Osteon

The bones and cartilages are a type of specialized connective tissue that is present in the human body. Osteon means bone and the process of formation of bone is known as osteogenesis. Bones and cartilage are a part of the specialised connective tissue. It is further divided into two types that are fluid connective tissue and skeletal connective tissues. Blood comprises fluid connective tissue and we will learn about skeletal connective tissue. The extracellular ground substance in the bones and cartilages is very hard in nature. They help in forming the endoskeleton and the internal framework of the body of organisms. They provide support to the body and also help in providing protection to the organs of the body. We will learn about bones and cartilages and also about primary osteon and compact bone osteon. 

Bones

The bones are made up of hard connective tissue. They are non-flexible and very strong in nature. They comprise 70% of the inorganic matter and 30% organic material is present in them. They are also known as primary osteon. The hydrochloric acid can make them soft and flexible but the potassium hydroxide does not affect the nature of the bones. As we read above, their matrix is very hard. This hard and non-pliable nature of the matrix is due to the presence of calcium phosphate and calcium carbonate. Some proteins like ossein and collagen fibres also help in the formation of bones and their ossification. Lamellae is the name of their matrix and it is present in the shape of layers. Lacunae are fluid-filled spaces that are present inside the lamellae. 

Osteocytes are the bone cells that are present in the lacunae and these osteocytes help in the formation of bones. Bone marrow is the cavity that is present in the long bones of the limbs. This bone marrow serves the purpose of formation of the blood cells that are red blood cells, white blood cells and platelets. This is fatty tissue and is soft in nature. Fibres are also present in the matrix of the bones and these are indistinguishable. Bidirectional growth is present in the bones and osteocytes help in the process of branching of these bones. Osteoblasts are the special bone-forming cells that are present in the outer and inner layers of the lacunae. 

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Bones Functions

As we read above that the bones hold great importance in our lives. We will learn about some of the functions of the bones. They are: 

• They help in providing a structural framework for the body.

• They help in protecting and supporting the softer tissues and organs of the body.

• Calcium and phosphate are stored in the bones so they also serve the purpose of storage of minerals. 

• They help in providing a surface for the muscles. 

• When a surface is provided for muscles, then only other bones can be joined together and this joining of bones helps in the locomotion process.

• A compact bone osteon structure is also exhibited by them. 

Types of Bone

As we read above the functions and structure of bones, now we will learn about the different types of bones. There are four different types of bones present in the human body and they serve different functions. The four types of bones are: 

• Cartilage Bones: They are also known as endochondral bone or replacing bone. As the name suggests, for the formation of these bones, the replacement of cartilage takes place. The cartilages are replaced by the bones. Examples of such bones are the bones of humerus, femur, vertebrae, ribs, girdle bones. The cells that help in this process are chondroblast cells. These cells are known as cartilage eater cells. 

• Membrane Bones: These bones are also known as investing bone or dermal bone. As the name suggests these bones are formed by the dermis of the skin. These bones are invested in the already present cartilages that are present in them. Skull bones and clavicles are an example of such types of bones. 

• Sesamoid Bones: These bones are formed from the tendon. There is an ossification of tendons and this enables the formation of such bones. These types of bones are seen in the knee cap or the patella bone. 

• Visceral Bones: As the name suggests, these bones are present near the visceral organs of the body. In these bones, the regular bones of the body are detached from the skeleton and then they are moved near to the visceral organs. 

Compact Bone

These bones are present in the dense outer layers of all the bones. They are composed of Haversian systems. These are parallel, longitudinal and column-shaped structures. They all are cemented to each other. Volkman canals help in connecting the Haversian canals to each other. Lamellae are the concentric layers that are present in the Haversian canals. They are bony in nature. They help in encircling the longitudinal Haversian canal. Blood vessels and nerves are carried out by this canal. Osteocytes are also present in between these canals. 

Cartilage

The cartilages are hard and dense connective tissue but they are softer than the bones. In the embryos, they are present in abundance and as the body grows into an adult some of them are transformed into bones. This process of formation of bones is known as osteogenesis. The matrix is solid and pliable in nature. The matrix helps in resisting compression. They also have fluid-filled spaces in the matrix and these spaces are known as lacunae. Chondrocytes are present in the lacunae and these cells are helpful in the formation of cartilages. 

These cartilage cells also help in maintaining the amount of matrix in the cell. They are present at the tip of the noses and outer ear joints. They are also present in between the vertebral column and limbs to provide support. Chondroblast cells are responsible for cartilage growth and their division. Their matrix is made up of homogenous mass. The perichondrium is a firm sheath that surrounds the cartilages. Unidirectional growth is present in the cartilages. Chondrin is a flexible material that is present in the matrix of the cartilage. 

Function of Cartilages

The cartilages are a part of the primary osteon and also compact bone osteon. The functions of the cartilages are: 

• They help in providing support to the parts of the body.

• They help in providing flexibility to the parts of the body. 

• They help in resisting compression.

• They help to smoothen the surface of joints.

• This smoothening of the surface of joints prevents them from the process of wear and tear. 

Fluid-Connective Tissue

This is another type of connective tissue apart from the skeleton connective tissue and is a part of dense connective tissue. Blood and lymph are examples of fluid connective tissue. They are made up of a fibre-free matrix and they also have the presence of some specialized living cells. These living cells are not able to divide and also are not able to secrete matrix. They are a type of vascular tissue and they are circulated all over the body and this helps them to bring about the transportation of nutrients, minerals and oxygen to the whole body. Blood platelets are also present in them and these are round and minute in nature. They are oval and biconvex fragments that are called megakaryocytes.