Fibrocartilage

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

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
• collagen
• proteoglycans
• glycosaminoglycans

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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Summary

Although not discussed in this article, the comparative table below gives an overview of the differences and similarities among the three types of cartilage: