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Bone

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Bone - A connective tissue that contains a hardened matrix of mineral salts and collagen fibers. Its cells include osteocytes, which are embedded within lacunae, and the free-roaming osteoblasts and osteoclasts.

http://www.sci.sdsu.edu/histology/gloss_a.htm

A contiguous amount of this hard connective tissue forms a complete structure, called a bone. Individual bones together are linked via ligaments attached to muscle. The sum of all the bones in an individual is termed a skeleton. The purpose of a skeleton is to provide a physical frame to hang muscles and other tissues on. Species with skeletal structures built from bone can support bigger individuals than are possible without them, especially when the animals are terrestrial. Terrestrial locomotion is critically dependendent on skeletal structures (provided by chitin in insects, but by bone in most other animal life) The young of most species have bones which are not very stiff at birth, and gradually harden as the individual matures. The tips of the bones in mammals remains slightly cartilagenous where growth occurs from childhood until maturity, and then progressivly harden in adulthood. The amount of cartilagenous tissue remaining is considered an important method of evaluating the age of death of individual fossils.

Studies indicate bone mass in a living body can be increased by exercise which stresses the skeleton. The study of bones and and related tissues are an important part of the part of anatomy


Evolution

Bone plays such a huge role in evolution: ultimately, it is one of the features that allowed the first tetrapods to hold themselves up on land without the buoyancy of water to support them against gravity. As a structural tissue, its capacity for flexibility of structure has made possible a huge number of shapes, sizes, and adaptations in movement and locomotion between species and allows one tissue to take on a number of functions within a single individual. ...

The possibilities of bone tissue itself for flexibility, strength, and most importantly remodeling and growth over other structural tissues gave the structures it made up the advantage of adaptability, which lead to the vast diversity of bone shapes and functions seen across the vertebrates. ... One of the most amazing things to realize, however, is how well conserved the signalling mechanisms that determine limb shape are. Though the very process by which bones become bone tissue is different in different parts of the skeleton, limb morphogenesis is directed by genes that are similare between humans, rats, and even drosophila. How the system has come to have such variety and yet such conservation of molecular mechanisms is something to wonder at.

from: http://www.brown.edu/Courses/BI0032/bone/evolve.htm

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