Cartilage is flexible connective tissue found throughout the whole body. It is stiffer than most other connective tissue types but flexible enough to withstand compressive forces without breaking or becoming permanently deformed.
This article will discuss the function, structure and different types of cartilage, as well as its neuromuscular supply. Relevant clinical conditions will be discussed at the end of the article.
Function of Cartilage
Cartilage has 3 main functions:
- It provides a supportive framework for the walls of the airways (nose, trachea, larynx and bronchi) which prevents them from collapsing.
- It forms the articular surfaces of bone.
- It functions as a template for the majority of the skeleton (hyaline cartilage is a precursor to bone in-utero and ossifies to form the foetal skeleton).
Structure of Cartilage
Cartilage consists of 3 main elements:
- Specialised cells
- The extracellular matrix
- Collagen fibres
Cartilaginous tissue is encased by a fibrous sheath called the perichondrium (which has a similar function to the periosteum in bone) which provides protection and mechanical support.
The Specialised Cells
There are two main cell types in cartilage:
The chondroblasts are found near the outer surface of the cartilage, under the perichondrium, and these secrete the extracellular matrix. As they do this, they become entrapped and mature into chondrocytes. As the cartilage grows, the chondrocytes divide and cluster together to form what is known as ‘lacunae’ (meaning little lakes or small pits).
Overall, the main function of chondroblasts is to create new cartilaginous tissue whilst the function of chondrocytes is to maintain it.
The Extracellular Matrix
The extracellular matrix (ECM) is made up of 3 main elements:
- Aggrecans (10%)
- Water (75%)
- Collagen fibres and other constituents (15%)
Aggrecans are a type of proteoglycan, which refers to a molecule made of carbohydrates (repeating disaccharide units known as glycosaminoglycans or GAGs) attached to a protein core. Aggrecan is specific to cartilage and forms large aggregates with hyaluronic acid. The aggregates are negatively charged and pull water into the ECM which allows cartilage to resist compressive forces more easily.
Collagen fibres provide the tissue with very high tensile strength and ensure that aggrecan molecules are trapped in the ECM. Elastic fibres provide elasticity and flexibility. Therefore, depending on the function of the cartilage the ratio of collagen fibres to elastic fibres changes.
Cartilage Growth and Repair
Growth of Cartilage
Cartilage can expand by producing new cartilaginous tissue both from inside and outside the formed cartilage:
- Appositional growth – Chondroblasts, found along the surface of the inner cellular layer of the perichondrium, continuously produce new matrix.
- Interstitial growth – During childhood and adolescence, new cartilaginous tissue is formed from within the cartilage as the chondrocytes divide and produce new matrix.
Cartilage does not receive a blood supply and is therefore avascular. The cells receive nutrition by diffusion from nearby capillaries in the perichondrium. This means that cartilage can never become too thick and the growth/turnover of cells in the cartilage is very slow.
Cartilage is aneural, it does not contain any nerves. Therefore, any pain associated with pathology is typically due to irritation of surrounding tissues.
Repair of Cartilage
Cartilage can be damaged by overuse of a joint or through direct injury but is difficult to repair because:
- The chondrocytes are fixed in the lacunae and unable to migrate to repair damaged areas
- Cartilage is avascular and relies on diffusion, so the deposition of new cartilage is often very slow
- Damaged hyaline cartilage is often replaced with fibrocartilage which is not as well suited to the function of cartilage at that site
Types of Cartilage
There are 3 types of cartilage
This is the most common form of cartilage in the body and also the weakest type. It is mostly made up of collagen (type II) with relatively few elastic fibres and is surrounded by a perichondrium.
Hyaline is a precursor for bone and so is found in the epiphyseal growth plates of children. It is also found in the ribs, nose, larynx and trachea.
Articular cartilage is a type of hyaline cartilage that is found on the surface of bones in synovial joints. It has a unique structure that is organised into specific zones.
The chondrocytes are flattened at the surface and in the deeper layers, chondrocytes are stacked in columns within a calcified matrix – increasing their resistance to compressive forces. Articular cartilage does not have an overlying perichondrium instead, it is covered by the synovial membrane.
Elastic cartilage contains collagen and an abundance of elastic fibres that are arranged into a threadlike network in which chondrocytes can be found.
It is resilient, flexible and found in structures that are commonly deformed for example the outer ear, larynx and epiglottis. Similar to hyaline, it also has a perichondrium.
This is the strongest type and is found in joint capsules, tendon insertions, ligaments and intervertebral disks. Fibro-cartilage is made up of alternating layers of hyaline matrix and layers of dense collagen fibres.
These thick layers of collagen fibres are orientated in the direction of pressure applied in order to further reinforce it. Fibro-cartilage does not always have a perichondrium and is usually connected directly to either hyaline cartilage, tendons or ligaments.
Clinical Relevance – Osteoarthritis
Osteoarthritis is characterised by the progressive loss of articular cartilage and remodelling of the underlying bone. Osteoarthritis affects the hip joint, small joints of the hands and feet and the knee joint.
Patients often present with pain and stiffness in these joints. Risk factors include advancing age, obesity, female gender and manual labour occupations.
Treatment of an osteoarthritic joint normally begins with lifestyle changes such as losing weight, then physiotherapy and painkillers (NSAIDs) are used for management and finally, when both conservative and medical interventions fail, surgical intervention such as a joint replacement may be considered.
Clinical Relevance – Costochondritis
Costochondritis is a condition where the costal cartilage (a type of hyaline cartilage) of the ribs becomes inflamed. This causes chest pain which can present with symptoms similar to a heart attack.
Costochondritis often resolves on its own and is managed with anti-inflammatory medication and rest.