Synonyms
Bone structure, bone formation, skeleton Medical: Os
- The braided bone and the
- Lamellar bones
- The periosteum is located on the outside,
- This is followed by the layer of compacta and then
- The layer of cancellous bone.
- The inner periosteum (endosteum) still lies on the inside.
The periosteum consists of a taut, mesh-like collagenous layer of connective tissue, the inner layer (cambium layer) of which is loosely built and is permeated by numerous blood vessels and nerves. This layer consists mainly of osteoblasts and their stem cells. The outer layer (stratum fibrosum) is made of elastic fiber mesh and tightly arranged collagen fiber bundles (Sharpey fibers).
Together with the collagen fibers of attached tendons, they radiate into the bone and thus anchor the tendon. The outer layer contains the artery and the nutricia vein, which lead through holes into the bone. The compacta is a densely packed bone substance from which approx.
80% of all skeletal mass is built. The remaining 20% of the skeletal mass is formed by cancellous bone. The compacta is located in the entire outer region of a long bone.
The compacta consists of small, circular bone structures, the so-called osteons, which are about 1 cm long and have a diameter of about 250-350 μm. In the center, there is a vessel, nerve fibers and loose connective tissue in the Havers canal, around which 5-20 layers of collagen fibers are embedded, running helically around the axis of the osteon. Each layer is 5-10μm thick and runs at varying angles to the one below.
The arrangement of the collagen fibers depends on the mechanical load and is aligned with it. If the angle of inclination of the collagen fibers is flat, the osteon is more resistant to compression; if the angle of inclination is steep, the osteon is resistant to tension. This specific arrangement of the collagen fibers and the high content of mineral salts in the extracellular matrix gives the bone its high dimensional stability.
The osteocytes are located between the collagen fiber layers, and their projections protrude far between the layers and communicate with each other. Via these projections, nutrients and oxygen from the blood vessels reach all cells and thus ensure their nutrition. The outer boundary of the osteon is the cement line with a thickness of 1-2μm.
Switching lamellae are fragments of old osteons between the other osteons. The outer general lamella is located directly under the outer periosteum, while the inner general lamella is located under the inner periosteum. The blood vessel in the Havers Canal runs perpendicular to the artery and vein nutricia, which lead into the bone from the outside.
In addition, the Havers channels running longitudinally in the bone are connected by short Volkmann channels running transversely and at an angle. The cancellous bone structure is structured like a sponge and provides a three-dimensional framework of lattice-like arranged thinner and thicker beams, rods and plates, the so-called cancellous bone grafts. This structure means that more than 60% of the bone surface is in the area of the cancellous bone.
The bone substance of the cancellous bone is also arranged in a lamellar pattern, but does not have any blood vessels. As a result, the cancellous bone trabeculae have a thickness of only 200-300μ, so that they are still nourished by diffusion from the adjacent medullary canal. The medullary cavity of the bone structure is filled either with fatty tissue or with hematopoietic tissue.
Due to the alignment of the cancellous bone calculi, the bone is capable of functional deformation. Thus, bending forces create compressive and tensile forces within the bone, which in turn lead to the formation of compression and tensile trabeculae. This function enables the bone to adapt structurally to the function and the changed static conditions throughout its life.
In cancellous bone, the rate of remodeling is approximately three times higher than in compact bone. While cultivation predominates in the growing age, degradation predominates after the age of 50, with age-related hormonal changes playing a decisive role here. Through these growth and conversion processes, old lamella systems are broken down and new ones are built up.
The breakdown is carried out by the osteoclasts. These are bone cells that are specifically targeted for degradation. The osteoblasts then build up the lamellae.The first generation of osteons, which are created by the remodelling of woven bones, are called primary osteons, those in the process of remodelling are called switchable lamellae, and those already remodeled are called secondary osteons.
The endosteum of the bone structure is a thin layer of cells that is formed from bone covering cells. The composition depends on age and localization. In adults, about 5% of the total area is covered with osteoblasts and osteoclasts, which are responsible for conversion and degradation processes, 95% is formed by the bone graft cells.
In addition to the osteocytes, osteoblasts and osteoclasts, the precursor cells of the osteoblasts are also found in the bone as stem cells. These stem cells can divide and develop into osteoblasts. The osteoblasts are located where bone is formed.
They are located in the tissue as a round layer connected by cellular processes and initially produce a non-mineralized matrix called osteoid and collagen fibers. After 8-10 days, calcium phosphate salts are deposited and the osteoblasts wall themselves in, so to speak. After that they further differentiate into osteocytes.
Osteoclasts are large multinucleated cells that develop from immigrated blood cells and have developed the ability to break down bone tissue. They are in close contact with the bone matrix and form resorption cavities (howship lacunae) on its surface in which the bone matrix is broken down by enzymatic means. In growing bone, osteoclasts are still present in relatively large numbers; in differentiated lamellar bone, they are only found in places of active bone remodeling.
These are about 1% of the inner bone surface. During the day, 40-70μm of osteoclast can eat into the bone and thus degrade as much tissue as about 100 osteoblasts have previously built up. All bone-building and degradation processes take place on the outer and inner surfaces of the bone, involving the outer (periosteum) and inner periosteum (endosteum).
With the exception of the cartilaginous joint surfaces and the tendon attachments, the bone is surrounded by the periosteum. The endosteum covers the inner surface of the compacta, the Havers- and Volkmann canals as well as all bone balls of the cancellous bone. The estimated surface area of the periosteum in adults is about 0.5 m2, that of the endosteum is about 11 m2.
