2.1 Bone classification and its cellular structure

There are two major types of bone. They denote different stages of a bone lifecycle.

1. Woven Bone – It is an unorganized and premature bone that is found in either growing bones or at fracture sites as newly-formed bone.
2. Lamellar Bone – It is a mature bone that results from the further remodeling of woven bone. Lamellar bone may be further divided into:
a. Cortical Bone – It is a dense or compact bone, which constitutes approximately 80% of the skeleton. It is approximately 20% porous and consists of a dense bundle of vascular channels containing blood vessels surrounded by mature bone. It forms the middle 80% of long bones of the body – tibia, fibula in the lower leg, femur in the upper leg, the radius and ulna in the lower arm, and the humerus in the upper arm.
b. Cancellous Bone – It is a spongy bone, which constitutes approximately 20% of the skeleton. It is approximately 70% porous, highly vascularized and consists of loosely formed matrix of beams designed to withstand the principal stresses and strains applied to the bone. The density of cortical bone is four to six times higher than cancellous bone. Cancellous bone constitutes the remaining 20% of bone located at the ends of the long bones and predominates in the pelvis and the 33 vertebrae from the neck to the tailbone. A fibrous membrane called the periosteum covers bone.

In general, bone constitutes minerals, proteins, hormones, water, other molecules such as sugar, and specialized bone cells. There are three types of bone cells:

1. Osteoblasts – They are bone forming cells, which line the surface of a bone structure.
2. Osteoclasts – They are bone resorbing cells that take bone through its degradation process.
3. Osteocytes are bone cells and are found within the bone structure.

The bone remodeling process is defined by the