24The skeleton is composed primarily of the skull, the vertebral column, and the notochord. The vertebrae are intermetameric and connected by elastic ligaments. Fishes have no sternum. The appendicular part of the skeleton includes pectoral and pelvic girdles with their related fin skeleton. Cartilage is a firm, resistant tissue found in all classes of vertebrates. The chondrocyte is the main constituent cell of cartilage that is isolated within a voluminous extracellular matrix. Covering most elements of cartilage is the perichondrium, which comprises chondroblasts that are capable of forming a new cartilage matrix. Cartilage forms the structural elements of the skeleton in embryonic and very young trout, and can almost disappear in older fishes. There are three main types of cartilages: hyaline, elastic, and fibrous. Of the three, hyaline is the most common. Fishes have a variety of specialized cartilages that are somewhat different from those found in higher vertebrates. Bone is formed in all vertebrates except living agnathans and chondrichthyes. Like cartilage, it is composed of a parenchymal cell, the osteocyte, embedded in a secreted acellular matrix. Bone is not confined to the internal skeleton but is also found as hard plates or scales in the integument. Histologically, fish bones are similar to those of higher vertebrates. Unlike mammals, fishes do not have any hematopoietic elements within the bone. There are two types of fish bones, cellular and acellular. Cellular bone contains osteocytes and is found in lower orders. Fishes of higher orders usually have acellular bones, which are characterized by a lack of osteocytes. Bone histogenesis takes place in two forms, direct and indirect. In direct bone formation, the bone is formed in association with the dermis; in indirect bone formation, it is formed by the perichondral ossification of the hyaline cartilage. In addition to osteocytes, two other bone cells can be observed: osteoblasts and osteoclasts. Besides intramembranous and endochondral ossification, we have to consider parachondral and perichondral ossifications.