Classification and localization
Cartilage tissue is subdivided into three main types, classified according to the structure of the extracellular substance.
- Hyaline cartilage tissue. The most widespread type. In the body, it covers the articular surfaces of bones, forms the anterior parts of the ribs and the walls of the airways (larynx, trachea, large bronchi). This type of cartilage also forms the metaphyses of tubular bones, acting as growth plates.
- Elastic cartilage tissue. The second most common type. Found in structures requiring flexibility: auricles, nasal cartilages, medium-sized bronchi, and part of the laryngeal cartilages (e.g., the epiglottis).
- Fibrocartilage tissue. Has a limited localization. It makes up the intervertebral discs (except for their central part—the nucleus pulposus), as well as the transition zones of tendons and ligaments directly into hyaline cartilage.
Cellular composition and growth
In mature cartilage tissue, there is only one cell type—chondrocytes, which are located in special cavities within the extracellular substance called lacunae.
Within a single cartilage, cells undergo regular differentiation depending on their depth:
- Young chondrocytes. Located superficially, directly beneath the perichondrium. Unlike fibrocytes, they retain the ability to undergo active cell division.
- Mature chondrocytes. Located in deeper layers. They no longer divide, but begin intensively synthesizing extracellular matrix components.
The metabolic activity of young cells results in the formation of isogenous groups—clusters of 2–6 mature chondrocytes originating from a single mother cell (clones). These groups lie within a single lacuna and are typical of hyaline and elastic cartilages. The combination of matrix synthesis and cell division ensures interstitial growth—an increase in cartilage mass from within.
Intercellular substance
The cartilage matrix consists of fibrous and amorphous components. The physical properties of a particular cartilage directly depend on which element predominates within it.
The fibrous component determines the tissue classification:
- In hyaline cartilage, collagen fibrils (type II collagen) with cross-striations are present. They do not form thick fibers and are highly hydrophilic, causing them to blend with the ground substance and remain invisible under standard hematoxylin-eosin staining.
- In elastic cartilage, collagen fibrils and elastic fibers are combined.
- In fibrocartilage, thick bundles of collagen fibers (type I collagen) are located, which are clearly visible under a microscope.
The amorphous component consists primarily of water (70–80%), organic compounds (10–15%), and minerals (4–7%). Proteoglycan aggregates (PGAs) play a critical role. These giant complexes of proteoglycans and proteins bind massive amounts of water, providing the tissue with pronounced turgor and resilience (particularly characteristic of hyaline cartilage).
Avascularity and immunological features
Cartilage tissue differs fundamentally from many other tissues by its avascularity—the complete absence of its own blood vessels.
Nutrients are delivered to chondrocytes exclusively via diffusion from surrounding structures (perichondrium, joint synovial fluid, or underlying bone tissue). To maintain its avascular status, chondrocytes continuously secrete an anti-angiogenic factor that physically blocks capillary ingrowth. During organism aging, the production of this factor drops, vessels begin to grow into the tissue, which inevitably leads to its mineralization (calcification).
Another unique property is immunological privilege. Cartilage can be transplanted from a donor to a recipient without the risk of a rejection reaction. This is due to the fact that antibodies (immunoglobulins) and immunocompetent cells of the recipient simply cannot penetrate the tissue and contact foreign antigens due to the absence of vessels and the high density of the extracellular matrix.
Briefly: answers to topical questions
What cell types are present in cartilage tissue during its development and functioning?
Sources indicate the following cells and differon stages for cartilage tissue:
- Connective tissue stem cells → prechondroblasts → chondroblasts.
- Chondroblasts — located in the inner cellular layer of the perichondrium, capable of proliferation and active synthesis of extracellular matrix components; they serve as an extrinsic cambial reserve.
- Chondrocytes — the only cell type in mature cartilage, located in lacunae of the extracellular matrix:
- Young chondrocytes — located beneath the perichondrium, capable of division.
- Mature chondrocytes — located in deep layers, do not divide, actively synthesize matrix; may be arranged in isogenous groups of 2–6 cells.
By what mechanisms does cartilage tissue growth occur?
Cartilage tissue growth occurs via two mechanisms:
- Interstitial growth — increase in cartilage mass from within. Provided by chondrocyte activity (matrix synthesis and cell division).
- Appositional (peripheral) growth — deposition of new layers from the outside. Carried out by perichondrial chondroblasts, which synthesize extracellular substance, wall themselves inside it, and turn into chondrocytes. This method is the primary one in embryogenesis and regeneration.
What layers does the perichondrium consist of?
The perichondrium (perichondrium) consists of two functional layers:
- Fibrous layer (outer) — contains blood vessels that provide nutrition (trophism) to the avascular cartilage tissue.
- Cellular layer (inner) — directly adjacent to the cartilage and contains extrinsic cambial elements—chondroblasts.
To remember the physical properties of the matrix: Minerals mean Hardness, Fibers mean Strength, Elastin means Elasticity, and PGAs (water) mean Turgor/Resilience.
Class and exam questions
Why are fibers not visible in hyaline cartilage specimens?
It contains fine collagen fibrils that are highly hydrophilic. Because of this, they merge with the ground amorphous substance and are not visible with standard hematoxylin-eosin staining.
How does the nutrition of cartilage tissue occur?
Nutrition occurs exclusively via diffusion. Substances penetrate the cartilage from synovial fluid, the perichondrium, or vessels of the underlying bone.
Why can cartilages undergo calcification with age?
During the aging process, chondrocytes decrease their secretion of the anti-angiogenic factor. This leads to the ingrowth of blood vessels into the cartilage tissue and its subsequent mineralization.
Why can cartilage be transplanted without the threat of rejection?
Cartilage has no vessels, and its dense matrix is impermeable to large proteins. The recipient's immune system simply has no physical access to the donor tissue antigens.
What else to study on this topic
- Mechanism of formation and structure of proteoglycan aggregates (PGAs)
- Stages of interstitial growth and formation of isogenous groups
- Differences between type I and type II collagen in cartilage tissue
- Mechanism of action of the chondrocyte anti-angiogenic factor
- The process of differentiation from young to mature chondrocytes
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