Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and protects the ends of long bones at the joints and nerves, and is a structural component of the rib cage, the ear, the nose, the bronchial tubes, the intervertebral discs, and many other body components.
Structure, Type, and Location of Cartilage
Cartilage is an avascular, flexible connective tissue located throughout the body that provides support and cushioning for adjacent tissues.
Key Points
Cartilage is a flexible connective tissue that differs from bone in several ways; it is avascular and its microarchitecture is less organized than bone.
Cartilage is not innervated and therefore relies on diffusion to obtain nutrients. This causes it to heal very slowly.
The main cell types in cartilage are chondrocytes, the ground substance is chondroitin sulfate, and the fibrous sheath is called perichondrium.
There are three types of cartilage: hyaline, fibrous, and elastic cartilage.
Hyaline cartilage is the most widespread type and resembles glass. In the embryo, bone begins as hyaline cartilage and later ossifies.
Fibrous cartilage has many collagen fibers and is found in the intervertebral discs and pubic symphysis.
Elastic cartilage is springy, yellow, and elastic and is found in the internal support of the external ear and in the epiglottis.
Key Terms
chondroitin sulfate: An important structural component of cartilage that provides much of its resistance to compression.
connective tissue: A type of tissue found in animals whose main function is to bind other tissue systems (such as muscle to skin) or organs. It consists of the following three elements: cells, fibers, and a ground substance (or extracellular matrix).
hyaline cartilage: A type of cartilage found on many joint surfaces; it contains no nerves or blood vessels, and its structure is relatively simple.
temporal-mandibular joint: A joint of the jaw that connects it to the temporal bones of the skull.
Chondrocytes: Cells that form and maintain the cartilage.
What is Cartilage?
Cartilage is a flexible connective tissue that differs from bone in several ways. For one, the primary cell types are chondrocytes as opposed to osteocytes. Chondrocytes are first chondroblast cells that produce the collagen extracellular matrix (ECM) and then get caught in the matrix. They lie in spaces called lacunae with up to eight chondrocytes located in each.
Chondrocytes rely on diffusion to obtain nutrients as, unlike bone, cartilage is avascular, meaning there are no vessels to carry blood to cartilage tissue. This lack of blood supply causes cartilage to heal very slowly compared with bone.
The base substance of cartilage is chondroitin sulfate, and the microarchitecture is substantially less organized than in bone. The cartilage fibrous sheath is called the perichondrium. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself.
Articular cartilage function is dependent on the molecular composition of its ECM, which consists mainly of proteoglycans and collagens. The remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix, which responds to tensile and compressive forces experienced by the cartilage.
Cartilage types: Images of microscopic views of the different types of cartilage: elastic, hyaline, and fibrous. Elastic cartilage has the most ECM; hyaline a middle amount; and fibrous cartilage has the least amount of ECM.
Types of Cartilage
There are three major types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage.
Hyaline Cartilage
Hyaline cartilage is the most widespread cartilage type and, in adults, it forms the articular surfaces of long bones, the rib tips, the rings of the trachea, and parts of the skull. This type of cartilage is predominately collagen (yet with few collagen fibers), and its name refers to its glassy appearance.
In the embryo, bones form first as hyaline cartilage before ossifying as development progresses. Hyaline cartilage is covered externally by a fibrous membrane, called the perichondrium, except at the articular ends of bones; it also occurs under the skin (for instance, ears and nose).
Hyaline cartilage is found on many joint surfaces. It contains no nerves or blood vessels, and its structure is relatively simple.
If a thin slice of cartilage is examined under the microscope, it will be found to consist of cells of a rounded or bluntly angular form, lying in groups of two or more in a granular or almost homogeneous matrix. These cells have generally straight outlines where they are in contact with each other, with the rest of their circumference rounded.
They consist of translucent protoplasm in which fine interlacing filaments and minute granules are sometimes present. Embedded in this are one or two round nuclei with the usual intranuclear network.
Fibrocartilage
Fibrous cartilage has lots of collagen fibers (Type I and Type II), and it tends to grade into the dense tendon and ligament tissue. White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions.
It owes its flexibility and toughness to the fibrous tissue and its elasticity to the cartilaginous tissue. It is the only type of cartilage that contains type I collagen in addition to the normal type II.
Fibrocartilage is found in the pubic symphysis, the annulus fibrosus of intervertebral discs, menisci, and the temporal-mandibular joint.
Elastic Cartilage
Elastic or yellow cartilage contains elastic fiber networks and collagen fibers. The principal protein is elastin.
Elastic cartilage is histologically similar to hyaline cartilage but contains many yellow elastic fibers lying in a solid matrix. These fibers form bundles that appear dark under a microscope. They give elastic cartilage great flexibility so it can withstand repeated bending.
Chondrocytes lie between the fibers. Elastic cartilage is found in the epiglottis (part of the larynx) and the pinnae (the external ear flaps of many mammals, including humans).
Cartilage Growth
Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue.
Cartilage is part of the skeletal system and is formed from condensed mesenchyme tissue that is mostly derived from the mesoderm germ layer while a fetus is developing inside the womb. This loose undifferentiated connective tissue undergoes differentiation to form the cells of the various connective tissues throughout the body such as bone and cartilage. The process of cartilage formation is called chondrification or chondrogenesis.
The precursor cells that become cells of bones and cartilage are called skeletal blast cells. Skeletal blast cells that express the transcription factor Sox9 along with the coexpression of Sox5 and Sox6 develop into chondroblast precursors and those with coexpression of transcription factors Runx2 and osterix become osteogenic precursors.
Differentiation of chondroblasts is favored in areas under compressive force and low partial pressure of oxygen (pO2) as these downregulate the expression of morphogenetic protein 3, a protein that usually inhibits cartilage differentiation.
By contrast, osteogenic differentiation is favored in regions under mild forces and with a relatively high pO2, conditions which upregulate the expression of BMP4, which induces bone formation.
The growth of cartilage is attributable to two processes
Interstitial growth includes:
- Cell division of the chondrocytes
- Synthesis of the extracellular matrix
- Expansion of the cartilage matrix from within
Appositional growth which includes:
- Differentiation of the chondroblasts or perichondrial cells
- Synthesis of the extracellular matrix
- Expansion of the girth of the cartilage
Key Points
The division of cells within cartilage occurs very slowly.
Articular cartilage function is dependent on the molecular composition of its extracellular matrix (ECM).
The remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix in response to forces experienced by the cartilage.
Cartilage growth mainly refers to matrix deposition, but can include both growth and remodeling of the extracellular matrix.
Key Terms
collagen matrix: The most abundant protein in the human body and accounts for 90% of bone matrix protein content.
mesenchyme: Undifferentiated cells of the early embryo are able to develop into the different tisue types, including bone and cartilage.
chondrocyte: A cell that makes up the tissue of cartilage.
chondrification: The process by which cartilage is formed from condensed mesenchyme tissue.
Formation
Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue.
Mesenchyme tissue differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix (ECM). Mesenchymal stem cells (MSCs) are undifferentiated, meaning they can give rise to different cell types. Under the appropriate conditions and at sites of cartilage formation, they are referred to as chondrogenic cells.
During cartilage formation, undifferentiated MSCs are highly proliferative and form dense aggregates of chondrogenic cells at the center of chondrification. These chondrogenic cells then differentiate to chondroblasts, which will then synthesize the cartilage ECM.
Cartilage: Hyaline cartilage showing chondrocytes and organelles, lacunae and matrix.
The extracellular matrix consists of ground substances (proteoglycans and glycosaminoglycans) and associated fibers, such as collagen. The chondroblasts then trap themselves in lacunae, small spaces that are no longer in contact with the newly-created matrix and contain extracellular fluid. The chondroblast is now a chondrocyte, which is usually inactive but can still secrete and degrade the matrix depending on the conditions.
Growth
The majority of body cartilage is synthesized from chondroblasts that are largely inactive at later developmental stages compared to earlier years (pre-pubescence). The division of cells within cartilage occurs very slowly.
Therefore, growth in cartilage is usually not based on an increase in size or mass of the cartilage itself. Remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix, which responds to tensile and compressive forces experienced by the cartilage. Cartilage growth thus mainly refers to matrix deposition, but can include both growth and remodeling of the ECM.
Early in fetal development, the greater part of the skeleton is cartilaginous. This temporary cartilage is gradually replaced by bone (endochondral ossification), a process that ends at puberty. In contrast, the cartilage in the joints remains permanently unossified during life.
Repair
Once damaged, cartilage has limited repair capabilities because chondrocytes are bound in lacunae and cannot migrate to damaged areas. Also, because cartilage does not have a blood supply, the deposition of the new matrix is slow.
Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue. Over the last few years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement.
These include marrow stimulation techniques, including surgeries, stem cell injections, and grafting of cartilage into damaged areas.
However, due to the extremely slow growth of cartilage and its avascular properties, regeneration, and growth of cartilage post-injury is still very slow.
Disease
Several diseases can affect cartilage. Chondrodystrophies are a group of diseases, characterized by the disturbance of growth and subsequent ossification of cartilage. Some common diseases that affect cartilage are listed below.
- Osteoarthritis: Osteoarthritis is a disease of the whole joint, however, one of the most affected tissues is the articular cartilage. The cartilage covering bones (articular cartilage—a subset of hyaline cartilage) is thinned, eventually completely wearing away, resulting in a “bone against bone” within the joint, leading to reduced motion, and pain. Osteoarthritis affects the joints exposed to high stress and is therefore considered the result of “wear and tear” rather than a true disease. It is treated by arthroplasty, the replacement of the joint by a synthetic joint often made of a stainless steel alloy (cobalt Chromoly) and ultra-high-molecular-weight polyethylene (UHMWPE). Chondroitin sulfate or glucosamine sulfate supplements have been claimed to reduce the symptoms of osteoarthritis, but there is little good evidence to support this claim.[7]
- Traumatic rupture or detachment: The cartilage in the knee is frequently damaged but can be partially repaired through knee cartilage replacement therapy. Often when athletes talk of damaged “cartilage” in their knee, they are referring to a damaged meniscus (a fibrocartilage structure) and not the articular cartilage.
- Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
- Costochondritis: Inflammation of cartilage in the ribs, causing chest pain.
- Spinal disc herniation: Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content. The hernia often compresses the adjacent nerves and causes back pain.
- Relapsing polychondritis: a destruction, probably autoimmune, of cartilage, especially of the nose and ears, causing disfiguration. Death occurs by asphyxiation as the larynx loses its rigidity and collapses.
Tumors made up of cartilage tissue, either benign or malignant, can occur. They usually appear in bone, rarely in pre-existing cartilage. The benign tumors are called chondroma, the malignant ones chondrosarcoma. Tumors arising from other tissues may also produce a cartilage-like matrix, the best-known being pleomorphic adenoma of the salivary glands.
The matrix of cartilage acts as a barrier, preventing the entry of lymphocytes or diffusion of immunoglobulins. This property allows for the transplantation of cartilage from one individual to another without fear of tissue rejection.
Function
Mechanical properties
The mechanical properties of articular cartilage in load-bearing joints such as the knee and hip have been studied extensively at macro, micro, and nanoscales. These mechanical properties include the response of cartilage in frictional, compressive, shear, and tensile loading. Cartilage is resilient and displays viscoelastic properties.[rx]
Frictional properties
Lubricin, a glycoprotein abundant in cartilage and synovial fluid, plays a major role in bio-lubrication and wear protection of cartilage.[rx]
Repair
Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore, cartilage damage is difficult to heal. Also, because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Over the last years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement. A tear of the meniscus of the knee cartilage can often be surgically trimmed to reduce problems.
Biological engineering techniques are being developed to generate new cartilage, using a cellular “scaffolding” material and cultured cells to grow artificial cartilage.[rx]
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