Bones

Synonyms

Bone structure, bone formation, skeleton Medical: Os

Bone forms

According to the form one differentiates: Independent of the form one still differentiates:

  • Long bones
  • Short bones
  • Plate Planar bone
  • Irregular bones
  • Aerated bones
  • Sesame bones and additional, so-called
  • Accessory bones

The long bones of the extremities are tubular bones and are formed by a shaft (diaphysis) and two ends (epiphyses). During the growth phase, a growth joint (epiphysis joint) consists of cartilage between the shaft and the epiphysis, which at the end of the growth phase ossifies into the so-called epiphysis joint. The part of the shaft directly adjoining the epiphyseal joint is called the metaphysis.

Protrusions of bone to which tendons and ligaments are attached are called apophyses. If the tendons and ligaments are attached to roughnesses, these roughnesses are called tuberosities. Bone edges that are comb-shaped or strip-shaped are called crest (Crista) or lip (Labrum) or linear roughness (Linea ).

These combs, lips and lines serve muscles, tendons, ligaments and joint capsules as attachment. The bone tissue consists of the bone cells (osteocytes), which are formed by an extracellular matrix: The basic substance and collagenous fibrils are also called intercellular substance. The collagen fibrils belong to the organic part of the bone and the salts belong to the inorganic part.

The most important salts in bone are: the less important are other compounds of calcium, potassium, sodium with chlorine and fluorine. The salts determine the hardness and strength of the bone. If the bone is free of salts, it becomes flexible.

The organic components of the bone provide the elasticity. The ratio of salts and organic components changes in the course of life. In newborns the proportion of organic parts of the bone is 50%, in old people only 30%.

In addition to the osteocytes, there are osteoblasts as bone-building cells and osteoclasts as bone-destroying cells. After dental tissue, bone tissue is the hardest substance in the human body and has a water content of 20%.

  • Basic substance
  • Collagen Fibrils
  • A putty substance and
  • Various salts is formed.
  • Calcium Phosphate
  • Magnesium phosphate and
  • Calcium carbonate,

Bones are formed in the human body in two different ways.

In both cases, the first bone units appear in the 2nd embryonic month with the collarbone and ends with the closure of the apo- and epiphyseal joints at the beginning of the 20th year of life. If the bone develops directly in the embryonic connective tissue (mesenchyme) from mesenchymal precursor cells, this is called desmal bone development. The resulting bones are called connective tissue bones.

Thus, skull bones, the lower jaw and parts of the clavicle are formed. If the bone does not develop from the connective tissue but from cartilage tissue, this is called chondral ossification. Initially, a cartilaginous skeleton (primary skeleton) develops, which is similar in shape to the later skeleton.

This “pre-skeleton” is then replaced by bone. In both forms, meshwork bone is formed first, which is then transformed into lamellar bone under stress. The meshwork bone has a greater growth potential than the lamellar bone and thus forms more groins and beams, with the help of which it can erect a spacious skeleton in a relatively short time.

Within the meshwork bone, the blood vessels and the course of the collagen fibers are disordered and the number of osteocytes is low and their arrangement is irregular. In addition, the mineralization content of the tissue is low. Therefore, the braided bone is not as resilient as the lamellar bone.

During growth into the 20s, the braided bone is transformed into lamellar bone. The first generation of osteones are called primary osteones and are formed during the fetal period. When these are replaced by new osteones through remodelling processes, they are now called secondary osteones.

This remodeling process takes place increasingly between the ages of 8 and 15. During remodeling, vessels first penetrate the braided bone and drive a vessel-bearing canal into the bone with the help of osteoclasts. This channel already has the diameter of the osteon.The osteoblasts then differentiate from the connective tissue accompanying the vessels, attach themselves to the canal wall and begin to form the matrix, which as an osteoid already arranges itself in the form of lamellae in the osteon.

Later, the osteoid is completely mineralized and the osteoblasts are walled in. The lumen of the canal is thus narrowed bit by bit until only the Havers canal remains.

  • In desmal bone development (ossification), the bone is formed directly, whereas in
  • Chondral bone development of the bones from cartilage tissue indirectly results.

The development of a tubular bone occurs through both direct and indirect ossification.

Within the bone shaft, the so-called perichondral bone cuff is formed via direct ossification. On this basis, the shaft grows in thickness. Further fibrous and braided bone balls are attached to the perichondral bone cuff until a loosely structured bony shaft has formed.

Initially, the ring forms only in the middle part of the shaft, but then expands over the entire length of the shaft. This leads to stiffening and the further bony remodelling processes do not lead to an interruption of the supporting function. With the appearance of braided bone, the perichondrium, which is temporarily surrounded by the bone, is converted to the periosteum, from which further growth of the bone thickness is initiated.

This is followed by strong cartilage growth in the area of the shaft, which provokes longitudinal growth of the shaft. Here the cartilage cells are already arranged in longitudinal cell columns, which then ossify. Due to a deteriorated supply of nutrients to the cartilage cells, these are then broken down by connective tissue penetrating from the vessels with the help of cartilage-degrading cells.

This creates a primary medullary cavity, in which the bone marrow with its mesenchymal cells is then formed. At the edges of the medullary cavity, osteoblasts begin to form bone mass, resulting in a primary bone nucleus. Starting from the primary medullary cavity, the cartilage is then gradually replaced by the meshwork bone, except for the epiphyses.

At a genetically determined time, secondary bone nuclei are then formed within the pineal gland, which then displace the cartilage tissue from the pineal gland. At the pineal gland joints, cartilage is increased by division, which results in longitudinal growth. The bony epiphysis is separated from the metaphysis by a cartilage plate.

The joint cartilage is connected to the growth zone. Within the epiphyseal fugue, four zones are distinguished. The proliferation zone is decisive for length growth.

This is where cell proliferation takes place. Characteristic cell columns are formed through cell division. With increasing size, the cells take up more water and are then located in the bladder cartilage zone.

This cell hypertrophy and cell division are beneficial to the growth of the length. In the bladder cartilage zone, cell activity increases, resulting in increased collagen formation, which forms longitudinal septa, and mineralization, resulting in stiffening. This is a prerequisite for the sprouting of vessels and the septa serve as a scaffold for the newly formed bone.

Via the vessels, cartilage-eating cells enter the tissue and build the cartilage, creating space for the newly formed bone. Bone formation then begins with the colonization by osteoblasts on the surface of the remaining mineralized septa.

  • The reserve zone (with resting cartilage),
  • The proliferation zone (with columnar cartilage cells),
  • The cartilage remodeling zone and the
  • Ossification.