Cartilage
Cartilage, or gristle is a fibrous connective tissue found in humans and vertebrate animals that provides support to the skeleton at specific sites throughout the body. Cartilage is composed of specialized cells, called chondrocytes, surrounded by a gelatinous matrix of collagen, a tough protein. The cartilage surface is covered by a membrane known as the perichondrium. The skeleton of vertebrate fetuses is composed largely of cartilage, which is eventually replaced by bone . Some cartilage persists into adulthood. It is fibrous and rubbery, providing support, flexibility, and elasticity to the ends of bone tissue and to the nose, ears, breastbone, trachea, larynx, joints, and other parts of the body. Some animal skeletons, such as that of the shark, are completely cartilaginous.