Chapter 53 - Engineering cartilage and other structural tissues: principals of bone and cartilage reconstruction

Abstract

Cartilage is a fiber-reinforced composite material composed of chondrocytes surrounded by specialized extracellular matrix (ECM) in unique architectures. There are three types of cartilage, namely, hyaline, elastic, and fibrocartilage. Hyaline cartilage is the most prevalent type of cartilage and can be found in synovial joints. It is a translucent stratified tissue composed mainly of type II collagen that consists of several distinct microscale cellular niches. Hyaline cartilage enables joints to articulate with low frictional forces by providing smooth and lubricated surface. One unique characteristic of hyaline cartilage is the lack of blood vessels, nerves, or lymphatics. It also has a limited capacity for intrinsic repair and healing. This type of cartilage is composed of a dense ECM with chondrocytes and the nutrients usually reach chondrocytes through diffusion. Hyaline cartilage chondrocyte homeostasis is achieved through cyclic compression, which facilitates the exchange of nutrients and waste products. Elastic cartilage is similar to hyaline cartilage in architecture, and it contains yellow elastic fiber networks and type II collagen. The matrix is dominated by elastin fiber networks, and chondrocytes lie between the fibers. Elastic cartilage is found in the ear, epiglottis, and pinnae. Fibrocartilage is composed of a mixture of types I and II collagen and is whitish in color. This type of cartilage is found mainly in soft tissue-to-bone attachments, symphysis pubis, annulus fibrosus, and menisci. Cartilage is largely populated by chondrocytes, which are specialized cells that can secrete proteins that form the ECM of cartilage. Collagen is the main constituent of cartilage and is made from amino acids wrapped together to form elongated triple-helix fibrils. Fibrils are structural materials that are usually in the scale of 10–100nm. Aggrecan, bone sialoproteins, and proteoglycans are also found in the ECM of cartilage.