Stem Cells: Scientific Progress and Future Research Directions. Bel Air, CA: University Press of the Pacific, 2004.
Scadden, David T., et al. “Overview of Stem Cells.” UpToDate (n.d.), http://www.uptodate.com/contents/overview-of-stem-cells.
Cartilage, Tendons, and Ligaments: Cell Types Composing the Tissue
Cartilage, Tendons, and Ligaments: Cell Types Composing the Tissue
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Cartilage, Tendons, and Ligaments: Cell Types Composing the Tissue
Cartilage is a firm and flexible type of connective tissue consisting of cells and intracellular fibers in a gel-like material. It has a smooth and resilient surface and a weight-bearing capacity exceeded only by that of bone. It is composed of cells called chondroblasts and chondrocytes and extracellular matrix. The matrix is made up of aggrecan (10 percent), water (75 percent), and a mix of collagen fibers and other constituents.
There are three types of cartilage: elastic cartilage, hyaline cartilage, and fibrocartilage. They differ mostly in the type of fibers they contain. Elastic cartilage contains some elastin in its intercellular substance. It is found in most areas such as the ear, where some flexibility is required. Fibrocartilage has intermediate characteristics between hyaline cartilage and dense connective tissue. It is primarily found in the intervertebral disks and in the symphysis pubis. Finally, hyaline cartilage is the most abundant type of cartilage and forms most of the fetal skeleton. In the adults, it forms the costal cartilages and respiratory, articular, and epiphyseal cartilages. The free surfaces of most hyaline cartilage are covered by a layer of fibrous connective tissue called the perichondrium:
Hyaline: most common, glassy appearance
Elastic: elastic plus collagen fibers
Fibrocartilage: parallel bundles of collagen fibers, least number of chondrocytes
Chondrocytes, being the only type of cells present, are found in healthy cartilage. The progenitor of chondrocytes arises in the bone marrow in the form of stem cells. When stem cells differentiate into cartilage cells, they are initially chondroblasts. These secrete chondrin, the substance in cartilage responsible for building and repairing of the tissue. The extracellular matrix, mainly collagen and proteoglycans, is also secreted by chondroblasts. These precursor cells are found in the outer layer of cartilage. The secreted matrix and fibers entrap the chondroblasts and eventually mature into cells called chondrocytes. These cells are located in lacunae, which are surrounded by an uncalcified gel-like intercellular matrix of collagen fibers and ground substance.
Lacunae consist of rounder chondrocytes daughter cells that remain close together in groups, forming nests of 2 to 4 cells. The active cells are large secretary cells with basophilic cytoplasm and abundant rough endoplasmic reticulum. The number of cells found in the cartilage tissue determines how bendy the structure is. They perform a number of functions, including facilitating the exchange of fluids through gelatinous layers. Since cartilage is an avascular structure, it relies on this exchange to receive nutrients and express waste materials. Chondrocytes undergo terminal differentiation during endochondral ossification. The cell exhibits major phenotypic changes, predominantly hypertrophy.
Under normal circumstances, wearing of cartilage is replaced and repaired by the chondrocytes. However, in some conditions the cartilage damage is so extensive that surgical intervention must be used to correct the problem. Transplants with patient’s own cells or a donor’s cell preparation can also be used to address problems with cartilage.
Tendons and Ligaments
Tendons and ligaments are specialized forms of dense fibrous connective tissue. They are composed of fibroblasts embedded in extracellular matrix (ECM) of ground substance and connective tissue fibers. Type 1 collagen is the main constituent in both tissues. The ECM determines 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.