Bones – a constantly changing structure

Bones – a constantly changing structure

Bones form the rigid and resistant part of the human skeleton. Some are linked together by ligaments and joint capsules that allow movement, others are linked by short fibers that are not very mobile, and others are welded together like the bones of the skull or the sacrum.

The skeleton

The human skeleton comprises 206 bones, including :

• 80 for the head-neck-trunk axis
  14 for the face, 8 for the skull, 24 mobile vertebrae, 9 fused vertebrae for the sacrum and coccyx, 24 ribs and a sternum.
• 126 for the limbs

It weighs about 20% of the body weight, or 13 to 16 kilos for an average person.
There are long, short, flat or irregular bones in the body. The smallest is the pisiform of the wrist, the size of a pea, and the longest is the femur, which can reach 60 cm.

Types of bones

Long bones

Like all long bones, the femur has a central shaft, or diaphysis, and two ends, or epiphyses, covered with articular cartilage.
Other long bones: clavicle, humerus, radius, ulna, metacarpals, phalanges, tibia, fibula, metatarsals, toe bones.

The flat bones

The parietal bone, which occupies the upper part of the back half of the skull, has two parallel sides. It is connected to the neighboring bones by elastic sutures in children and fixed welds in adults.
Other flat bones: the frontal, the temporal and the occipital of the cranial vault,
the ribs (despite their elongated shape), the sternum, the lower jaw, the scapula.

The short bones

The patella and calcaneus, like all short bones, are curved in on themselves. The shape of the short bones can be round, pyramidal, ovoid, cubic or irregular.

Other short bones: wrist bones, neck bones of the foot.

Irregular bones

The sphenoid, located at the base of the skull, is one of the most irregular bones in the body with its wings, needles or apophyses and slots around a more massive central body. Some have cavities or sinuses.

Other irregular bones: vertebrae, pelvis.

The structure of bones

Bone is a living material that grows and is renewed and repaired as needed throughout life.

External structure In the long bones :

• The diaphysis is a compact bone shaft whose central cavity contains red marrow in children and yellow marrow in adults.
• The epiphyses are large blades
  of compact bone surrounding cancellous bone filled with red marrow; they are covered with cartilage.
• The metaphyses are the intermediate parts between epiphysis and diaphysis.
• The short bones are a mass of compact bone around cancellous bone.
• The flat bones are formed by two blades of compact bone surrounding a blade of cancellous bone.
• Irregular bones combine these different structures.

Internal structure A bone has 6 different types of tissue:

• The periosteum is a fibrous membrane that covers the bones, except for the joints.
• Compact bone, very dense and uniform, is composed of cylindrical elementary units or osteons, made up of juxtaposed lamellae like in a paper roll.
• Cancellous bone resembles a sponge with its bony lamellae delimiting countless cavities.
• The articular or hyaline cartilage, which covers the extremities, appears under the microscope as a rigid but still elastic jelly.
• The bone marrow or red marrow occupies all the cavities of the spongy bone, producing 100 to 150 billion red blood cells and 1 to 30 billion white blood cells every day.
• The yellow marrow, a fatty mass that occupies the center of the diaphysis of long bones in adults.

Chemical composition

The living part of the bone consists of 1% proteins that form a matrix, bone cells or osteocytes
bone cells or osteocytes, collagen fibers and marrow cells. It represents one third of the weight of the bone.
The mineral part of the bone includes a large proportion of calcium phosphate and some iron, fluorine and mineral trace elements. It represents two thirds of the weight of the bones.

The functions of bone

Bone has 4 main functions:

1. Support: the skeleton serves as an anchor point for all muscles and soft organs. It supports the weight of the body in all positions.
2. Protection: the skull protects the brain; the rib cage protects the heart and lungs; the vertebrae enclose the spinal cord.
3. Storage: the bone contains 99% of the body’s calcium and phosphorus reserves.
4. The production of red and white blood cells in the red marrow.