Cartilage is a flexible type of connective tissue found in various parts of the body, providing support and cushioning. It is softer and more pliable than bone, yet possesses considerable strength. Cartilage develops through two distinct processes, one of which is interstitial growth, characterized by its expansion from within the existing tissue.
The Mechanism of Interstitial Growth
Interstitial growth begins with specialized cartilage cells called chondrocytes, which reside within small cavities in the cartilage matrix known as lacunae. These chondrocytes are active secretory cells, containing abundant rough endoplasmic reticulum to produce the components of the extracellular matrix.
The process unfolds as these existing chondrocytes undergo mitosis, producing two or more daughter cells within the same lacuna. These newly formed chondrocytes then begin to synthesize and secrete new extracellular matrix components, such as collagen fibers and proteoglycans like aggrecan. This continuous secretion of new matrix material from the inside out causes the cartilage tissue to expand, pushing the cells further apart and increasing the overall volume of the cartilage. The extracellular matrix allows for this internal expansion and provides the tissue’s flexible and semi-rigid nature.
Appositional Growth as a Counterpart
In contrast to interstitial growth, cartilage also grows through appositional growth, which means growth on its outer surface. This process involves a membrane covering most cartilage surfaces called the perichondrium. The inner layer of the perichondrium contains specialized stem cells that differentiate into chondroblasts.
These chondroblasts then secrete new extracellular matrix components onto the existing surface of the cartilage. As new layers of matrix are added, the cartilage increases in girth or width. This external layering mechanism differs fundamentally from interstitial growth.
Occurrence and Significance in the Body
Interstitial growth is particularly significant during early development, especially in the epiphyseal plates of long bones. These plates are responsible for the lengthening of bones during childhood and adolescence. Chondrocytes within these growth plates rapidly divide and enlarge, pushing the ends of the bone away from the shaft, a process that is later replaced by bone tissue in endochondral ossification.
Interstitial growth also contributes to the development and maintenance of articular cartilage. However, in adult cartilage, particularly articular cartilage, interstitial growth becomes very limited. This limited capacity for growth and repair in adults is largely due to cartilage’s avascular nature, meaning it lacks a direct blood supply, which hinders nutrient diffusion and the delivery of cells necessary for significant repair. Consequently, mature cartilage exhibits poor self-repair capabilities.