Bone tissue
Bone tissue is the one tissue that the bone gives its stability. Bone tissue is classified as connective and supporting tissue and consists of a network of bone cells ( osteocytes ) that is embedded in an extracellular matrix made up of 25% water , 30% organic and 45% inorganic substances. The inner and outer surfaces of the bone tissue are covered with endosteum and periosteum , respectively ; These bone membranes contain the osteoclasts and osteoblasts , which break down or rebuild the bone tissue in the course of bone remodeling .
Bone matrix
Organic ingredients
The organic components of the bone matrix consist of 95% collagen type I and 5% proteoglycans as well as several other non-collagenous proteins, for example osteonectin , osteopontin and osteocalcin . Bone matrix, which consists solely of these organic components, is called osteoid and occurs as a preliminary stage to the mineralized matrix in the development of bone tissue and in small quantities immediately below the endosteum .
Inorganic components
Longitudinally are connected to the tensile collagen fibrils hydroxyapatite - crystals attached, giving the bone its compressive strength; the attachment and defined alignment of the crystals is due to their affinity to the collagen molecules. The phosphate ions in hydroxyapatite have been replaced to a small extent by carbonate ions and hydroxy groups are rarer than in pure hydroxyapatite. The crystallites are only 3 nm thick. About one percent of the citrate molecules are also built into the bone.
Lakuno-canalicular system
The osteocytes run through the bone matrix with long cell processes with which they record mechanical stress and exchange nutrients and information with one another via gap junctions . The mineralized bone matrix is not directly adjacent to this network: the cell bodies lie in small cavities (lacunae) and the cell processes in bone canals (canaliculi) , the resulting cavity system filled with interstitial fluid is called the lacuno-canalicular system .
Classification
According to the spatial organization of the collagen fibrils, a distinction is made between woven bones and lamellar bones .
Braided bone
In contrast to the lamellar bone, the osteocytes are irregularly distributed in the braided bone and the collagen fibers of the bone matrix are apparently disordered in coarse bundles. Braided bone is formed during osteogenesis and in the first phase of bone healing ; usually it is later converted into the biomechanically higher quality lamellar bone as part of the natural bone remodeling. In adults it occurs only in the petrous bone , in the ossicles , on the tooth sockets and the edges of the cranial sutures . Also, certain bone tumors and bone cysts may be composed of woven bone. Woven bone contains a relatively large number of osteoblasts, is well supplied with blood and is less mineralized than lamellar bone. It is very tensile and flexible.
Lamellar bone
Lamellar bone is more stable than braided bone and replaces it in humans from the age of 2. It consists of 3 µm to 5 µm thick layers in which the collagen fibrils are aligned in approximately the same way. This direction of progress changes between lamellas lying on top of one another, in that the component perpendicular to the load direction changes its sign. The angle between the fibrils and the direction of loading is smaller, the more the bone is subjected to tension.
In the cancellous bone , the lamellae are predominantly arranged parallel to the trabecular surface. The trabeculae are vascular so that the osteocytes in them have to be nourished by diffusion from the vessels of the bone marrow, which limits the thickness of the trabeculae to 300 µm as a rule.
The lamellae also run parallel to the surface on the outside of the cortex , creating an outer general lamella that completely surrounds the bone. Some bones also have an inner general lamella on the side of the cortex facing the medullary cavity . Inside, the cortex is made up of osteons and switching lamellae .
Osteons
An osteon (from the Greek ὀστέον " bone "; also Havers system according to Clopton Havers ) consists of a central Havers canal that contains small blood vessels . About 5 to 20 bone lamellae are arranged concentrically around this canal. Volkmann canals , which also contain small vessels and thus create anastomoses between the Haversian vessels, run perpendicular to the Haversian canals .
Switching blades
In the context of bone tissue remodeling , new osteons are constantly driven through the cortex in the direction of loading, with existing structures being broken down by osteoclasts and new lamellae being built from the outside inwards onto the tunnel wall by subsequent osteoblasts. Osteons that are no longer complete because they have been partially replaced by new osteons fill the space between the osteons as switching lamellae. The result of the remodeling process gives the bone density .
Individual evidence
- ↑ Dorothea Schleuter, Susanne Ueberlein, Elke Brunner: Biominerals and biomaterials. In: GIT laboratory journal. May 2013, p. 297.
- ↑ a b Jörg Jerosch among others: Bones: curasan pocket atlas special . Georg Thieme Verlag, 2002, ISBN 3-13-132921-1 .
- ↑ a b Karl-Josef Moll, Michaela Moll: Anatomy: Short textbook for the catalog of objects 1. 18th edition. Elsevier, Urban & Fischer Verlag, 2005, ISBN 3-437-41743-6 , p. 96.
- ^ Roche Lexicon Medicine. 5th edition. Urban & Fischer, Munich 2003, ISBN 3-437-15156-8 . "Lamellar bone" entry.
- ↑ Renate Lüllmann-Rauch: Pocket textbook histology . 5th edition. Thieme, Stuttgart 2015, ISBN 978-3-13-129245-2 , p. 169 .