Bone ossification is the formation of new bone, which begins as an embryo and continues until early adulthood. It can occur in two ways; through intramembranous or endochondral ossification.
This article will discuss both forms of bone ossification, and will consider the clinical relevance of this important physiological process.
Intramembranous ossification is a process that forms flat bones such as the skull and the clavicle, through the remodelling of mesenchymal connective tissue.
Intramembranous ossification begins in-utero and continues into adolescence. So, at birth, the skull and clavicles are not completely ossified and the cranial sutures (junctions between the skull bones) are not closed. This is important because it allows the skull and shoulders to deform during the passage through the birth canal.
Mesenchymal connective tissue is found mostly during embryonic development and develops into the tissues of the lymphatic, circulatory and musculoskeletal systems.
Intramembranous ossification begins when mesenchymal stem cells within the matrix differentiate into a variety of different specialised cells. Some cells differentiate into blood vessels, while others differentiate into osteoblasts and osteogenic cells. The osteoblasts aggregate in what is called the ossification centre.
Here, osteoblasts synthesise and secrete the osteoid, which is the unmineralised, organic portion of the bone matrix. As this secreted matrix becomes calcified with the binding of calcium, the matrix hardens and entraps the osteoblasts.
Then, osteoblasts become osteocytes, which are involved in the routine turnover of the bony matrix. At the edge of the newly formed and growing bone, the osteogenic cells differentiate into new osteoblasts. As osteoblasts continue to secrete osteoid, these surround the blood vessels and form trabecular (or cancellous) bone. These vessels eventually form the red bone marrow.
Mesenchymal cells on the outer surface of the newly formed bone from the periosteum. Whilst mesenchymal cells on the inner surface of the periosteum differentiate into osteoblasts and secrete osteoid, forming layers of compact (or cortical) bone.
Endochondral ossification is a process where bone forms by replacing a hyaline cartilage precursor. This occurs in the long bones.
The hyaline cartilaginous precursor of the bones start to form in the 6-8 week embryo; mesenchymal cells differentiate into chondroblasts that secrete extracellular matrix. Chondroblasts become encased in the cartilage matrix and become chondrocytes. The perichondrium forms around the cartilage model.
The chondrocytes in the centre of the cartilage model hypertrophy (increase in size), and some burst releasing cell contents that trigger calcification. Chondrocyte cell death within the calcifying matrix forms small cavities for osteoblasts to move into.
The perichondrium changes to periosteum.
The perichondrium (surrounding the cartilage) contains blood vessels that contain nutrients which diffuse into the cartilage precursor. These blood vessels also bring osteoblasts. The osteoblasts begin to deposit bone around the diaphysis (shaft of the bone) and forms a bone collar. This collar of bone prevents nutrients from diffusing into the hyaline cartilage and leads to chondrocyte death at the centre of the hyaline cartilage model.
Primary ossification centre
With impaired diffusion of nutrients to the hyaline cartilage precursor, there is chondrocyte death and cavities forms; blood vessels penetrate these cavities and bring osteogenic cells into these spaces. These spaces combine and become the medullary cavity and osteoblasts bought in by the blood vessels begin to deposit bone into these space, this is known as the primary ossification centre.
The cartilage template continues to grow as cartilage is laid down by chondrocytes above and below the primary ossification centre, which is eventually replaced by bone.
Secondary ossification centre
After birth, secondary ossification centres then develop in each end of long bones and mesenchymal cells and blood vessels are carried in by periosteal buds.
The primary and secondary ossification centres have a thin cartilage between them, called the epiphyseal growth plate. The chondrocytes in this plate continues to proliferate and form new cartilage which is replaced by bone until early adulthood, when bones reach their maximal length and this point of union is termed the epiphyseal line.
The growth plate itself is split into several zones to based on function. These zones and functions are described in Figure 3 and the table below. This allows bones to elongate in a process similar to endochondral ossification.
|Reserve zone||No cellular proliferation|
|Proliferative zone||Chondrocytes divide rapidly and form columns within the matrix. This division is what pushes the diaphysis away from the epiphysis creating growth.|
|Hypertrophic zone||Enlargement of chondrocytes leads to the beginning of calcification.|
|Calcification zone||Calcification of the matrix causes chondrocytes begin to die as it restricts nutrient diffusion. This leaves empty spaces.|
|Ossification zone||Osteoblasts and vessels penetrate the empty spaces and creating new bone whilst osteoclasts break down the calcified cartilaginous matrix.|
As people reach adulthood, the growth plate matures and chondrocytes stop dividing until eventually the physis itself ossifies. As a result, the bone is no longer able to grow in length and the only place where cartilage remains after puberty is at the articular surfaces of joints.
Clinical Relevance – Epiphyseal Growth Plates
A bone age study can be used to help estimate the maturity of a child’s skeletal system; it is done by taking an X-ray of bones that contain epiphyseal growth plates and comparing these with data on other children of the same gender and age.
The epiphyseal growth plates, as shown by Figure 3, appear darker because cartilage is less dense than bone.
It is also important to note that whilst the growth plate is supported to allow it to remain structurally sound, it is still a point of weakness for fractures. These fractures require prompt attention because they can result in limbs of unequal length.
The Salter-Harris System, as shown in Figure 4, is widely used to categorise the different types of growth plate fractures.