the mechanical properties of these structures. Tendons function to attach muscles to bones, and ligaments typically connect bone to bone at joints. Tendons are more organized and can withstand more tension than ligaments. In contrast, ligaments are more elastic structures and readily resume their original shape after tension is decreased.
Histologically, tendons consist of dense regular connective tissue fascicles encased in dense irregular connective tissue sheaths. The collagen fibers run parallel in a closely packed manner and are produced by tenocytes. Tenocytes or tendon cells are elongated fibroblast-type cells. The cytoplasm of these cells is stretched between the collagen and they have a central cell nucleus with a prominent nucleolus. Tenocytes have well-developed RER and they are responsible for synthesis of tendon fibers and ground substance. The epitenon—a loose connective tissue sheath—surrounds the tendon externally, and delicate connective tissue septae divide the tendon into bundles.
These bundles are smaller and less organized in ligaments than in tendons. Some tendons also have synovial sheaths, particularly in areas of friction. The fibroblastic cells composing the ligaments are round to oval and small in size (12–15 µm long). Ligaments also have less total collagen content and glycosaminoglycans per unit area in the matrix as compared to tendons.
Ayesha Irum
Muhammad Saad Faiz
Army Medical College, National University of Sciences and Technology
See Also: Cartilage, Tendons, and Ligaments: Major Pathologies.
Further Readings
Ovalle, William K. and Patrick C. Nahirney. Netter’s Essential Histology, 2nd ed. Philadelphia: Saunders, 2013.
Walsh, William R., Ed. “Repair and Regeneration of Ligaments, Tendons, and Joint Capsule. New York: Springer-Verlag, 2010.
WiseGeek. “What Are Chondrocytes.” http://www.wisegeek.org/what-are-chondrocytes.html (Accessed May 2014).
Cartilage, Tendons, and Ligaments: Current Research on Isolation or Production of Therapeutic Cells
Cartilage, Tendons, and Ligaments: Current Research on Isolation or Production of Therapeutic Cells
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Cartilage, Tendons, and Ligaments: Current Research on Isolation or Production of Therapeutic Cells
Tendons and ligaments contribute to motion and are connective, elastic, fibrous tissues. Tendons attach muscles to the bone, and in the case of the eye, muscles to the eyeball, so they aid in movement of the structure or the bone. Ligaments, on the other hand, attach bone to other bones, aiding in stabilizing the structure.
Cartilage is present at the joints of bones and is flexible but tough. The cartilage protects the tips of bones from wearing down and is also responsible for the shape of certain parts of the body, like the nose and ears.
Injury or Damage
There are many factors that could contribute to the damage or the wear and tear of tendons, ligaments, and cartilage. A tear or damage from a sports injury or while dancing is among the most severe and can lead to acute pain and immobility among injured patients. Genetic factors also contribute to wear and tear of cartilage, tendons, and ligaments, along with factors like the development of arthritis. Current therapies do not allow complete recovery and are normally slow, with many interventions required.
Tendon, cartilage, and ligament injuries are of particular interest, as treatment strategies require prolonged periods of recuperation with a heightened chance of recurrence and a drastic effect on the quality of life of the injured individual. This is especially true among athletes, who risk foregoing their sporting career as a result of injury.
The need to provide a more permanent solution that would restore the quality of life and is reliable is a necessity and instrumental in the quest to use stem cells in the treatment of damage to tendons, ligaments, or cartilage. Stem cell therapy has been found to increase the rate of recovery, with a reduction in recurrence and with no recorded side effects. This has resulted in considerable interest in the use of stem cells for treatment, as it can provide affordable treatment methods in both developing and developed nations, reducing the global health equity.
To understand the importance of stem cells in the treatment of damaged tendons, ligaments, or cartilage, it is important to understand the molecular changes that occur in the embryonic stem cells that lead to the formation of these tissues and the molecular pathways that are followed in adult stem cells.
Important Molecular Factors in the Formation of Tendons
During embryonic development, syndetome, which is a separate part of the somite, gives rise to progenitor cells that form tendons, aided by the expression of transcription factor scleraxis (SCX). However, the introduction of SCX alone will not result in the formation of tendons or ligaments under laboratory conditions; other factors are required, like SIX1, SIX2, and proteins called Eya, Eya 1, and Eya 2.
The effects of these molecular factors have been studied on animal tendons after trans-section or in injured human tendons, so their role in development is not fully understood. This area of research is creating a lot of interest, as an injured tendon heals with the growth of scar tissue that contains collagen, but the mechanical properties of the healed tendon are never the same as an intact tendon. There is a need to identify the exact mechanisms involved in the development of tendons so injured tendons can be treated with stem cells to gain complete recovery. During embryogenesis, stem cells develop into specialized cells on exposure to specific growth factors. Scientists attempt to create similar conditions for the growth and development of stem cells under laboratory conditions.
Sources of Stem Cells for the Treatment of Ligament, Tendon, or Cartilage Damage
Bone marrow-derived stem cells for cartilage repair. The bone marrow is an important source of stem cells and is used extensively in the treatment of various conditions. Bone marrow is present in the center of long bones, with the highest concentration of stem cells present in the pelvic bone. Other sources of bone marrow stem cells include the long bones of the arm, leg, and the thigh. The bone marrow stem cells are aspirated using a needle and then concentrated using a centrifuge. Nucleated cells are separated using flow cytometry and 15 percent of this cellular population will consist of stem cells. Platelets are also constituents of this population of cells and, as platelets contain growth factors that aid in the synthesis of collagen matrix, like fibroblast growth factor, insulin-like growth factor-1 and transforming growth factor-B, they are essential to the cellular population. Finally, thrombin is added to the cellular mixture to cleave fibrinogen, which would then give rise to a fibrin scaffold that is essential to hold the stem cells and the growth factors together. This mixture is then injected into the damaged or the injured site. Though the body signals the movement of stem cells from the bone marrow to the injured site on its own, injecting stem cells speed recovery and also increases the concentration of stem cells at the site of injury.
Bone marrow-derived stem cells for tendon repair. Bone marrow stem cells are collected and then centrifuged to obtain the cellular concentrate, while the supernatant is stored for later use. The nucleated cells
