Human skeletons can be classified based on their position, shape, and structure. In long bones, the epiphysis is the section between the expanded end of bone and its growth plate (also known as the physis or epiphyseal plate). The physis disappears by 20 years of age. The epiphysis consists of a hyaline cartilage joint surface, compact exterior (cortex), and spongy interior (medulla), and it is the region of secondary ossification. The diaphysis is the central shaft of the long bone with compact bone surrounding marrow cavity, and it is the region of primary ossification. The metaphysis is the junctional portion lying between the growth plate and the diaphysis. Cartilage is composed of chondrocytes embedded in a prominent ground substance of extracellular matrix, which is made of proteoglycan aggregates in hyaluronic acid. The varying proportions of collagen types and elastic fibers embedded within the ground substance give rise to three types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage.