Ossification (bone formation) can happen in two different ways. There is chondral and desmal ossification. In chondral bone formation, there is already a basic framework of cartilage tissue. In a second step, this is transformed into bone tissue during ossification. All long bones and the spinal column are formed by chondral ossification.

In the case of desmal ossification, however, a cartilaginous skeleton is not preformed. It is characterized by direct bone formation from embryonic connective tissue. The bones of the skull, facial skull and collarbone are built up via desmal ossification. These bones are also known as woven, covering, covering or connective tissue bones.

Direct bone healing also takes place via desmal ossification. If there is still intense contact of the bone ends via the periosteum after a fracture has occurred , accelerated bone healing occurs without callus formation . In this process, connective tissue cells from the periosteum or the endosteum are converted into bone cells.

Function & task

As already mentioned, chondral and desmal ossification represent the two basic forms of bone formation. Most of the skeleton is formed by chondral ossification. This is an indirect bone formation, because in a first step in embryogenesis, a cartilage model of the skeleton is created, which in a further step is converted into a bone skeleton.During desmal ossification, embryonic connective tissue converts directly into bone. No joint bones or the bones of the spine are formed via the desmal ossification, but the cranial, facial skull and collarbone bones. The bone building processes of both forms of ossification are basically the same. In the case of desmal ossification, however, there is no preformed basic framework of cartilage tissue.

While in chondral ossification cartilage degradation and bone formation take place simultaneously, in desmal ossification only bone formation from so-called osteoblasts takes place. Bone healing in fractures can occur through chondral or desmal ossification, depending on the type of injury. In this case, desmale ossification only takes place if there is still closer contact between the two bone fragments. Bone cells can form directly from the osteoblasts of the periosteum or endosteum. The detour via a cartilage-like callus tissue is no longer necessary. However, if these intensive contacts are no longer present, healing takes place via the callus (scar tissue) as part of a chondral ossification, which is gradually converted into a bone structure.

In both forms of bone formation, woven or fibrous bone initially develops from the osteoblasts of the embryonic connective tissue. Calcium vesicles develop in the osteoblasts, which rupture and release calcium crystals. The calcium crystals enlarge during the formation of the bone substance from hydroxyapatite. The small bone cores form the starting point for further deposits of osteoblasts, which continue the mineralization.

While this process uses the pre-formed matrix of cartilage tissue in chondral ossification, in dermal ossification the bone build-up is continued appositionally (through further attachment to existing bone substance). The fibrous bones that initially form do not yet have great mechanical strength, since the collagen fibrils of the basic bone substance are disordered. Mechanical stimuli cause bone remodeling in the first few years of life or after a bone fracture has healed, resulting in stable and organized lamellar bones.

The modeling of bone remodeling is accomplished through the joint work of osteoclasts and osteoblasts. Osteoclasts are multinucleated bone marrow cells that perform tasks similar to macrophages . They break down old bone cells and make way for new osteoblasts, which in the process form more stably organized lamellar bone.

Diseases & Ailments

Some rare bone formation disorders are known in the context of desmal ossification. The clinical picture of craniosynostosis is characterized by premature ossification of the cranial sutures. As a result, the usual growth of the skull is no longer possible. A so-called compensatory growth of the skull bone occurs. When multiple cranial sutures are affected, surgical correction is often necessary to allow the brain room to grow. This skull malformation is common in children whose mothers smoked during pregnancy .However, craniosynostosis also occurs in the context of certain hereditary diseases such as Baller-Gerold syndrome , Jackson-Weiss syndrome or Muenke syndrome .

Rickets is a typical disorder of ossification . The disease affects both chondral and desmal ossification. Rickets is a calcium absorption disorder . The disease is triggered by a severe lack of vitamin D in early childhood. For example, through metabolic disorders , lack of sunlight or poor nutrition.

Vitamin D is imperative for the absorption of calcium from food. Rickets causes muscle weakness and soft skull bones. This leads to a malformation of the head shape. At the same time, crookedness of the legs develops, which later promotes postural defects. The most important therapy for the disease is an adequate supply of vitamin D.

Another ossification disorder is the so-called brittle bone disease (osteogenesis imperfecta). Both desmal and chondral ossification are affected in osteogenesis imperfecta. This disease is characterized by an unusual fragility of the bones caused by a gene mutation of type I collagen in the connective tissue.