The cellular matrix: a feature of tensile bearing dense soft connective tissues

Directing ligament-mimetic bi-directional cell organization in scaffolds through zone-specific microarchitecture for ligament tissue engineering

Ligament tissues exhibit zone-specific anisotropic cell organization. The cells in ligament-proper are longitudinally oriented, whereas, the cells in epiligament are circumferentially oriented. Therefore, scaffolds developed to regenerate ligament tissues should possess adequate architectural features to govern ligament-mimetic bi-directional cell organization. The scaffold architectural features along with ligament-mimetic cell organization may ultimately yield neo-tissues with ligament-like extracellular matrix (ECM) structure and biomechanical properties. Towards this goal, we fabricated a silk/gelatin-based core-shell scaffold (csSG) with zone-specific anisotropic architectural features, wherein, the core of the scaffold possessed longitudinally aligned pores while the shell of the scaffold possessed parallel microgrooves that are aligned circumferentially around the surface of the scaffold. The ligament-mimetic architectural features significantly improved the mechanical properties of the scaffold. Moreover, architectural features of the csSG scaffold governed zone-specific anisotropic organization of cells. The cells in the core were longitudinally oriented as observed in the ligament-proper and the cells on the shell were circumferentially oriented as observed in epiligament. This bi-directional cell orientation partially mimicked the complex cellular network in native ligament tissue. Additionally, both the core and the shell individually supported fibrogenic differentiation of stem cells which further improved their potential for ligament tissue engineering. Further, the aligned pores of the core could govern unidirectional organization of ECM deposited by cells which is crucial for regenerating anisotropic tissues like ligaments. Finally, when implanted subcutaneously in mice, the scaffolds retained their anisotropic architecture for at least 2 weeks, were biocompatible, supported cell infiltration and governed anisotropic organization of cells and ECM. Taken together, the fabricated biomimetic csSG scaffold, through its zone-specific architectural features, could govern ligament-mimetic cellular and ECM organization which is ultimately expected to achieve regeneration of ligament tissues with native-like hierarchical structure and biomechanical properties. Consequently, this study introduces bi-directional structural parameters as design criteria for developing scaffolds for ligament tissue engineering.

Cell migration: implications for repair and regeneration in joint disease

Connective tissues within the synovial joints are characterized by their dense extracellular matrix and sparse cellularity. With injury or disease, however, tissues commonly experience an influx of cells owing to proliferation and migration of endogenous mesenchymal cell populations, as well as invasion of the tissue by other cell types, including immune cells. Although this process is critical for successful wound healing, aberrant immune-mediated cell infiltration can lead to pathological inflammation of the joint. Importantly, cells of mesenchymal or haematopoietic origin use distinct modes of migration and thus might respond differently to similar biological cues and microenvironments. Furthermore, cell migration in the physiological microenvironment of musculoskeletal tissues differs considerably from migration in vitro. This Review addresses the complexities of cell migration in fibrous connective tissues from three separate but interdependent perspectives: physiology (including the cellular and extracellular factors affecting 3D cell migration), pathophysiology (cell migration in the context of synovial joint autoimmune disease and injury) and tissue engineering (cell migration in engineered biomaterials). Improved understanding of the fundamental mechanisms governing interstitial cell migration might lead to interventions that stop invasion processes that culminate in deleterious outcomes and/or that expedite migration to direct endogenous cell-mediated repair and regeneration of joint tissues.

New look at about nature, structure and function of Trietz ligament

Background:

Trietz ligament connects the duodeno-jejunal flexure to the right crus of the diaphragm. There are various opinions regarding the existence of the smooth muscle fibers in the ligament. We want to resolve this complexity with microscopic study of this part in cadavers.

Materials and Methods:

This study done on three cadavers in the medical faculty of Isfahan University of Medical Sciences. Three samples of histological specimens were collected from the upper, the central, and the lower parts of Trietz ligament and were stained by H and E staining and Mallory's trichrome stain. Three samples were collected from the regions of exact connection of the main mesentery to the body wall, the intestine, and the region between these two connected regions, and these specimens were stained.

Results:

In the microscopic survey, no collagen bundles were observed in the collected samples of the Trietz ligament after the dense muscular tissues. In the samples which were collected to work on collagen tissues stretching from the Trietz ligament to the main mesentery of intestine, no collagen bundles were observed.

Conclusion:

Trietz ligament is connected to the right crus of the diaphragm from the third and the fourth parts of the duodenum. Number of researchers state that there are smooth and striated muscular tissues and some others, with regard to observations of histological phases made from the samples of Trietz muscles, conclude that it can probably be noted that muscular bundles or the dense connective tissue bundles of collagen cannot be observed in the way we imagine.

Comparative transcriptional analysis of three human ligaments with distinct biomechanical properties

One major aim of regenerative medicine targeting the musculoskeletal system is to provide complementary and/or alternative therapeutic approaches to current surgical therapies, often involving the removal and prosthetic substitution of damaged tissues such as ligaments. For these approaches to be successful, detailed information regarding the cellular and molecular composition of different musculoskeletal tissues is required. Ligaments have often been considered homogeneous tissues with common biomechanical properties. However, advances in tissue engineering research have highlighted the functional relevance of the organisational and compositional differences between ligament types, especially in those with higher risks of injury. The aim of this study was to provide information concerning the relative expression levels of a subset of key genes (including extracellular matrix components, transcription factors and growth factors) that confer functional identity to ligaments. We compared the transcriptomes of three representative human ligaments subjected to different biomechanical demands: the anterior cruciate ligament (ACL); the ligamentum teres of the hip (LT); and the iliofemoral ligament (IL). We revealed significant differences in the expression of type I collagen, elastin, fibromodulin, biglycan, transforming growth factor β1, transforming growth interacting factor 1, hypoxia-inducible factor 1-alpha and transforming growth factor β-induced gene between the IL and the other two ligaments. Thus, considerable molecular heterogeneity can exist between anatomically distinct ligaments with differing biomechanical demands. However, the LT and ACL were found to show remarkable molecular homology, suggesting common functional properties. This finding provides experimental support for the proposed role of the LT as a hip joint stabiliser in humans.

Gap junctions of the medial collateral ligament: structure, distribution, associations and function

Ligaments are composed of two major components: cells and extracellular matrix. The cells express gap junction proteins and are arranged into a series of rows that traverse the tissue, suggesting that all the cells of the tissue are functionally interconnected. The results of our study demonstrate that medial collateral ligament (MCL) cells do not have a uniform fusiform morphology or placement along a row of cells as previously suggested, but rather display a complex placement and form that weaves within the collagen matrix in a manner that is far more extensive and complex than previously appreciated. Within this morphological context, we find that MCL cells in vivo contain functional gap junctions (verified using fluorescence recovery after photobleaching) that are localized to sites of close cell-cell contact, and this pattern imparts or reflects a bipolarity inherent to each cell. When we studied ligament cells in conventional tissue culture we found that this bipolarity is lost, and the placement of gap junctions and their related proteins, as well as general cell morphology, is also altered. Finally, our study demonstrates, for the first time, that in addition to gap junctions, adherens junctions and desmosomes are also expressed by MCL cells both in vivo and in vitro and map to sites of cell-cell contact.

Communication between paired chondrocytes in the superficial zone of articular cartilage

The regeneration and repair of cartilage damaged by injury or disease, a major goal of orthopaedic science, depends on understanding the structure and function of both the extracellular matrix and the chondrocytes. In this study, we explored the in situ organization and potential interactions between chondrocytes in the superficial zone of adult rabbit articular cartilage. Some chondrocytes in this zone were observed close together and appeared to be paired whereas others were solitary. The shared surfaces of a chondrocyte pair were separated by a narrow plate of extracellular matrix, into which extended small cytoplasmic projections from both cells. Furthermore, the spatial distribution of major cellular landmarks, such as the nucleus and centrosome as well as some intracellular proteins such as connexin-43, tended to be mirrored about this matrix plate. Fluorescence recovery after photobleaching revealed the fluorescent dye calcein-AM dye can pass between paired cells, and that the passage of this dye can be inhibited by the gap junction blocker octanol. These results illustrate that rapid cellular communication is possible between cells in the superficial layer of adult articular cartilage, which challenges the current thinking that these chondrocytes function in isolation.

Regional variations in the cellular matrix of the annulus fibrosus of the intervertebral disc

The three-dimensional architecture of cells in the annulus fibrosus was studied by a systematic, histological examination using antibodies to cytoskeletal components, in conjunction with confocal microscopy. Variations in cell shape, arrangement of cellular processes and cytoskeletal architecture were found both within and between the defined zones of the outer and inner annulus. The morphology of three, novel annulus fibrosus cells is described: extended cordlike cells that form an interconnected network at the periphery of the disc; cells with extensive, sinuous processes in the inner region of the annulus fibrosus; and cells with broad, branching processes specific to the interlamellar septae of the outer annulus. The complex, yet seemingly deliberate arrangement of various cell shapes and their processes suggests multiple functional roles. Regional variations in the organization of the actin and vimentin cytoskeletal networks is reported across all regions of the annulus. Most notable is the continuous, strand arrangement of the actin label at the disc's periphery in contrast to its punctate appearance in all other regions. The gap junction protein connexin 43 was found within cells from all regions of the annulus, including those which did not form physical connections with surrounding cells. These observations of the cellular matrix in the healthy intervertebral disc should contribute to a better understanding of site-specific changes in tissue architecture, biochemistry and mechanical properties during degeneration, injury and healing.