ABSTRACT
The skeleton is composed primarily of the skull, the vertebral column, and the notochord. 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. There are six main types of cartilages: hyaline-cell cartilage, elastic-cell rich cartilage, fibro-cell cartilage, matrix-rich hyaline cartilage, cell-rich hyaline cartilage, and scleral cartilages. Hyaline-cell cartilage is the most common. Bone is formed in all vertebrates except living agnathans and Chondrichthyes. The bone is composed of osteocytes embedded in a secreted acellular matrix. Bone is not only confined to the internal skeleton but also found as hard plates or scales in the integument. Unlike mammals, fishes do not have any hematopoietic elements within the bone. Macroscopically, three bone types are distinguishable: compact bones, spongy bones, and chondroid bones. There are two types of fish compact bones: cellular and acellular. Cellular bone contains osteocytes and is found in lower orders. Fishes of higher orders usually have acellular bones that are characterized by a lack of osteocytes. In addition to osteocytes, two other bone cells can be observed: osteoblasts and osteoclasts. Two types of ossification could be identified in fish: intramembranous and endochondral ossification. The vertebra bears processes above and below the centrum (a vertebral body), which encloses remains of the notochord. These processes are the neural and hemal arches, which protect the spinal cord. The notochord consists of inner vacuolated cells surrounded by an epithelial-like sheath of cells. It contains very tumescent cells lying closely pressed together. The notochord cells are thick walled and their cytoplasm is filled with a homogeneous, semifluid 40content. The dorsal fin consisted of a series of longitudinally arranged bones (pterygiophores) embedded in the dorsal musculature that articulated with the fin rays. The caudal fins were supported by a series of flattened hypural bones that contained hyaline cartilage surrounded by perichondral bone. The teleost scale comprises an outer osseous part and an inner layer that consists of parallel collagen fibers embedded in an organic matrix.
The histological diversity of the skeletal tissues of fishes is impressive compared with that of other vertebrate groups. The skeleton is composed primarily of the skull, the vertebral column, and the notochord, incomplete in most fishes. Fishes have no sternum. The appendicular part of the skeleton includes pectoral and pelvic girdles with their related fin skeleton.
