Chondral ossification refers to bone formation from cartilage tissue. Along with desmal ossification, it represents one of the two basic forms of bone formation. A well-known disorder of chondral ossification is achondroplasia (short stature).
What is chondral ossification?
Chondral ossification refers to the formation of bone from cartilage tissue. In contrast to desmal ossification, chondral ossification refers to indirect bone formation. Whereas in desmal ossification embryonic connective tissue is converted into bone substance, in chondral ossification bone formation takes place via an initially built-up cartilage skeleton. In this process, cartilage tissue is degraded in parallel with bone formation. Furthermore, a distinction is made between perichondral and enchondral ossification. Perichondral ossification is characterized by an ossification of the diaphysis (bone shaft) of the bone from the outside to the inside. In enchondral ossification, ossification occurs from the inside. It usually takes place at the epiphyseal joints and is responsible for the longitudinal growth of the bones while the epiphyseal joints are still open. After completion of ossification, however, the epiphyseal joints close. The length growth of the bones then comes to a halt. This state marks the end of the human growth process. Now, only thickness growth of the bones at the diaphysis occurs through perichondral ossification.
Function and task
Chondral ossification is responsible for almost all of the bone skeleton’s structure. Only the bones of the cranial vault, facial skull, and clavicle are built up via desmal ossification. In chondral ossification, the human skeleton is initially built up as a cartilaginous skeleton during embryogenesis. Therefore, these bones are also called replacement bones. During further development, ossification takes place on this cartilaginous tissue. Ossification is not complete until the end of the human growth process. With the complete transformation of cartilage tissue into bone tissue at the epiphyses, the last epiphyseal joints then also close. The longitudinal growth of the bones and thus the entire human growth process thereby comes to a conclusion. Chondral ossification can be divided into two subtypes. As already mentioned, a distinction is made between perichondral and enchondral ossification. Perichondral ossification usually takes place at the bone shaft (diaphysis). As part of this process, osteoblasts are formed on the outer skin of the bone and attach themselves in a ring around the cartilage model. This results in the formation of a bone cuff around the cartilage. The ossification moves from the outside to the inside. Inside, the cartilage tissue is broken down by chondroclasts, while at the same time further bone tissue is built up by osteoblasts. This results in ossification of the cartilage tissue and simultaneous thickening of the bone. Enchondral ossification begins inside the cartilage tissue. For this purpose, blood vessels grow into the cartilage tissue, which are accompanied by mesenchymal cells. These mesenchymal cells also differentiate into chondroclasts and osteoblasts. In this process, chondroclasts constantly degrade cartilage cells, while osteoblasts build bone cells. Enchondral ossification takes place mainly at the epiphyses. As long as the epiphyses consist of cartilaginous tissue, the epiphyseal joints are open. However, due to bone growth from the inside, the bone cells spread longitudinally because the joints do not allow width and size growth. Thus, the longitudinal growth of the bones is the result of an evasive growth. Only when the epiphyses have ossified do the epiphyseal joints also close. Then length growth finally comes to a halt. Bone growth then only occurs after bone fractures or injuries. However, bone cells are formed and degraded throughout life. In chondral ossification, as in desmal ossification, the bone cells also arise from the mesenchyme. However, in the chondral form of bone formation, a cartilaginous skeleton is first built up, which already fulfills the most important basic functions of a bone skeleton. The actual bone formation takes place here as a second step, whereby a conversion of the cartilage tissue into bone tissue then takes place.This results in the breakdown of cartilage cells and the simultaneous buildup of bone cells.
Diseases and complaints
In the context of chondral ossification, disorders can occur that significantly affect bone growth. A typical growth disorder is the so-called achondroplasia. In achondroplasia, the epiphyseal joints become prematurely closed. The length growth of the bones stops. However, the thickness growth of the bones does not stop. At the same time, desmal ossification continues so that the head continues to grow normally. The rib bones and the vertebrae are also not affected by the closing of the epiphyseal joints. Because of this differential growth, there are shifts in body proportions: The trunk and head show normal growth, whereas the length growth of the extremities comes to a premature halt. This growth disorder is genetically determined. However, it does not have a negative effect on health. Another disorder of chondral ossification manifests itself in excessive bone formation. This clinical picture is also called heterotopic ossification. This term expresses the fact that bone formation takes place at a different site than usual. This results in ossification in places where only connective tissue should actually be present. This heterotopic ossification is often triggered by accidents and injuries. In this case, the body is animated by the tissue damage to produce messenger substances that can induce the conversion of bone precursor cells via cartilage into bone. In most cases, this additional bone formation does not lead to further symptoms. A genetic disease that leads to progressive fossilization is fibrodysplasia ossificans progressiva. In this condition, all of the body’s connective and supportive tissue is gradually converted to bone.
