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Biology > The Human Skeleton

The human skeleton is made up of around 300 different bones at birth. As the body develops, some of these bones fuse together to leave a total of 206 individual bones in adult humans.

Bones are made from calcified connective tissue and contain many essential minerals, vitamins and protein.

Bones consist of several layers:
  • The outer surface of bones consists of a thin dense membrane called the periosteum. It contains blood vessels which nourish the bone and also contains nerves.
  • Beneath the periosteum is compact bone. This is smooth to the touch and hard. This compact bone is the outer layer visible when you look at dead, dry bones.
  • Inside the compact bone are layers and layers of a sponge like substance, known as cancellous or 'spongy' bone.
  • Within, and being protected by the 'spongy' bone is the bone marrow which is a jelly like substance which creates new blood cells.
The diagram below shows the major bones in the human body.

Fig 1: The human skeleton
Fig 1:
The human skeleton

Cells within bone

There are several types of cells which make up animal bones:

Osteoblasts are bone forming cells which produce a mixture of protein known as Osteoid which turns into new bone. They produce an enzyme which helps create new bone, called alkaline phosphatase. Osteoblasts are essentially immature bone cells.

Inactive Osteoblasts cover the outer bone surface and are sometimes referred to as 'bone lining cells'

Osteocytes are mature bone cells and their functions include formation of new bone, bone matrix maintenance and calcium homeostasis. They originate from Osteoblasts.

Osteoclasts are large cells which are located on the bone surfaces and are responsible for bone resorption (the reshaping of bone in order to reduce its volume).

Types of bone

There are five types of bone in the human body; short, flat, long, irregular and sesamoid:

Short bones can be found in the wrists and ankles and consist of an almost cubic formation and made up of a thin layer of compact bone surrounding spongy bone.

Flat bones can be found in the skull and the sternum and are made up of a thin layer of spongy bone surrounded by two thin layers of compact bone.

Long bones generally make up the limbs and consist of long shafts with rounded ends known as epiphyses. They are longer than they are wide and are formed from mainly compact bone with spongy and marrow bone within.

Irregular bones are pretty self explanatory - they are irregularly shaped and include spinal and hip bones.

Sesamoid bones hold tendons in place and help muscles by increasing their effectiveness. An example is the patella.