An average adult skeleton has 206 bones, which weigh only 20 pounds but yet stronger than concrete. On the other hand, men and women both have the same number of muscles (around 600). Still, men look more muscular than women, and they have 40 per cent of their weight in the form of muscles as compared to 23 per cent of the women. As mentioned earlier, the skeleton system not only supports the body but also protects the vital internal organs like the brain which is safe and sound within the skull and the heart and lungs within the rib cage. Bones along with their attachments with muscles, tendons, and ligaments perform the most important function of providing movement and that too with precise coordination.

Bone is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxyapatite. It has relatively high compressive strength but poor tensile strength. While bone is essentially brittle, it does have a degree of significant elasticity contributed by its organic components (chiefly collagen). Bone has an internal mesh-like structure, the density of which may vary at different points.

Bone can be either compact or cancellous (spongy). Cortical (outer layer) bone is compact; the two terms are often used interchangeably. Cortical bone makes up a large portion of skeletal mass; but, because of its density, it has a low surface area. Cancellous bone is trabecular (honeycomb structure). It has a relatively high surface area, but forms a smaller portion of the skeleton.

Bone can also be either woven or lamellar. Woven bone is put down rapidly during growth or repair. It is so called because its fibers are aligned at random, and as a result has low strength. In contrast lamellar bone has parallel fibers and is much stronger. Woven bone is often replaced by lamellar bone as growth continues.

• Long bones are tubular in structure (e.g. the tibia). The central shaft of a long bone is called the diaphysis, and has a hollow middle—the medullar cavity filled with bone marrow. Surrounding the medullar cavity is a thin layer of cancellous bone that also contains marrow. The extremities of the bone are called the epiphyses and are mostly cancellous bone covered by a relatively thin cortical of compact bone. In children, the bones are filled with red marrow, which is gradually replaced with yellow marrow as the child ages.  

• Short bones (e.g. finger bones) have a similar structure to long bones, except that they have no medullar cavity.  

• Flat bones (e.g. the skull and ribs) consist of two layers of compact bone with a zone of cancellous bone sandwiched between them.  

• Irregular bones are bones, which do not conform to any of the previous forms (e.g. vertebrae).  

All bones consist of living cells embedded in a mineralized organic matrix that makes up the main bone material.

Bone cells are called osteoblasts, osteocytes and osteoclasts.

Osteoblasts

Osteoblasts are typically viewed as bone forming cells. They are located near to the surface of bone and their functions are to make osteoid and manufacture hormones such as prostaglandin, which act on bone itself. Osteoblasts are mononucleate.

Osteoclasts

If osteoblasts can be described as bone forming cells, the osteoclasts can be described as bone destroying cells. Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae. These lacunae, or resorption pits, are left behind after the breakdown of bone and often present as scalloped surfaces.

Osteocyte

An osteocyte, a star-shaped cell, is the most abundant cell found in bone. Once osteoblasts become trapped in the matrix they secrete, they become osteocytes. Osteocytes are networked to each other via long processes that occupy tiny canals called canaliculi, which are used for exchange of nutrients and waste. The space that an osteocyte occupies is called a lacuna (Latin for a pit). Their main function involves maintaining the bone tissue.

Bone cells

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