bone, rigid bodytissue consisting of cellsembedded in an abundant hard intercellular material. The two principal components of this material,collagen andcalcium phosphate, distinguish bone from such other hard tissues aschitin,enamel, and shell. Bone tissue makes up the individual bones of thehuman skeletal system and theskeletons of othervertebrates.

The functions of bone include (1) structural support for the mechanical action of soft tissues, such as the contraction of muscles and the expansion of lungs, (2) protection of soft organs and tissues, as by theskull, (3) provision of a protective site for specialized tissues such as the blood-forming system (bone marrow), and (4) amineral reservoir, whereby the endocrine system regulates the level of calcium and phosphate in the circulating body fluids.

Evolutionary origin and significance

Bone is found only invertebrates, and, among modern vertebrates, it is found only inbony fish and higher classes. Although ancestors of thecyclostomes andelasmobranchs had armoured headcases, which served largely a protective function and appear to have been true bone, modern cyclostomes have only an endoskeleton, or innerskeleton, of noncalcifiedcartilage and elasmobranchs a skeleton of calcified cartilage. Although a rigid endoskeleton performs obvious body supportive functions for land-living vertebrates, it is doubtful that bone offered any suchmechanical advantage to the teleost (bony fish) in which it first appeared, for in a supporting aquaticenvironment great structural rigidity is not essential for maintaining body configuration. Thesharks andrays are superb examples ofmechanical engineeringefficiency, and their perseverance from theDevonian Period attests to the suitability of their nonbony endoskeleton.

In modern vertebrates, true bone is found only in animals capable of controlling the osmotic and ioniccomposition of their internal fluid environment. Marine invertebrates exhibitinterstitial fluidcompositions essentially the same as that of the surrounding seawater. Early signs of regulability are seen in cyclostomes and elasmobranchs, but only at or above the level of true bone fishes does the composition of the internal body fluids become constant. The mechanisms involved in this regulation are numerous and complex and include both thekidney and thegills. Fresh and marine waters provide abundant calcium but only traces of phosphate; because relatively high levels ofphosphate are characteristic of the body fluids of higher vertebrates, it seems likely that a large, readily available internal phosphatereservoir would confer significant independence of external environment on bony vertebrates. With the emergence of terrestrial forms, the availability of calcium regulation became equally significant. Along with the kidney and the various component glands of theendocrine system, bone has contributed to development of internal fluidhomeostasis—the maintenance of a constant chemical composition. This was a necessary step for the emergence of terrestrial vertebrates. Furthermore, out of the buoyancy of water, structural rigidity of bone afforded mechanical advantages that are the most obvious features of the modernvertebrate skeleton.