Desmal ossification involves the conversion of embryonic connective tissue into bone. In comparison to chondral ossification, direct bone formation takes place here. In particular, the skull, facial skull, and clavicle are formed via desmal ossification.
What is desmal ossification?
During desmal ossification, embryonic connective tissue is transformed into bone. Figure shows embryo with recognizable spine. Ossification (bone formation) can occur in two different ways. There is chondral and desmal ossification. In chondral bone formation, a basic framework of cartilage tissue already exists. This transforms into bone tissue in a second step during ossification. All long bones and the spine are formed by chondral ossification. In desmal ossification, however, a cartilaginous skeleton is not preformed. It is characterized by direct bone formation from embryonic connective tissue. Via desmal ossification, the bones of the skull, facial skull, and clavicle are built. These bones are also referred to as braided, cover, occupancy or connective tissue bones. Direct bone healing also occurs 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 formation of the callus. In this process, connective tissue cells are converted into bone cells from the periosteum or endost.
Function and task
As mentioned, chondral and desmal ossification represent the two basic forms of bone formation. Most of the skeleton is formed via chondral ossification. This is an indirect bone formation, because in a first step during embryogenesis a cartilage model of the skeleton is first formed, which in a further step is transformed into a bone skeleton. In desmal ossification, embryonic connective tissue transforms directly into bone. Via desmal ossification, no articular bones or the bones of the spine are formed, but the bones of the skull, facial skull and clavicle. The bone-building processes of both forms of ossification are basically the same. In desmal ossification, however, there is no preformed basic framework of cartilaginous tissue. Whereas in chondral ossification cartilage degradation and bone formation occur simultaneously, in desmal ossification only bone formation from so-called osteoblasts takes place. Bone healing in fractures can proceed by chondral or desmal ossification, depending on the type of injury. In this case, desmal ossification only takes place if there is still closer contact between the two bone fragments. Thus, bone cells can form directly from the osteoblasts of the periosteum or the endosteum. The detour via a cartilage-like callus tissue is omitted. However, if these intensive contacts are no longer present, healing takes place via the callus (scar tissue) in the course of chondral ossification, which is gradually transformed into a bone structure. In both forms of bone formation, woven or fibrous bones are initially formed from the osteoblasts of the embryonic connective tissue. Calcium vesicles develop in the osteoblasts and burst, releasing calcium crystals. In the process, the calcium crystals enlarge under the formation of the bone substance from hydroxyapatite. The small bone nuclei form the starting point for further accumulation of osteoblasts, which continue the mineralization. While in chondral ossification this process uses the pre-formed matrix of cartilage tissue, in desmal ossification bone formation continues appositionally (by further apposition to existing bone substance). The initially formed fibrous bones do not yet possess great mechanical strength because the collagen fibrils of the basic bone substance are disordered. Mechanical stimuli cause bone remodeling during the first years of life or after healing of a bone fracture, resulting in stable and organized lamellar bones. The modeling of bone remodeling is accomplished by 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 in the process form a more stably organized lamellar bone.
Diseases and ailments
Some rare bone formation disorders are known in the context of desmal ossification. For example, the clinical picture of craniosynostosis is characterized by premature ossification of the cranial sutures. As a consequence, 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 room for the brain to grow. This malformation of the skull often occurs in children whose mothers smoked during pregnancy. However, craniosynostosis also occurs in the context of certain hereditary disorders 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 vitamin D deficiency in early childhood. This can be caused by metabolic disorders, lack of sunlight or inadequate nutrition. Vitamin D is absolutely necessary for the absorption of calcium from food. Rickets leads to muscle weakness and soft skull bones. This results in malformation of the shape of the head. At the same time, curvatures of the legs develop, which promote later postural deformities. The most important therapy of the disease is an adequate supply of vitamin D. Another ossification disorder is the so-called brittle bone disease (osteogenesis imperfecta). Osteogenesis imperfecta also affects both desmal and chondral ossification. This disorder is characterized by unusual fragility of bone caused by a gene mutation of type I collagen in connective tissue.
