Enchondral ossification is the indirect ossification from within, which occurs through the intermediate stage of cartilage. Connective tissue and mesenchyme are the basic materials for ossification. If the connective tissue is altered in structure, then it can cause severe ossification disorders.
What is enchondral ossification?
Enchondral ossification is the indirect ossification from within, which occurs through the intermediate stage of cartilage. Ossification or osteogenesis is the formation of bone tissue. It takes place in the human organism on the one hand during growth and on the other hand for regeneration after bone fractures. New bone is either formed directly from connective tissue or is formed via an intermediate stage. Cartilage elements usually act as an intermediate stage. Chondral ossification is the indirect process, which is realized via intermediate stages. The final product is also called replacement bone. Chondral ossification occurs either from the inside or outside. When ossification occurs from the outside, it is referred to as perichondral osteogenesis. Enchondral ossification, on the other hand, is ossification from the inside. The opposite of chondral types of ossification is desmal ossification, in which bone is formed directly from connective tissue. A third type of bone tissue formation is appositional ossification, which accounts for the thickness growth of bone. In this type of growth, bone tissue attaches to pre-existing bone material. For example, perichondral ossification is an appositional type of ossification.
Function and task
Together with the gelatinous connective tissue, the so-called mesenchyme makes up the embryonic connective tissue. Mesenchyme is the basic material for the development of loose and tight reticular connective tissue. It is involved in the development of smooth muscle and heart muscle, contributes to the kidneys and adrenal cortex, and is required for the formation of the hematopoietic system including all blood and lymphatic vessels. Apart from this, bones and cartilage are formed from the mesenchyme. During chondral ossification, cartilaginous elements are formed from the material, which are called the primordial skeleton. Because of this intermediate stage, the process is also called indirect ossification. The resulting bones are the replacement bones. Ossification from the outside is perichondral ossification. In this process, osteoblasts split off from the skin of the cartilage (perichondrium) and accumulate in a ring shape around the model of the cartilage. Thus, a bone cuff is formed, which contributes to the thickness growth of the bone and is therefore counted as appositional osteogenesis. Enchondral ossification is distinct from this type of bone growth in that ossification in this process is performed from within. Blood vessels grow into the tissue of the cartilage piece by piece during this process. Accompanied by the blood vessels, mesenchymal cells also migrate into the cartilage. Differentiation of the cells then takes place. Some of the migrated mesenchymal cells become chondroclasts. Others mature into osteoblasts. Chondroclasts break down bone. Osteoblasts are responsible for bone formation. Length growth, also known as interstitial growth, occurs in the epiphyseal joints because of the permanent buildup and breakdown processes. An inner space is formed inside the bone. This internal space is known as the primary medulla and is involved in the formation of the actual bone marrow. In both enchondral and perichondral ossification, osteoblasts release what is known as osteoid as the basic substance. Under the influence of the osteoblast enzymes, calcium salts are deposited on the bone, whereupon the osteoblasts differentiate into osteocytes. The starting points of each ossification are called ossification centers or bone nuclei.
Diseases and disorders
The best-known clinical pictures related to ossification are the so-called ossification disorders, which are mainly dealt with by orthopedics. One of the best-known diseases in this group is Osgood-Schlatter disease. In this disease, free bone fragments detach as part of the disturbed ossification. Many athletes are affected. The cause is a lack of balance between load-bearing capacity and actual stress on the cartilage. Heavy loads in the knee joints are particularly frequently associated with the phenomenon.In Osgood-Schlatter disease, the apophysis of the tibia is overloaded by mechanical stress, so that all types of ossification are disturbed. Close to the ossification front is the insertion site of the fibrous tendon components. At this site, thickening of the tibial tuberosity occurs. Because of the osteochondronecrotic processes, smaller areas of the affected region detach from the tissue in the form of free ossicles. Brittle bone disease is also related to impaired osteogenesis. Affected individuals suffer from abnormally light and easily brittle bones due to genetic changes in their type one collagens. These collagens are a major component of connective tissue. Since both enchondral and perichondral ossification make use of connective tissue, 90 percent of the bone matrix is altered in patients with this hereditary disease. The cause is thought to be a point mutation on chromosomes 7 and 17. Leading symptoms include skeletal deformities, bending of the spine, and hyperextensible joints. Sometimes ossification occurs not only in cartilage but also in soft tissue. This is also a pathological phenomenon most often associated with so-called myositis. Muscle ossification with calcium salt deposits is one of the main symptoms of this phenomenon. Meanwhile, an autoimmunological cause is considered for this disease.
