At the edge of the cartilage is the perichondrium and the progression of cells as they differentiate from perichondrial fibroblasts (top) to chondrocytes (bottom) (HA18). The perichondrium is composed of dense irregular CT and has flattened perichondrial fibroblasts (HA18, p). These fibroblasts differentiate into chondroblasts and start secreting the components of the cartilage matrix. When chondroblasts are completely surrounded by matrix, they are called chondrocytes. This process is called appositional growth or growth from the outside (addition of cells and matrix to the surface of the cartilage). During their transformation, perichondrial fibroblasts stop secreting type I collagen, fibronectin, and predominantly hyaluronic acid and start secreting type II collagen, chondronectin, and predominantly chondroitin sulfate.