Endoskeletons first evolved about 350–500 million years ago when the first vertebrates evolved cartilaginous endoskeletons. Lampreys are descendants of these early vertebrates. Subsequently, bony fishes evolved a more rigid mineralized internal skeleton, a defining feature of this diverse group of vertebrates. Sharks and other elasmobranchs (the cartilaginous fishes) evolved unusual skeletons of calcified cartilage, losing the earlier mineralized bony skeletons of their bony fish ancestors.

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A bony or cartilaginous endoskeleton lies internal to most of an animal’s soft tissues. In contrast to exoskeletons, the endoskeletons of vertebrate animals can grow extensively and, when broken, can be repaired. Endoskeletons also provide protection for certain key internal organs, such as the brain, lungs, and heart.

The bones of vertebrate skeletons consist of a variety of tubular, rodlike, and platelike elements that form a scaffold to which the muscles attach. Because bones are rigid and hard, there are joints between adjacent bones to enable movement. Muscles attach to the skeleton by connective tissue and specialized tendons made of collagen. Tendons transmit muscle forces, allowing the forces to be redirected and transmitted over a wide range of joint motion. Tendons, like a spring, also store and recover elastic energy.

The vertebrate skeleton can be divided into axial and appendicular regions (Fig. 37.17). The axial skeleton consists of the skull and jaws of the head, the vertebrae of the spinal column, and the ribs. Bones of the skull protect the brain and sense organs of the head. The appendicular skeleton consists of the bones of the limbs, including the shoulder and pelvis. The axial and appendicular regions reflect the evolutionary ancestry of vertebrates. The axial skeleton formed first, to protect the head and provide support and movement of the animal’s body by bending its body axis. As a result, the axial skeleton is the main skeletal component of fishes. The appendicular skeleton includes the pectoral and pelvic fins of fish, which evolved into limbs when vertebrate animals first moved onto land. In a special group of lobed-fin fish, the fin bones became adapted for an amphibious lifestyle, evolving into the limb bones that terrestrial vertebrates use for support against gravity and for movement on land.

Skeletal tissues have relatively few cells for their volume because they consist mostly of an extensive extracellular matrix that is secreted by specialized cells, forming connective tissue external to the cells (Chapter 10). In addition to tendon, three main extracellular tissues are produced in the formation of the vertebrate skeleton: bone, tooth enamel, and cartilage.

Bone tissue–forming cells called osteoblasts synthesize and secrete calcium phosphate as hydroxyapatite mineral crystals in close association with the protein collagen. Generally, bone consists of two-thirds hydroxyapatite mineral and one-third type I collagen protein. The combination of mineral and protein makes bone a composite material, like exoskeletons. Bone is rigid (because of the properties of the mineral), but also hard to break because it absorbs much energy before fracture (because of the properties of the collagen). Bone gradually becomes more mineralized and brittle as a normal process of aging. The genetic bone disorder osteogenesis imperfecta leads to bones that are brittle and fragile because the osteoblasts produce insufficient collagen in the extracellular matrix.

Articular cartilage located at the joint surfaces of a bone forms a cushion between bones meeting at a joint. The cartilage is a gel-like matrix that resists fluid being squeezed out when forces press on the cartilage during movement. Articular cartilage consists of 70% water, 15% large, branched molecules called proteoglycans, and the remaining 15% type II collagen. The collagen fibers reinforce the cartilage, allowing the cartilage to cushion and distribute loads as the joint moves. Cartilage may degenerate for a variety of reasons, including rheumatoid arthritis and repetitive physical injury, but in general it lasts over an individual’s lifetime.

Quick Check 5 Glass is strong and rigid but can be easily broken. In contrast, a bone is strong and rigid but hard to break. How can this be?