Perichondral ossification corresponds to the growth in thickness of bone. This growth occurs via the intermediate step of cartilage formation. Perichondral bone formation disorders are present, for example, in vitreous bone disease.
What is perichondral ossification?
Perichondral ossification corresponds to the growth in thickness of bone. Ossification or osteogenesis is a process of bone formation. The organism engages in osteogenesis for both length and thickness growth. Ossification is also relevant after fractures and other bone injuries. In ossification, a distinction is made between a desmal and a chondral form. Desmal ossification is direct osteogenesis. That is, the bone material is formed from connective tissue without any intermediate steps. In contrast, chondral ossification corresponds to indirect osteogenesis. In this process, bone is formed via an intermediate step. This intermediate step corresponds to cartilage formation. The product of indirect ossification is called replacement bone. Chondral ossification can be further subdivided into perichondral and enchondral ossification depending on its direction of attachment. In the perichondral form, growth occurs in width. Bone tissue is attached to existing tissue from the outside. Enchondral ossification, on the other hand, takes place from the inside. As a thickness growth, perichondral ossification is a form of appositional osteogenesis.
Function and task
Bones are alive. That this is so is noticed by humans predominantly after a bone fracture, which can heal again by growth processes. Ossification processes are as crucial to this phenomenon as they are to the growth processes of the early years of life. The most important material for bone formation is mesenchyme. This is supporting connective tissue that arises from the mesoderm. From the mesenchyme, the body initially forms cartilaginous skeletal elements during chondral ossification, also known as the primordial skeleton. Indirect osteogenesis continues with the ossification of this cartilaginous tissue. Ossification from the inside corresponds to enchondral ossification. In this process, blood vessels accompanied by mesenchymal cells grow into the cartilage. The migrated mesenchymal cells undergo a differentiation process and become either chondroclasts or osteoblasts. Chondroclasts degrade cartilage. Osteoblasts, on the other hand, are involved in bone formation. Thus, in the epiphyseal joints, build-up and degradation processes take place permanently, causing the bone to grow in length. This growth is also called interstitial growth. Thus, an inner space is formed inside the bone, which is called the primary medulla. After replacement with pluripotent mesenchymal cells, this primary marrow becomes the actual bone marrow. In addition to length growth, thickness growth also takes place. This process corresponds to ossification from the outside, i.e. perichondral ossification. Osteoblasts detach from the skin of the cartilage (perichondrium) during this process. After detachment, they accumulate in the form of a ring around the model of the cartilage. This results in the formation of a so-called bone cuff. Perichondral ossification always occurs at the midshaft (diaphysis) of long tubular bones and corresponds to their appositional growth. The ossification points within the ossification process are also called ossification centers or bone nuclei. In both perichondral and enchondral ossification, the osteoblasts involved release osteoid. Osteoblast enzymes take influence and support the deposition of calcium salts. After these processes, osteoblasts become osteocytes. During the healing of bone fractures, ossification processes produce woven and fibrous bones that become increasingly resilient through bone remodeling processes. During bone growth, length growth takes place in the section of the growth plate at the midsection, around the edge of which lie the perichondral bone cuffs. The chondrocytes eventually proliferate toward the epiphysis. In the reserve zone, a supply of undifferentiated chondrocytes is present. The proliferation zone contains active chondrocytes that proliferate in a mitotic manner to form longitudinal columns. In the hypertrophic zone, the columnar chondrocytes grow hypertrophically and mineralize the longitundinal septa. Only in the opening zone are enzymes secreted that build the transverse septa.The longitudinal septa are ossified in the opening zone by osteoblasts. At the end of the growth phase, the dia- and epiphysis grow together bony.
Diseases and disorders
Diseases related to osteogenesis are also known as bone formation disorders. For example, mutational achondroplasia, which is known to be the most common cause of genetic short stature, falls into this group. A point mutation in the growth factor receptor gene FGFR-3 disrupts cartilage formation. Thus, the bone growth zone ossifies prematurely, restricting length growth of the arms and legs. This disorder is an enchondral ossification disorder. Most other bone growth disorders also primarily affect enchondral rather than perichondral ossification. A second example from the same group of diseases is fibrodysplasia ossificans progressiva, in which the connective tissue ossifies prematurely. This is caused by a missing switch-off signal for the gene that controls skeletal growth during fetal development. In addition to enchondral ossification, brittle bone disease also directly affects perichondral osteogenesis. Type I collagens are a major element of connective tissue and are appropriately relevant to any bone matrix formation. In brittle bone disease, a point mutation of type I collagen on chromosomes 7 and 17 alters the structure of collagens. For this reason, the most important amino acids of the collagen are interchanged with other amino acids. Collagen synthesis is thus reduced and the twisting of the triple helix is impeded. The collagens therefore lose their stability. Affected bones are therefore glassy in structure and break at the slightest stress.
