In the desmal ossification embryonic connective tissue is converted into bone. In comparison to chondral ossification, there is direct bone formation here. In particular, the skull, facial skull and collarbone develop through desmal ossification.
What is desmal ossification?
Ossification (bone formation) can take place in two different ways. There is chondral and desmal ossification. In chondral bone formation, there is already a basic structure of cartilage tissue. In a second step, this is converted into bone tissue during ossification. All long bones and the spine are formed by chondral ossification.
However, a cartilage skeleton is not pre-formed in desmal ossification. It is characterized by the direct bone formation from embryonic connective tissue. The bones of the skull, the facial skull and the collarbone are built up via desmal ossification. These bones are also known as braided, cover, or connective tissue bones.
Direct bone healing also takes place via desmal ossification. If there is still intensive contact between the ends of the bones via the periosteum after the formation of a bone fracture, accelerated bone healing occurs without the formation of the callus. In the process, connective tissue cells are converted into bone cells from the periosteum or the endosteum.
Function & task
As 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 the first step in embryogenesis is a cartilage model of the skeleton, which is converted into a bone skeleton in a further step.
In desmal ossification, embryonic connective tissue is converted directly into bone. Through the desmal ossification, no joint bones or the bones of the spine are formed, but the skull, facial and clavicular bones. The bone-building processes of both forms of ossification are basically the same. In the case of desmal ossification, however, there is no pre-formed basic structure made of cartilage tissue.
While cartilage degradation and bone formation take place simultaneously with chondral ossification, with 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, the desmal ossification only takes place when the two bone fragments are in closer contact. This allows bone cells to be formed directly from the osteoblasts of the periosteum or the endosteum. The detour via a cartilage-like callus tissue is no longer necessary. If these intensive contacts are no longer given, however, 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, braided or fiber bones arise from the osteoblasts of the embryonic connective tissue. Calcium vesicles develop in the osteoblasts, which burst and release calcium crystals. The calcium crystals enlarge with the formation of the bone substance from hydroxyapatite. The small bone nuclei 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 desmal ossification the bone structure is continued appositionally (through further deposition on existing bone substance). The fiber bones initially formed do not yet have great mechanical strength, since the collagen fibrils of the basic bone substance are disordered. Mechanical stimuli lead to 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 room for new osteoblasts, which thereby form a more stable, organized lamellar bone.
Illnesses & ailments
In the context of desmal ossification, some rare bone formation disorders are known. The clinical picture of craniosynostosis is characterized by premature ossification of the skull sutures. As a result, the normal growth of the skull is no longer possible. So-called compensatory growth of the skull bone occurs. If multiple cranial sutures are affected, surgical correction is often necessary to give the brain room to grow. This malformation of the skull 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.
A typical disorder of ossification is rickets. The disease affects both chondral and desmal ossification.Rickets is a calcium absorption disorder. The disease is triggered by a severe deficiency in vitamin D in early childhood. For example due to metabolic disorders, lack of sun exposure or poor nutrition.
Vitamin D is essential for the absorption of calcium from food. Rickets leads to muscle weakness and soft skull bones. This leads to a malformation of the head shape. At the same time, curvatures develop on the legs, which later lead to poor posture. The most important therapy for the disease is an adequate supply of vitamin D.
Another ossification disorder is the so-called glass bone disease (osteogenesis imperfecta). Both the 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 connective tissue.
