Bone Formation and Ossification in Humans

Bone is the rigid tissue that is a part of the skeleton system in vertebrates. They help in the production of the red and white blood cells, provides structure, it stores minerals, enables mobility, and provides support to the body. It has complex internal and external structures, now let us see how bones are formed. Bone tissue is also known as osseous tissue, thus the process of formation of the bone is called Bone ossification. Now let us define ossification and the steps involved in it. 

The process of remodeling the bone is called ossification; it is also known as osteogenesis. It is the process of remodeling the bone by laying the particles of the new bone material in the cells called osteoblasts. There are several hypotheses that propose the evolution of the bone as a structural element. According to one of the hypotheses, bones are evolved in order to store the minerals. In the cartilage, the specialized calcium minerals were stored from this classified cartilage the bone forms. There are two processes of bone formation, they are Intramembranous ossification and endochondral ossification. 

Bone Formation

1. Intramembranous Ossification: In this process, the formation of the compact and spongy bone takes place directly from the sheets made of the undifferentiated mesenchymal connective tissue. By this process the bones that are located in the face such as flat bones, the collar bones or clavicle, and cranial bones. 

There are 4 Stages of Bone Formation and Growth in Intramembranous ossification:

• When the mesenchymal cells that are present in the embryonic skeleton gather together then the process begins in order to differentiate into specialized cells. Some of these cells become osteogenic cells and some of them differentiate as capillaries. The osteogenic cells are further converted into osteoblasts. The early osteoblasts in a cluster of cells called the ossification centre. 

• The osteoblasts secrete osteoid, it is an uncalcified matrix that consists of collagen precursors and other types of organic molecules. Within a few days, mineral salts are deposited on the matrix and it gets hardened by entrapping the osteoblasts. When these are entrapped the osteoblasts get converted into osteocytes. The osteogenic cells that are present in the surrounding of the connective tissue get differentiated to form new osteoblasts at the edge of the growing bone tissue. 

• Osteoid clusters get united around the capillaries in order to form a trabecular matrix whereas the osteoblasts that are present on the surface of the newly formed spongy bone that becomes the cellular layer of the periosteum. 

• This periosteum then secretes the superficial of the compact bone to spongy bone. Nearby the blood vessels the spongy bone crowds that eventually condenses to form new red bone marrow. Under the action of osteoclasts, the new bone gets remodeled.

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1. Formation of the ossification centre

2. Formation of osteocytes

3. Trabecular matrix and periosteum form

4. Development of compact bone

2. Endochondral Ossification: In this process, bone develops by replacing cartilage. Cartilage does not become bone. Instead, cartilage serves as a template to be completely replaced by new bone. Endochondral ossification takes much longer than intramembranous ossification. Bones that are present at the base of the skull and long bones form through the process of endochondral ossification.

In a long bone, a number of the mesenchymal cells differentiate into chondroblasts or cartilage cells at about 6 to eight weeks after conception, which forms the hyaline cartilaginous skeletal precursor of the bones. This cartilage is a flexible, semi-solid matrix that is produced by cartilage cells and consists of chondroitin sulfate, collagen fibres, hyaluronic acid, and water. They're also called chondrocytes as the matrix surrounds and isolates the chondroblasts. Unlike most connective tissues, cartilage is avascular which means that it has no blood vessels that are supplying the nutrients and removes the metabolic wastes. All of those functions are carried on by the process of diffusion through the matrix from vessels within the surrounding perichondrium, a membrane that covers the cartilage.

As more and more matrix is produced, the cartilaginous model grows in size. Blood vessels that are present in the perichondrium bring the osteoblasts to the edges of the structure and these arriving osteoblasts deposit bone in a ring around the diaphysis this process is called a bone collar. The bony edges of the developing structure prevent the nutrients from diffusing into the middle of the cartilage. This leads to the death of chondrocytes and disintegration within the centre of the structure. 

Without cartilage inhibiting vessel invasion, blood vessels penetrate the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many of which can become osteoblasts. These enlarging spaces eventually combine to become the medullary cavity. Bone is then deposited within the structure that is creating the first ossification centre.

While these deep changes are occurring, chondrocytes and cartilage still grow at the ends of the structure, which increases the structure’s length at an equivalent time bone is replacing cartilage in the diaphyses. This continued growth is the reason for the remodeling inside the medullary cavity and the overall lengthening of the structure. Cartilage remains at the epiphyses and the joint surface as articular cartilage when the fetal skeleton is formed completely.

After birth, this same sequence of events occurs within the epiphyseal regions, and each of these centres of activity is referred to as a secondary ossification centre. Throughout childhood and adolescence, there remains a skinny plate of cartilage between the diaphysis and epiphysis referred to as the expansion or epiphyseal plate. Eventually, these cartilages are going to be removed and replaced by bone to become the epiphyseal line.

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Bone Development

While bones are increasing long, they are also capable of increasing in diameter, whereas growth in diameter can continue even after the cease of the longitudinal growth. This growth by adding to the free surface of the bone is also known as appositional growth. This appositional growth is found at the endosteum or periosteum where the osteoclasts resorb old bone that is present as a lining to the medullary cavity, whereas the osteoblasts produce to form new bone tissue. The erosion of old bone along the medullary cavity and hence the deposition of the latest bone takes place under the periosteum. 

This not only increases the diameter of the diaphysis but also increases the diameter of the medullary cavity. This remodeling of bone primarily takes place during the process of bone growth. However, in adult life, bone undergoes constant remodeling, during which resorption of old or damaged bone takes place on an equivalent surface where osteoblasts lay new bone to exchange that which is resorbed. Injury, exercise, and other activities lead to remodeling. Those influences are discussed later in the chapter, but even without injury or exercise, about five to ten percent of the skeleton is remodeled annually just by destroying old bone and renewing it with fresh bone.

Conclusion

All bone formation processes are replacement processes. During bone development, by the ossification process tissues are replaced by bone. In the intramembranous type of ossification, the bone develops directly from sheets that are made of mesenchymal animal tissue. In the endochondral type of ossification, the bone develops directly by replacing the cartilage. Activity within the epiphyseal plate enables bones to grow long. Appositional growth allows bones to grow in diameter. The remodeling process occurs as bone is resorbed and replaced.