Desmal Ossification

The desmal ossification is made of connective tissue. This is formed by mesenchymal cells. During ossification, the cells are first positioned close to each other and are then increasingly better supplied with blood.

Then the mesenchymal cells change into osteoblasts, the cells that form bone. These then first form the organic parts of the new bone such as collagen. Then calcium bubbles are formed in the osteoblasts and released.

These bubbles then burst and calcium crystals are released. These crystals enlarge and finally become hydroxyapatite. The osteoblast is finally completely surrounded by bone substance and is then called an osteocyte.

Further osteoblasts then attach themselves to the tiny bone that has now been formed and in turn form bone material, so that the bone finally grows “appositionally”, i.e. by attachment. Typically, bones of the skull are formed by desmal ossification. Bone fractures also heal first through desmal ossification.

Chondral Ossification

In contrast to the previous mechanism, bone is formed from cartilage during chondral ossification. The bone is therefore first created as cartilage and is only replaced by bone in the course of development. Because the bone is first created as cartilage, chondral ossification is also called indirect ossification.

A further distinction is made between perichondral and enchondral ossification.Perichondral ossification takes place, for example, in the child at the diaphysis, the shaft of the humerus. Here, osteocytes first form a bone cuff around the cartilage model of the bone. Strictly speaking, perichondral ossification is actually a desmal ossification, since it does not require cartilage cells.

However, the typical chondral ossification takes place directly in the cartilage and is called enchondral growth. In the humerus, this ossification takes place at the level of the epiphysis. Here, the cartilage cells divide in a so-called proliferation zone.

Because the bone cuff around the cartilage prevents it from expanding in width, the new cartilage cells are arranged in a longitudinal direction. In this way, the bone grows lengthwise. Further towards the end of the bone, the cartilage cells become larger and calcify.

Finally, the cartilage cells die and the osteoblasts, i.e. bone-building cells, begin to ossify. The zone in which the bone grows is called the epiphyseal fissure. As long as there are still cartilage cells in the epiphysis joint, the bone can grow in length. Normally, the epiphyseal fugue closes in the 19th year of life. Bone fractures caused by the joint can significantly disrupt the ossification, and the growth in length will lag behind that of the rest of the body.