an animal; rather, it serves a similar function at the cellular level. Vertebrate skeletons In most vertebrates, the main skeletal component is bone. Bones compose a unique skeletal system for each type of animal. Another important component is cartilage which in mammals is found mainly in the joint areas. In other animals, such as the cartilaginous fishes, which include the sharks, the skeleton is composed entirely of cartilage. The segmental pattern of the skeleton is present in all vertebrates, with basic units being repeated, such as in the vertebral column and the ribcage. Bones are rigid organs that form part of the endoskeleton of vertebrates. They provide structural support for the body, assist in movement by opposing muscular contraction, and create a protective wall around internal organs. Bones are primarily made of inorganic minerals, such as hydroxyapatite, while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones are either osteoblasts, osteoclasts, or osteocytes. Bone tissue is a type of dense connective tissue. One of the types of tissue that makes up bone tissue is mineralized tissue and this gives it rigidity and a honeycomb-like three-dimensional internal structure. Bones also produce red and white blood cells and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. During embryonic development, bones are developed individually from skeletogenic cells in the ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone. Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased in perichondrium tissue. Ligaments are elastic tissues that connect bones to other bones, and tendons are elastic tissues that connect muscles to bones. Amphibians and reptiles The skeletons of turtles have evolved to develop a shell from the ribcage, forming an exoskeleton. The skeletons of snakes and caecilians have significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards. Birds The