)collagen in type I collagen fibrillogenesis: in vitro assembly analysis

An arginine to glutamine substitution in the triple helix of proalpha2(I)collagen (R618Q) was first reported in a patient with a variant of Marfan syndrome and later identified in conjunction with a second mutation in a patient with osteogenesis imperfecta (OI). The presence of the R618Q proalpha2(I)collagen allele in unaffected or mildly affected family members suggests that the R618Q allele is either a non-affecting polymorphism or a potential genetic modifier. Conservation of arginine618 across species and fibrillar collagen types suggests it is functionally significant. To investigate the functional significance of the R618Q proalpha2(I)collagen allele, we isolated type I collagen from cultured dermal fibroblasts of control and two unrelated individuals heterozygous for the R618Q proalpha2(I)collagen allele and evaluated helical stability and fibrillar assembly. Type I collagen thermal stability analyzed by protease susceptibility and CD spectroscopy demonstrated no statistical difference between control and R618Q containing collagen molecules. In vitro fibril assembly analyses demonstrated that R618Q containing collagen exhibits rapid fibrillar growth with minimal fibril nucleation phase. Further, electron microscopy demonstrated that the diameter of assembled R618Q containing collagen fibrils was approximately 20% of control collagen fibrils. These findings suggest the R618Q variant does not impact triple helical stability but has a role in collagen fibril assembly, supporting the hypothesis that the R618Q proalpha2(I)collagen variant is a modifier of connective tissue structure/function and is potentially involved in disease pathogenesis.

Determination of collagen fiber orientation in human tissue by use of polarization measurement of molecular second-harmonic-generation light

Based on the reflection-type polarization measurement of second-harmonic-generation (SHG) light induced by collagen molecules, we are able to determine the collagen fiber orientation in human tissues taken from a cadaver. The resulting SHG radar graph shows the direction of the absolute orientation and the degree of organization of collagen fibers. To evaluate the probing sensitivity to the collagen orientation, we compared the proposed method with other polarimetric methods. Use of the proposed method revealed characteristic orientation differences among collagen fibers and demonstrated significant inhomogeneity with respect to the distribution of collagen orientation in human dentin. The proposed method provides a powerful research and diagnostic tool for examining the collagen orientation in human tissues.

Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Characterization of collagen orientation in human dermis by two-dimensional second-harmonic-generation polarimetry

We have proposed an optical probe that can be used to characterize the orientation of collagen fibers in human dermis. A specific probing ability for collagen results from the use of second-harmonic-generation (SHG) light induced by collagen molecules in the tissue. Based on the concept of collagen SHG light, a reflection-type polarization measurement system (named SHG polarimetry) with a probe light spot of 15 microm in diameter has been constructed, and the human reticular dermis has been measured using this system. Resultant data exhibit that the reticular dermis possesses approximately uniaxial orientation of the collagen fibers. Furthermore, we demonstrated a nondestructive measurement of the collagen orientation in the papillary dermis across an epidermis layer. For distribution measurement of the collagen fiber orientation in the reticular dermis, we have extended the SHG polarimetry to one- (1-D) and two-dimensional (2-D) measurement. By the macroscopic 2-D SHG polarimetry, we have observed that the orientation angle and organization degree of collagen fibers vary widely depending on the discrete probing positions in the reticular dermis. Furthermore, microscopic 1-D SHG polarimetry indicated a swell of the orientation angle and a large variance of the organization degree in the collagen fibers in the microscopic region. These results imply that the reticular dermis posses a tangled structure of collagen fibers, which is highly consistent with the result of the anatomical examination of the skin. The proposed method will be a powerful tool for monitoring the microscopic distribution of the collagen fiber orientation in the human dermis.

Ultrastructural study of cutaneous lesions in feline eosinophilic granuloma complex

The purpose of this study was to investigate the ultrastructural appearance of flame figures, reported to comprise a mixture of degenerate collagen and degranulated eosinophils, in feline eosinophilic granuloma complex (EGC). Skin specimens from eight cats with EGC and from two clinically healthy cats were examined by transmission electron microscopy. Flame figures appeared to comprise ultrastructurally normal collagen fibrils separated by oedema and surrounded by large numbers of degranulating eosinophils. Longitudinal sections of collagen fibrils displayed the characteristic cross-striation of normal dermal collagen. Feline eosinophils, analogous to human eosinophils, degranulated both by cytolysis and piecemeal degranulation. The results of this study suggest that flame figures form in feline EGC due to eosinophil recruitment and degranulation, and that collagen fibres are partially disrupted but collagen fibrils are not damaged. These findings suggest that eosinophil accumulation and the release of granule contents represent the primary events in feline EGC.

A forehead mass present for 53 years after trauma in childhood

A 60-year-old man developed a mass on his forehead in his childhood. Histologic examination revealed a subcutaneous encapsulated cellular neoplasm consisting of mostly polygonal epithelioid cells, with abundant amphophilic cytoplasm and pleomorphic nuclei. There was one focal area of spindle cells. Numerous blood vessels of various sizes, small vacuoles and eosinophilic hyalinized material were also present. The morphologic features of the neoplasm did not fit with the typical features of any recognized pathologic entity. The subsequent immunohistochemical and electron microscopical evidence supported the diagnosis of epithelioid schwannoma, a rare variant of schwannoma that has not been reported in oral and maxillofacial pathology publications. The purpose of this report is to document a case and increase the awareness of this entity.

Hierarchical assembly and the onset of banding in fibrous long spacing collagen revealed by atomic force microscopy

The mechanism of formation of fibrillar collagen with a banding periodicity much greater than the 67 nm of native collagen, i.e. the so-called fibrous long spacing (FLS) collagen, has been speculated upon, but has not been previously studied experimentally from a detailed structural perspective. In vitro, such fibrils, with banding periodicity of approximately 270 nm, may be produced by dialysis of an acidic solution of type I collagen and alpha(1)-acid glycoprotein against deionized water. FLS collagen assembly was investigated by visualization of assembly intermediates that were formed during the course of dialysis using atomic force microscopy. Below pH 4, thin, curly nonbanded fibrils were formed. When the dialysis solution reached approximately pH 4, thin, filamentous structures that showed protrusions spaced at approximately 270 nm were seen. As the pH increased, these protofibrils appeared to associate loosely into larger fibrils with clear approximately 270 nm banding which increased in diameter and compactness, such that by approximately pH 4.6, mature FLS collagen fibrils begin to be observed with increasing frequency. These results suggest that there are aspects of a stepwise process in the formation of FLS collagen, and that the banding pattern arises quite early and very specifically in this process. It is proposed that typical 4D-period staggered microfibril subunits assemble laterally with minimal stagger between adjacent fibrils. alpha(1)-Acid glycoprotein presumably promotes this otherwise abnormal lateral assembly over native-type self-assembly. Cocoon-like fibrils, which are hundreds of nanometers in diameter and 10-20 microm in length, were found to coexist with mature FLS fibrils.

Polarization-modulated second harmonic generation in collagen

Collagen possesses a strong second-order nonlinear susceptibility, a nonlinear optical property characterized by second harmonic generation in the presence of intense laser beams. We present a new technique involving polarization modulation of an ultra-short pulse laser beam that can simultaneously determine collagen fiber orientation and a parameter related to the second-order nonlinear susceptibility. We demonstrate the ability to discriminate among different patterns of fibrillar orientation, as exemplified by tendon, fascia, cornea, and successive lamellar rings in an intervertebral disc. Fiber orientation can be measured as a function of depth with an axial resolution of approximately 10 microm. The parameter related to the second-order nonlinear susceptibility is sensitive to fiber disorganization, oblique incidence of the beam on the sample, and birefringence of the tissue. This parameter represents an aggregate measure of tissue optical properties that could potentially be used for optical imaging in vivo.