and Jürgen Roth2
Medical University of Vienna, Vienna, Austria
University of Zurich, Zurich, Switzerland
Cartilage represents highly specialized connective tissues. Three types can be distinguished, which are found in different anatomical locations and differ with regard to the amount and arrangement of chondrocytes and fibers: hyaline (articular) and elastic cartilage and fibrocartilage. All three have in common a low metabolic rate, the absence of blood vessels, and the ability to grow. Cartilage consists of chondrocytes and a matrix, the latter making up the bulk of cartilage mass. The matrix consists of collagens and proteoglycans and has a 60–80 % water content. Cartilage is elastic and flexible and has a high tensile strength. This characteristic is mainly due to the matrix composition and architecture. Hyaline cartilage is the most abundant type of cartilage and, for example, covers the bone surfaces of joints, hence the name articular cartilage.
A most detailed analysis of articular cartilage, especially its matrix, became possible through high-pressure freezing fixation, which preserved the cartilage in a near-native vitrified state. Panels A and B show electron micrographs of bovine articular cartilage fixed by high-pressure freezing followed by freeze-substitution and Epon-embedding. Pairs of chondrocytes (the so-called chondrons) can be seen. They are actively synthesizing and secreting the matrix components. Young, as compared to old, chondrocytes contain abundant endoplasmic reticulum and well-developed Golgi apparatus. Mature chondrocytes, as seen in panel A, are spherical and lie in nests.