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Good day, today we are going to discuss fully on the bones:

learning outcomes

After studying this section about the bones , you should be able to:

  • State the functions of bones
  • list five types of bone and give an example of each
  • outline the general structure of a long bone
  • describe the structure of compact and spongy bone tissue

Although bones are often thought to be static and permanent, they are highly vascular living structures that are continuously being remodelled.

Functions of bones

The functions of bones include:

  • Providing the body framework
  • giving attachment to muscles and tendons
  • allowing movement of the body as a whole and of parts of the body, by forming joints that are moved by muscles
  • forming the boundaries of the cranial, thoracic and pelvic cavities,and protecting the organs they contain
  • haemopoiesis, the production of blood cells in red bone marrow
  • mineral storage, especially calcium phosphate the mineral reservoir within bone is essential for maintenance of blood calcium levels, which must be tightly controlled.

Types of bones

Bone is classified as long, short, irregular, flat and sesamoid.

long bone. These consist of a shaft and two extremities. As the name suggests, these bones are longer than they are wide. Most long bone are found in the limbs ; examples include the femur, tibia and fibula.

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Short,irregular,flat and sesamoid bones. These have no shafts or extremities and are diverse in shape and size. Examples include:

  • Short bone- carpals (wrist)
  • irregular bone- vertebrae and some skull bones
  • flat bone- sternum, ribs and most skull bones
  • sesamoid bone- patella (knee cap).

Bone structure

Long bones

These have a diaphysis (shaft) and two epiphyses (extremities). The diaphysis is composed mainly of compact bone with a central medullary canal, containing fatty yellow bone marrow. The epiphyses consist of an outer covering of compact bone with spongy (cancellous) bones inside. The diaphysis and epiphyses are separated by epiphyseal cartilages, which ossify when growth is complete.

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A mature labeled structure of bones
A mature long bones

Blood and nerve supply

One or more nutrient arteries supply the bones shaft; the epiphyses have their own blood supply, although in the mature bones the capillary networks arising from the two are heavily interconnected. The sensory nerve supply usually enters the bone at the same site as the nutrient artery, and branches extensively throughout the bone. Bone injury is, therefore, usually very painful.

short, irregular, flat and sesamoid bone

These have a relatively thin outer layer of compact bone, with spongy bone inside containing red bone marrow. They are enclosed by periosteum except the inner layer of the cranial bones where it is replaced by dura mater.

Microscopic structure of bone

Bone is a strong and durable type of connective tissue. Its major constituent (65%) is a mixture of calcium salts, mainly calcium phosphate. This inorganic matrix gives bone great hardness, but in its own would be brittle and prone to shattering. The remaining third is organic material, called osteoid, which is composed mainly of collagen. Collagen is very strong and gives bone slightly flexibility. The cellular component of bone contributes less than 2% of bone mass.

Bone cells

There are three types of bone cell:

  • osteoblast
  • osteocyte
  • osteoclast.


These bone-forming cells are responsible for the deposition of both inorganic salts and osteoid in bone tissue. They are therefore present at sites where bone is growing, repairing or remodeling, e.g.:

  • in the deeper layers of periosteum
  • in the centres of ossification of immature bone
  • at the ends of the diaphysis adjacent to the epiphyseal cartilages of long bone
  • at the site of a fracture.

As they deposit new bone tissue around themselves, they eventually become trapped in tiny pockets (lacunae) in the growing bone, and differentiate into osteocytes.


These are mature bone cells that monitor and maintain bone tissue, and are nourished by tissue fluid in the canaliculi that radiate from the central canals.


These cells break down bone, releasing calcium and phosphate. They are very large cells with up to 50 nuclei, which have formed from the fusion of many monocytes. The continuous remodelling of healthy bone tissue is the result of balanced activity of the bones osteoblast and osteoclast populations. Osteoclasts are found in areas of the bone where there is active growth, repair or remodeling, e.g.:

  • Under the periosteum, maintaining bone shape during growth and to remove excess callus formed during healing of fractures
  • round the walls of the medullary canal during growth and to canalise callus during healing.

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