Bones in Human Body, types of bones and structure of bones?

Definition of Bones:

Osteology is the study of bones and teeth. It is usually applied in the fields of anthropology, forensic science, and archaeology for various functions like the determination of nutritional value, health, and also to determine the position of bone within the body.

Types of Bones in Human Body:

Bones are categorised into five classes on the basis of the following salient features:

1) Long Bones: 

These bones are characterised by the presence of a shaft. They have longer lengths as compared to the widths. They end in a number of extremities, e.g., femur, tibia, fibula, humerus, ulna, and radius. They are usually slightly curved, thus impart high strength to these bones.

2) Short Bones: 

These bones are characterised by an almost equal length and width, e.g., ankle and wrist bones. They are more or less cubical in shape. 

3) Flat Bones: 

These bones are characterised by a thin shape or structure. They offer wide surfaces for muscle attachments and thus provide substantial mechanical protection, e.g., cranial bones (protecting the brain), the sternum and ribs (protecting the organs in the thorax), and the scapula (shoulder blades). 

4) Irregular Bones: 

As the name suggests these bones are asymmetrical in shape and therefore are said to have complicated shapes. They are shaped to fulfill certain specific functions within the body. For example, the vertebrate and some facial bones; provide major mechanical support to the body while also protecting the spinal cord. 

5) Sesamoid Bones: 

These bones develop in some tendons in areas where there is considerable friction, tension, and physical stress. Therefore, these bones are formed in the palms of the hands and the soles of the feet [patellae (knee caps)]. 

6) Sutural Bones: 

These bones are very small and are found within the sutural joints in between the cranial bones. They are categorised on the basis of their location and not shape. For example, suture bones are present in the cranium.

Macroscopic Structure of Bones:-

Internally, bones differ in their structural composition and comprise the following structures: 

1) Compact Bone: 

It is the hard outermost layer of bone. This bone is named compact as it has minimum voids and gaps and hence is also known as dense bone or cortical bone. The tissue is responsible for providing a smooth, white, and sturdy outlook to the bones. 

Haversian systems or osteons are the main characteristics of compact bone (figure 4.2). These are minute cylindrical structures. Osteons consist of concentric layers of collagen called Haversian lamellae; within these lamellae a central Haversian canal is present. These Haversian canals carry blood vessels, lymph vessels, and nerve fibres. A transverse canal known as Volkmann’s canal is found to connect Haversian systems with one another. Small tiny spaces known as lacunar are present in compact bones. These lacunae have tissue fluid and spider-shaped osteocytes. 

These osteocytes have long processes known as canaliculi, which join with each other and form a network. Periosteum covers the outer surface of bones. It is made up of a dense layer of vascularised connective tissue. 

It has two layers, the internal layer is osteogenic while the outer layer is fibrous in nature. Periosteum does not cover the joints. It is heavily innervated with nerves, blood vessels, and lymph vessels.

Cross-Sectional Structure of Compact and Cancellous Bone

2) Trabecular Bone: 

It is also known as spongy bone or cancellous bone. The interior of the bone is filled with trabecular bone tissue. This tissue is a network of open cells containing spongy or cancellous bones. The trabecular bone contains rods and plate-like elements which make the overall network of bones lighter and provides space for blood vessels and bone marrow. This lighter mass constitutes 20% of the total mass of bone but has ten times greater surface area than the compact bone.

Microscopic or Cellular Structure:

Bones are made up of different types of cells such as: 

1) Osteoblasts: 

These are mononucleate cells formed by the precursor osteoprogenitor cells. They are present on the outer portion of osteoid seams and lead to the formation of a proteinaceous mixture called osteoid. The osteoid gets mineralised by the addition of calcium and phosphorus to form the bone. 

2) Osteocytes: 

Osteoblasts that have migrated into the bony matrix gets trapped and give rise to osteocytes or mature bone cells. They occupy specific spaces called lacunae. They are the mature cells of bone. They tend to regulate the response of bone to stress, probably due to the presence of mechano-sensory receptors. 

3) Osteoclasts: 

These are large, multinucleated cells responsible for remodelling of bone in order to reduce their volume. This process is also known as bone resorption. These cells are located in small depressions on the surface of bones which are called Howship’s lacunae or resorption pits. In case of fracture or breakdown of bones, these resorption pits are left behind and appear as scalloped surfaces. 

i) They also consist of collagen fibres lying parallel to other fibres present in that layer.

ii) In lamellar bone, the direction of fibres is opposite in each alternate layer. This arrangement gives strength to bones so that they can withstand torsion forces. 

iii) When a bone breaks, the lamellar bone is replaced with a woven bone (since it grows rapidly) and this is termed as a bony substitution. It takes place on the hyaline cartilage which is then calcified.