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Ossification (from Latin Os " bone ") is the formation of bone tissue during growth , after fractures or in the event of pathological (diseased) ossification ( heterotopic ossification ). Osteogenesis refers to the formation of an individual bone. During development, bones can arise in different ways:

  • from connective tissue : desmal osteogenesis (ossification),
  • from cartilage tissue : chondral osteogenesis (ossification),
  • by the accumulation of bone tissue on the existing: appositional ossification.

Desmal ossification

In desmal ossification, the bone tissue arises directly from the embryonic connective tissue ( mesenchyme ), which is why one speaks of direct ossification . Bones formed in this way are called connective tissue , mesh , cover or evidence bones . In this way, the bones of the skull and the facial skull and the collarbone are formed .

Mesenchymal cells condense to form precursor cells like islands. These differentiate into osteoblasts and build up the basic bone substance ( osteoid ), which then mineralizes. In doing so, they gradually wall themselves in through the accumulation of further osteoid layers and become osteocytes ( resting cells ). This creates individual ossification points that combine to form bone braces and thus form the finished bone.

If a bone fracture occurs , new bone tissue is always formed first, which is later converted into lamellar bone (→ bone healing ).

Chondral ossification

Course of the chondral ossification

In this form, the mesenchyme initially develops cartilaginous skeletal elements, the hyaline primordial skeleton, through the differentiation of mesenchymal cells into chondroblasts , which is why it is also known as indirect ossification . These bones are also called replacement bones .

There is ossification from the inside ( enchondral ossification ), with blood vessels growing into the cartilage tissue, accompanied by mesenchymal cells. These differentiate into chondroclasts (cartilage degradation) and osteoblasts (for bone formation). In the area of ​​the epiphyseal plates there is a growth in length (interstitial growth) due to the constant build-up and breakdown.

With ossification from outside ( perichondral ossification ), osteoblasts separate from the cartilage ( perichondrium ). These are placed in a ring around the cartilage model, creating a bone cuff . Perichondral ossification takes place on the middle shaft (diaphysis) of the long tubular bones. The perichondral ossification thus serves to increase the thickness (appositional growth). An interior space (primary marrow) is also created inside the bone, which is replaced by pluripotent mesenchymal cells and thus represents the actual bone marrow .

In both forms of chondral ossification, the osteoblasts secrete a basic substance, the osteoid. Calcium salts are deposited due to the influence of the osteoblastic ferments. The osteoblasts then differentiate into osteocytes . The points that pose the ossification, called ossification centers or centers of ossification . The ossification centers arise in the case of those who flee the nest , and in the case of the nestlings mostly only after birth .

Appositional ossification

In appositional ossification , bone tissue is attached to existing tissue. This is how bones grow in thickness.

Bone remodeling

Both desmal and chondral ossification and the healing of broken bones initially result in woven or fiber bones. Here the collagen fibrils of the basic bone substance are still disordered . The fiber bones have a great growth potential, but a low mechanical strength. Mechanical stress (formative stimuli) leads to remodeling in the more stable and more strictly organized lamellar bones in the first years of life.

A lamellar bone in cross section (scheme)

The structure of the lamellar bone is most clearly pronounced in the substantia compacta . It is initially created by osteoclasts , which expose larger, vascular ducts in the longitudinal direction of the (braided) bone by paving a way through the tissue with the secretion of strong proteolytic enzymes. Osteoblasts then attach to the walls of this canal, known as Haversian canal , and produce new ground substance until they have "walled in" (from then on they are called osteocytes ). As further layers of osteoblasts accumulate, the diameter of the canal gradually becomes smaller. This creates a so-called osteon - a system of around 5–20 mineralized bone lamellae, which are arranged concentrically around a Haversian canal with smaller ("Haversian") blood vessels. There are cross-connections between the longitudinally aligned Haversian vessels and the vessels of the periosteum, the Volkmann canals .

The collagen fiber bundles within the lamellae run helically around the canal, the direction of rotation changing with each adjacent lamella. They are stretched here by pretension and not - as is usually the case in connective tissue - wavy. This structure of counter-rotating, networked spirals converts pressure and tensile loads into surface pressures and gives the bone its special stability.

Due to the constant remodeling that takes place in the finished bone, new osteons are repeatedly formed (see also bone tissue remodeling ). Remnants of older osteons that can be found between the younger, intact osteons are called switching lamellae.

Bone growth

Short and flat bones grow through the external accumulation of bone substance. This is how the long bones grow in thickness.

Long bones grow in length in the area of ​​a growth plate between the endochondrally ossified diaphysis (middle section), around the edge of which is a perichondrally created bone cuff, and the enchondrally created epiphysis (end piece). Later, when the diaphysis and the epiphysis have approached, this growth plate is also called the epiphyseal plate . The growth is based on the fact that the chondrocytes multiply in the direction of the epiphysis at the same speed as they are "chased" by the ossification from the center of the diaphysis. The growth plate can be divided into four zones (counted against the direction of growth, i.e. from epiphysis to diaphysis):

  • Reserve zone : Here there is still a supply of undifferentiated chondrocytes , from which replenishment for the proliferation zone can be obtained.
  • Proliferation zone : Active chondrocytes are located here, which multiply rapidly mitotically . However, they multiply and grow in such a way that longitudinal columns are formed. These columns are separated by the longitudinal septa, while the chondrocytes in a column are separated by the transverse septa.
  • Hypertrophic zone : The chondrocytes arranged in columns grow hypertrophically and then mineralize the longitundinal septa, but not the transversal septa.
  • Opening zone : The chondrocytes now secrete enzymes that break down the transverse septa. Then they die ( apoptosis ). This creates holes in which macrophages penetrate to remove the remains. These cavities between the longitudinal septa are now occupied and ossified by osteoblasts .

Fractures or mechanical trauma in this area can cause the joint to stop growing and the bone to stop growing. With the completion of growth, this cartilage plate disappears completely and the dia- and epiphyses grow together as bones. The border can often still be recognized as a joint line ( Linea physealis ).

See also