Ossification

Ossification (from Latin os, " bone ") is the formation of bone tissue in growth, after fractures or pathological ( pathological ) ossification. Osteogenesis refers to the formation of an individual bone. During the development of bone can occur in two ways:

• Of connective tissue: intramembranous osteogenesis ( ossification ),
• Of cartilage tissue: chondral osteogenesis ( ossification ),
• By deposition of bone tissue to existing: appositional ossification.

Intramembranous ossification

In intramembranous ossification, the bone tissue directly from the embryonic connective tissue ( mesenchyme ), which is why it is also called direct ossification occurs. Bone thus formed is called connective tissue, braid, deck or covering bones. In this way the bones of the cranial vault and the facial skull and the clavicle occur.

Mesenchymal cells condense like an island to progenitor cells. These differentiate into osteoblasts and build the bone matrix ( osteoid ), which subsequently mineralized. They substantiate due to the accumulation of further Osteoidschichten gradually itself and become osteocytes ( resting cells ). This gives rise to individual Verknöcherungspunkte which unite to extra bone to form the finished bone.

If there is a broken bone so new bone tissue is always first been formed, which is later converted to lamellar bone (→ bone healing).

Chondral ossification

In this form arise from the mesenchyme, first cartilaginous skeletal elements, the Primordialskelett - it is therefore also referred to as indirect ossification. One calls this bone also spare bones.

There is the ossification of the inside ( endochondral ossification ), thereby grow into the cartilage tissue blood vessels a, in their company are mesenchymal cells. These differentiate into chondroclasts (cartilage degradation) and osteoblasts ( for bone formation ). In the epiphyses occurs by the continuous assembly and disassembly to a linear growth ( interstitial growth). Also formed in the interior of the bone, an internal space (primary Mark ), which is replaced by pluripotent mesenchymal cells, and thus represents the actual bone marrow.

The ossification of the outside ( perichondrial ossification ) but from the perichondrium ( perichondrium ) from osteoblasts. This store in a ring around the cartilage model and there is such a bone cuff. The perichondrial ossification takes place of the long bones at midshaft ( diaphysis ). The perichondrial ossification thus serves the thickness growth ( appositional growth).

In both forms of chondral ossification but the osteoblasts a basic substance, the osteoid from. Through the influence of Osteoblastenfermente lime salts are deposited. The osteoblasts then differentiate into osteocytes. The points that pose the ossification is called ossification centers or centers of ossification. The ossification centers arise before at Nestflüchtern in the nest stools usually only after birth.

Appositional ossification

In the appositional ossification bone tissue is attached to an existing one. In this way, the thickness of bone growth takes place.

Bone remodeling

Initially formed both by intramembranous and endochondral ossification by and in the healing of bone fractures mesh or woven bone. Here, the collagen fibrils of the bone matrix are still disordered. The woven bone have a large growth potency, but a low mechanical strength. Due to mechanical stress (formative stimuli) occurs in the first years of life for the conversion to the more stable and strict organized lamellar bone.

The structure of the lamellar bone is most clearly pronounced in the substantia compacta. It is formed initially by osteoclasts, exposing the larger vessel conducting channels in the longitudinal direction of the ( braided ) bone by and carve a path through the tissue with the secretion strong proteolytic enzymes. Subsequently, osteoblasts attach to the walls of that channel, which is called the Haversian canal, and produce new basic substance until they have " bricked " ( henceforth referred to them as osteocytes ). Through the addition of further layers of osteoblasts, the diameter of the channel becomes gradually smaller. In this way, a so-called osteon - a system of about 4-20 mineralized bone lamellae, which are arranged concentrically around a Haversian canal with smaller ( " Haversian " ) blood vessels. Between the longitudinally oriented Haversian vessels with each other and the vessels of the periosteum there are cross-connections, the Volkmann's canals.

The collagen fiber bundle within the fins extend helically around the channel, thereby changing the direction of rotation with each adjacent blade. You are here stretched by bias and not - as is the connective tissue - curled. This construction of counter-rotating spirals networked converts compressive and tensile loadings into surface pressures and gives the bone its exceptional stability.

Because of the constant remodeling that takes place in the final bone, it always comes back to the formation of new osteons (see also Knochengeweberemodellierung ). Remnants of older osteons that are found between the younger intact osteons are called lamellae.

Bone growth

Short plate and bone grow by external addition of bone substance. In this way, the increase in thickness of the long bones occurs.

Linear growth of long bones occurs in the region of a growth plate between the endochondral ossified diaphysis ( middle piece ) to the edge of which is a perichondral formed bone cuff, and the resulting endochondrally epiphysis ( end ). One calls this growth plate later if the diaphysis and epiphysis have converged, also epiphyseal plate. The growth is due to the fact that the chondrocytes with the same velocity direction epiphysis multiply as they are " hunted " by the ossification of the Diaphysenmitte. The growth plate can be divided into four zones ( listed opposite to the growth direction, ie from epiphysis to diaphysis ):

• Reserve zone: There is still a supply of undifferentiated chondrocytes can be obtained from the supplies for the proliferative zone.

• Proliferative zone: Here are active chondrocytes rapidly proliferate mitotically. They multiply and grow it but so that longitudinal columns are formed. These columns are separated by the Longitudinalsepten. During which the chondrocytes are isolated in a column by the Transversalsepten.

• Hypertrophic zone: The parent column in chondrocytes grow hypertrophic and then mineralize the Longitundinalsepten, but not the Transversalsepten.

• Opening zone: The chondrocytes but now from enzymes by which the Transversalsepten be dismantled. Then they die (apoptosis). This creates holes into which penetrate macrophages to remove the residues. These caves between the Longitudinalsepten are now occupied and ossified by osteoblasts.

Fractures or mechanical trauma in this area can cause the joint stop growing and the bones are no longer growing. With the growth of financial statements, these cartilage plate disappears completely and grow slide and epiphyseal bone together. The limit is, however, often still be seen as a joint line ( Linea physealis ).