Classification and Localization
Cartilage tissue is divided into three main types based on the structural composition of its extracellular matrix.
- Hyaline cartilage. The most widespread type. In the body, it covers the articular surfaces of bones, forms the anterior parts of the ribs, and lines the walls of the respiratory tract (larynx, trachea, large bronchi). This cartilage type also forms the metaphysis of long bones, acting as growth plates.
- Elastic cartilage. The second most common type. Found in structures requiring flexibility: the auricles, nasal cartilages, medium bronchi, and parts of the laryngeal cartilages (e.g., the epiglottis).
- Fibrocartilage. Has a restricted distribution. It forms intervertebral discs (except for their central part, the nucleus pulposus), as well as the transition zones where tendons and ligaments attach directly to hyaline cartilage.
Cellular Composition and Growth
Mature cartilage tissue contains only one cell type — chondrocytes, which reside in specialized cavities within the extracellular matrix called lacunae.
Within a given cartilage, cells undergo predictable differentiation depending on their depth:
- Young chondrocytes. Located superficially, directly beneath the perichondrium. Unlike fibrocytes, they retain the capacity for active cell division.
- Mature chondrocytes. Located in deeper layers. They no longer divide, but actively synthesize components of the extracellular matrix.
The activity of young cells results in the formation of isogenous groups — clusters of 2–6 mature chondrocytes derived from a single parent cell (clones). These groups share a single lacuna and are typical of hyaline and elastic cartilages. The combination of matrix synthesis and cell division drives interstitial growth — the expansion of cartilage mass from within.
Extracellular Matrix
The cartilage matrix consists of fibrous and amorphous components. The physical properties of a specific cartilage depend directly on which element predominates.
The fibrous component determines tissue classification:
- In hyaline cartilage, collagen fibrils (type II collagen) with cross-banding are present. They do not form thick fibers and are highly hydrophilic, causing them to blend with the ground substance and remain invisible on standard hematoxylin-eosin staining.
- In elastic cartilage, collagen fibrils and elastic fibers are combined.
- In fibrocartilage, thick bundles of collagen fibers (type I collagen) predominate and are clearly visible under a microscope.
The amorphous component consists primarily of water (70–80%), organic compounds (10–15%), and minerals (4–7%). A critical role is played by proteoglycan aggregates (PGAs). These giant complexes of proteoglycans and proteins bind massive amounts of water, providing the tissue with pronounced turgor and resilience (especially characteristic of hyaline cartilage).
Avascularity and Immunological Features
Cartilage tissue differs fundamentally from many other tissues by its avascularity — the complete absence of intrinsic blood vessels.
Nutrients reach chondrocytes solely via diffusion from surrounding structures (the perichondrium, synovial fluid of joints, or underlying bone). To maintain its avascular status, chondrocytes continuously secrete an anti-angiogenic factor that physically blocks capillary ingrowth. During aging, the production of this factor declines, blood vessels begin to sprout into the tissue, inevitably leading to its mineralization (calcification).
Another unique property is immunological privilege. Cartilage can be transplanted from a donor to a recipient without the risk of rejection. This occurs because antibodies (immunoglobulins) and immunocompetent recipient cells cannot penetrate the tissue and contact foreign antigens due to the absence of blood vessels and the high density of the extracellular matrix.