Developing cartilage occurs as two large, oval structures near the midline of this specimen. Cartilage development begins with the aggregation of mesenchymal cells that differentiate into chondroblasts. With the initiation of matrix production, the cells become surrounded by matrix and are called chondrocytes.
Cartilage growth continues by two processes:
Appositional Growth - forms new cartilage on the surface of existing cartilage. The perichondrium is a layer of dense irregular connective tissue that surrounds cartilage. It is divided into two layers:
Outer Fibrous Layer - contains fibroblasts that produce the type I collagen on the outer surface of the perichondrium.
Inner Chondrogenic Layer - contains fibroblast-like cells that differentiate into chondroblasts, initiate matrix production (type II collagen), and become immature chondrocytes. The new matrix increases cartilage mass.
Interstitial Growth - forms new cartilage within the existing cartilage. Chondrocytes divide and the daughter cells move apart from each other. This is only possible when the surrounding matrix is distensible.
Unlike the basophilia of mature cartilage, developing cartilage is pale staining because sulfated glycosaminoglycans (GAGs) have not been added to the matrix.