Bones are living, growing tissue



Bones have the strength of cast iron but the weight of wood. Click the image to see at full size.
The adult human skeleton comprises 206 connected bones. The largest bone in the body is the thigh bone (femur), and the smallest is the stapes (pronounced ‘staypiz’), in the middle ear.

Bones provide a frame to keep the body supported and protect various organs. They enable movement to occur, serve as an attachment point for skeletal muscles, tendons and ligaments, and connect via joints.

Bone tissue is a type of dense connective tissue. It is comprised of collagen, a protein that provides a soft framework, and calcium phosphate, a mineral that adds strength and hardens the framework.
This combination of collagen and calcium makes bone strong and flexible enough to withstand stress.
While bone is essentially brittle, it does have a significant degree of elasticity, contributed by the collagen. Bones are lightweight yet strong – they have the same strength as cast iron but are as light as wood.

Blood vessels run near the surface of the bone, under which is the layer called ‘compact bone’. Within that is the spongier layer, with the jelly-like bone marrow in the centre of the bone. Bone marrow produces red blood cells, which carry oxygen, white blood cells, which fight infection, and platelets, which help the blood to clot. Bones also store minerals – more than 99 per cent of the body’s calcium is contained in the bones and teeth. Other types of tissue found in bones include endosteum, periosteum, nerves and cartilage.
Nearly all bone growth occurs during childhood and teenage years.
As a result, bones become larger, heavier, and denser. From the mid-thirties we start to lose more bone than we grow. Bone is renewed through a two-part process called remodelling, during which new bone tissue is laid down to replace the old. This process consists of reabsorption and formation.

Osteoblasts are involved in the creation and mineralisation of bone; osteocytes and osteoclasts are involved in the reabsorption of bone tissue. Several hormones, including calcitonin, parathyroid hormone, vitamin D, oestrogen (in women), and testosterone (in men), regulate this remodelling function.