Whenever locomotion is discussed, the default anatomical components that are addressed are usually bones, muscles, and ligaments. However, there are other supporting structures that contribute to the tenacity of the musculoskeletal system that is crucial for optimum functionality. Cartilage is a blended group of supportive tissue that provides structural support and shape to the tissues throughout the body. Most anatomical structures can be distinguished from each other by their unique traits.
Researchers are able to tell smooth muscle apart from striated or cardiac muscle based on the histological architecture and distinct biochemical markers found on each tissue. Cartilage, however, is more difficult to distinguish as there is significant overlap among the three subtypes that exists. However, there are specific features that can be identified that facilitate discernment of the varying types of cartilage.
This article focuses on one of the three subtypes of cartilage – fibrocartilage. The embryology, histological architecture and clinical significance of this tissue subtype will be discussed.
- What is cartilage?
- Embryology of cartilage
- Histology of fibrocartilage
- What is in the extracellular matrix?
- Clinical significance
What is cartilage?
Cartilage is an avascular, aneural tissue of the musculoskeletal system. Its primary role is to provide additional reinforcement at strategic weight-bearing areas in order to reduce the erosion that would occur if bony surfaces had direct articulation. Cartilage is also a major support system for several soft tissue structures within the auditory and upper digestive tract. Additionally, one particular subtype of cartilage also acts as scaffolding for subsequent osteogenesis.
Owing to the fact that cartilage is an avascular tissue; it obtains its nutrients from adjacent perichondrium that has a rich blood supply, is well innervated and is equipped with a definitive lymphatic system. The nutrients diffuse from the well-perfused perichondrium to the deeper cartilaginous tissue. It also has a lower metabolic rate in order to compensate for the slow delivery of nutrients to its constituent cells.
Cartilage is quite a ubiquitous structure during embryonic development. It forms the template that will be subsequently ossified during endochondral ossification. However, in other areas of the body, such as the synovial joint surfaces, tracheal rings or the laryngeal and epiglottis structures, cartilage persists throughout adulthood (only calcifying pathologically or in late adulthood). Cartilage is able to offer such great biomechanical support to weight-bearing areas due to the composition of its extracellular matrix. The arrangement of the composite macromolecules increases the tenacity and the flexibility of the structure. It is, therefore, able to withstand great amounts of mechanical stress while allowing a significant degree of flexibility.
The cells that result in the production of cartilage are known as chondroblasts. They also produce the extracellular matrix of cartilage; which is filled with different subtypes of collagen. These chondroblasts are irregular, flat cells that have numerous cytoplasmic projections extending into the extracellular matrix that it produces. As chondroblasts age, they lose their ability to replicate and form daughter cells. This coincides with a decrease in the metabolic activity of these cells. They are subsequently referred to as chondrocytes.
Embryology of cartilage
Cartilage is a mesenchymal derivative that begins to form during the fifth gestational week. Recall that from the third gestational week the notochord and neural tube continues to develop and laterally there is thickening of the intraembryonic mesoderm on either side of these structures, forming the paraxial mesoderm. By the fourth week of gestation the paraxial mesoderm becomes segmented and forms a continuous chain of bead-like elevations called somites. Each somite has a dorsolateral and ventromedial component. The former is known as the dermomyotome, while the latter is called the sclerotome. Loose aggregation of sclerotome cells gives rise to the embryonic connective tissue known as mesenchyme.
Both the sex-determining region Y box 9 (Sox-9) and bone morphogenetic proteins (BMP) have been implicated in the differentiation of mesenchymal cells into precondrocytes. These precondrocytes usually exist in a resting phase until they are stimulated by members of the hedgehog protein family (i.e. Indian hedgehog [Ihh] or sonic hedgehog [shh] proteins). This activation results in the formation of hypertrophic proliferating chondrocytes that produce cartilaginous matrix.
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Condensation of mesenchymal tissue results in the formation of chondrificationcentres and by extension, the beginning of chondrogenesis. One of the hallmark changes of this process is that as mesenchymal cells form these clusters, there is recession of their cytoplasmic extensions and the cells become more spherical. Additionally, there is a reduction of the cytoplasm to nucleus ratio, the nuclei become oval and adjacent cells maintain their gap junction communication pathways. The resulting precondrocytes are more tightly packed than their precursors.
As they undergo rapid replication, the precondrocytes are referred to as chondroblasts. Each cell continues to replicate and also produce extracellular matrix that results in further separation of the cells into their individual lacunae. The isolated cells retain their ability to replicate and subsequently there are clusters of chondrocytes within each lacuna known as isogenous cell aggregates.
It should also be noted that the pattern of differentiation of cartilage occurs centrifugally. In other words, cells at the centre of the tissue are usually older than those in the periphery. Therefore, the centrally located cells will become mature chondrocytes before the cells in the periphery. Consequently, the pattern of distribution of cells is such that chondrocytes are seen in the centre and chondroblasts are observed in the periphery. However, surrounding the developing cartilage is a double layer of perichondrium, which also develops from mesenchyme. The parietal sheath forms fibroblasts that produce matrix rich in collagen and is enclosed by vascular mesenchyme. The parietal layer contains a reservoir of prechondroblasts that will facilitate growth of the cartilage during extrauterine life.
The development of fibrocartilage, specifically, occurs later than other cartilage subtypes. There are two proposed developmental pathways for fibrocartilage:
- Differentiation from immature hyaline cartilage
- Differentiation from dense fibrous connective tissue.
In addition to bone morphogenetic protein and hedgehog proteins, parathyroid hormone-related peptide influences the transformation that results in fibrocartilage production. In additional to genetic involvement, mechanical forces also drive the formation of fibrocartilage. Formation of new fibrocartilage during adulthood is undoubtedly a metaplastic process.
Histology of fibrocartilage
Cartilage has been subdivided into:
Fibrocartilage is a transition tissue that should be viewed as a blend between hyaline cartilage and dense fibrous connective tissue. It is a white, densely arranged, opaque, tufted tissue with a mixture of both chondrocytes and fibroblasts. Its composition varies depending on the anatomical location and intended function of the fibrocartilage in this area. For example, fibrocartilage found in intervertebral discs has greater tensile strength and elasticity. In contrast, the fibrocartilage found in the glenoid or acetabular labra are more resistant to repetitious stress and provide strength and elasticity to the musculoskeletal attachment.
On the basis of their presence and functional significance, fibrocartilage can be categorized into four different types:
- intra-articular fibrocartilage (e.g., menisci) present at the joints where flexion and extension are associated with gliding and they act as thrust-pads and help to prevent instability of the joints;
- connecting fibrocartilage (e.g., intervertebral disks) present in the limited-motion joints, acting as a cushion to distribute stresses;
- stratiform fibrocartilage in the form of a thin layer over the bone where the tendons may glide, (e.g., tendons of the peroneus longus and tibialis posterior), helpng to minimize the friction between bone and tendon;
- circumferential fibrocartilage (e.g., glenoid and acetabular labrum) present in the form of a ring without a centre to protect the joint margins and improve the bony fit.
While it is difficult to differentiate fibrocartilage from hyaline or elastic cartilage at times, it should be known that fibrocartilage is more commonly observed at enthesis organs and wrap-around regions. The wrap-around regions represent areas where tendons of limb muscles course around pulley systems within the joint and change directions (e.g. the action of the quadriceps femoris across the knee joint). On the other hand, the enthesis organs represent points of increased stress within a fibrocartilaginous joint (e.g. the insertion of the Achilles tendon into the calcaneus).
There are periosteal and sesamoid forms of the fibrocartilaginous enthesis. The former is formed from the attachment from tendon to the periosteum, while the latter is closer to the tendinous part of the insertion. The organ functions primarily as risk reduction unit as it spreads the stress within; making it less likely for tendon rupture and joint destabilization to occur.
Cells that produce fibrocartilage are often referred to as fibroblasts or chondrocytes. However, because of their unique development, they should be referred to as fibrocartilage cells or even fibrochondrocytes. The cells within the enthesis units (i.e. enthesis fibrocartilage cells) are similar to chondrocytes such that they possess a round to oval shape and are isolated within lacunae in the extracellular matrix. Furthermore, these cells lack gap junctions among, and by extension there is little to no communication among the cells. This is quite the opposite when compared to normal fibroblast cells or osteocytes, which are known to have an elaborate communication system.
Otherwise, the organelles in the enthesis fibrocartilage cells are quite similar to those seen in the average chondrocyte. These cells possess lipid droplets, glycogen granules that arise from the rough endoplasmic reticulum and multiple intermediate filaments. The intermediate filaments are particularly significant as they are most likely there to reinforce the other biomechanically active components of the surrounding tissue (i.e. it helps the cells to tolerate compressive forces).
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What is in the extracellular matrix?
The extracellular matrix is an amalgamation of multiple substances that gives cartilage its biomechanical properties. Its composition varies with each subtype of cartilage. Generally, it consists of varying amounts of:
- elastic fibres
The mixture contains ground substance; which is a carbohydrate-based salts gel that is also substantially hydrated. The ground substance is held together within tightly packed strands of proteoglycans. Within cartilage, the proteoglycans contain glycosaminoglycan (GAG) molecules that have keratin sulphate and chondroitin sulphate side chains. The negatively charged sulphate components attract the water molecules (also called solvation water) to the proteoglycan strands. This contributes to the shock absorbing capacity of cartilage. However, proteoglycans are relatively scarce in fibrocartilage and as such; the tissue appears relatively more acidophilic when compared to hyaline or elastic extracellular matrix.
Collagen is another ubiquitous substance that also contributes to almost half of the composition of cartilage. There are numerous subtypes of collagen distributed throughout the body. However, the ones most commonly found in cartilage are types I and II. The quantity, size and type of collagen fibres observed in a given sample vary depending on the subtype of cartilage being evaluated.
In light of this fact, it should be noted that the relatively low quantity of type II – and the abundance of type I – collagen in fibrocartilage is a unique histological feature. However, in other areas, there can be a total absence of type II collagen from the extracellular matrix. Non fibrillar collagen (including types IX & XII) have also been documented in some fibrocartilages. The integrity of cartilage is dependent on the electrostatic attraction that exists among the subtypes of glycosaminoglycans (i.e. proteoglycans and hyaluronic acid) that are produced by chondroblasts (and chondrocytes) and secreted into the extracellular space. Histologically, the striations of the extracellular matrix are concentrically arranged. Visibility of the fibres on histology helps to distinguish between hyaline cartilage and fibrocartilage.
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Although not discussed in this article, the comparative table below gives an overview of the differences and similarities among the three types of cartilage:
There are very little primary pathologies directly linked to fibrocartilage. Most disorders are traumatic in nature and involve joints that are rich in fibrocartilage. Below are a few examples where trauma can affect joints rich in fibrocartilage:
Bankart and Hill-Sachs lesions
Bankart and Hill-Sachs lesions are associated with anterior shoulder dislocations, resulting in damage to the glenoid labrum and humeral head, respectively. Sudden anterior dislocation of the humeral head may be associated with retraction of the proximal humerus by the rotator cuff muscles and ligaments that are still intact. As a result, the posterolateral aspect of the head of the humerus makes a forceful impact on the anteroinferior part of the glenoid labrum. Therefore, both injuries often occur concomitantly. A reverse Bankart and reverse Hill-Sachs can also occur in posterior shoulder dislocations. It should also be noted that patients with a Bankart injury are at increased risk for recurrent dislocations.
Acetabular labral tears
Acetabular labral tears have numerous aetiological factors. These vary from laxity or hypermobility within the acetabulofemoral articulation, to traumatic damage, or femoroacetabular impingements. With age, the concept of joint degeneration or dysplastic changes can also be entertained. While the acetabular labrum is a fibrocartilaginous structure, remember that it is circumferentially incomplete. The structure is completed by the acetabular ligament. Therefore, either structure can become damaged or defective, leading to joint injury.
Intervertebral disc degeneration and herniation
Intervertebral discs degeneration and herniation is multifactorial and is characterized by the loss of proteoglycans and hydration. Consequently, the disc becomes more fibrous and is unable to distribute stresses equally. Disc herniation is a serious pathology that can cause long standing back and radiating lower limb pain. Recall that the fibrocartilaginous intervertebral disc has an inner nucleus pulposus and an outer annulus fibrosus. Trauma is most often associated with precipitating disc herniation.
Triangular fibrocartilage complex
The triangular fibrocartilage complex (TFCC) in an amalgam of several fibrocartilaginous and ligamentous structures located at the distal radioulnocarpal interface. The articular disc which forms the triangular fibrocartilage proper is supported on the volar and dorsal surfaces by radioulnar ligaments. The ulnocarpal meniscus and tendon sheath of the extensor carpi ulnaris also contribute to the complex.
The triangular fibrocartilage complex functions as a major stabilizer of the distal radiocarpal articulation. It also acts as a shock absorber for the ulnocarpal articulation as well. The TFCC extends from the ulnar aspect of the radius in the lunate fossa and travels toward the base of the styloid process of the ulna, over the ulnar head. The final insertion of the complex is at the hamate, triquetrum and the proximal part of the fifth metacarpal.
The most popular mechanism of injury to the TFCC is a fall on the outstretched hand (FOOSH). Additionally, torsional forces that dramatically rotate the wrist with the forearm relatively stable (i.e. power tool injuries) and distal radial fractures can also damage the complex. Injuries to TFCC is classified as either traumatic (class 1) or degenerative (class 2) (see Table 1). Patients with this form of injury often complain of pain to the ulnar aspect of the wrist that may be associated with a clicking sound.
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