Differentiation of the secondary elastic cartilage in the external ear of the rat

The cartilage in the external ear of the rat belongs to the group of secondary cartilages and it has a unique structural organization. The chondrocytes are transformed into typical adipose cells, the proteoglycan cartilage matrix is reduced to thin capsules around the cells and the rest of the extracellular matrix is occupied by a network of coarse elastic fibers. It appears late in development (16-day fetus) and needs more than one month for final development. The differentiation proceeds in several steps which partly overlap: the appearance of collagen fibrils, elastin fibers, the proteoglycan matrix, and the adipose transformation of chondrocytes. The phenotype of this cartilage and the course of its differentiation are very stable, even in very atypical experimental environmental conditions. The only exceptions are explants in organ culture in vitro and perichondrial regenerates. In these conditions the development of elastic fibers is slow and poor while the production of the proteoglycan matrix is abundant. The resulting cartilage then displays structural characteristics of hyaline cartilage rather than those of the initial elastic one.