Bones are living tissue which have their own blood vessels and are made of various cells, proteins, minerals and vitamins. This structure enables them to grow, transform and repair themselves throughout life.







The anatomical structure of bone specializes it for its various functions. Compact (cortical) bone provides a rigid frame work for support. Spongy (trabecular or cancellous) bone reinforces bone’s strength while managing the weight of the tissue.







Bone is protected by a durable connective tissue. Here again, structure denotes function. The outer layer of the periosteum is fibrous, meaning it is composed of many strong collagen fibers specializing it for protection. The inner layer is composed of a less dense connective tissue with specialized cells that add and remove bone tissue.







Blood cells and immune cells, through a mitotic and maturation process, must be continuously replenished . This process occurs in the bone marrow.





Microscopic Anatomy of Compact Bone
Though compact bone appears smooth to the naked eye, it is actually riddled with holes organized in functional units known as osteons or Haversian Systems.







The osteon is the basic unit of compact bone.

Each osteon is composed of a series of concentric tubes one surrounding another comprised of bone tissue. These concentric tubes are called concentric lamellae. These lamellae run the length of the diaphysis or the depth of the compact bone in short and irregular bones.


In the center of each osteon is the central or Haversian canal. The central canal contains blood and lymph vessels, as well as nerves.

Incomplete lamellae known as interstitial lamellae fill in the spaces between the osteon. Rather than long tubes, they are more like thin sheets of bone that run the length of the diaphysis or the depth of the compact bone in short and irregular bones.













The prefix osteo means bone. There are four basic bone cells with distinct functions, they include: osteogenic cells, osteoblasts, osteocytes, and osteoclasts.





Osteogenic Cells

These cells are located in the endosteum, inner layer of periosteum, and central canals. These cells are stem cells that differentiate into osteoblasts.


Osteoblasts

These non-mitotic cells are located in the endosteum and the inner layer of the periosteum. Osteoblasts are cells that synthesize bone matrix.


Osteocytes

These cells are located in the lacunae of compact bone. Osteocytes act as strain sensors. They communicate with osteoblasts and signal osteoclasts. Some osteoblasts mature and become osteocytes.


Osteoclasts

These cells are located on the inner bone surface. These cells dissolve/breakdown bone matrix.