Can acid attack the skull or bones

  Thumbnail overview bones (Ossa) + Bone formation (Ossification):
Illustrations that have already been labeled can be called up by clicking on the text! The total of 210 bone (Terminologia histologica: Ossa, English: bones) of humans consist of Bone tissue (Terminologia histologica: Textus osseus, English: bone tissue). A distinction is made between flat bones with a solid = hard outer and inner layer from the elongated, hollow bones on the inside. These have at their ends (Epiphysisn) Joint surfaces covered with hyaline cartilage, followed by a maximum 3 mm thick layer of compact bone tissue, the substantia corticalis, which merges into cancellous bone tissue. Thereafter, until the end of the longitudinal growth of the affected bone, a growth plate consisting of slowly secondary ossifying hyaline cartilage appears (Epiphyseal plate; Terminologia histologica: Centrum ossificationis secundarium, Centrum ossificationis epiphysiale; English: secondary ossification center, epiphysial ossification center). This is followed by cancellous bone in the transition area to the hollow shaft, which is called Metaphysis and finally the only up to a maximum of 15 mm thick compact bone tissue, which is called substantia compacta, as Diaphysis Bone shaft called the corpus, the interior of which is filled by bone marrow (in adults, fat marrow consisting of univacuolar adipose tissue).
Cells of the bone:
All bone tissue consists of bone cells and the surrounding ossified connective tissue substance. A distinction is made between the following cells:
Osteoblasts (Terminologia histologica: Osteoblasti, English: osteoblasts) are very metabolically active and build bones. They arise from mesenchymal stem cells, which are stimulated by bone morphogenitic proteinsbipotent progenitor cells become. The latter show a morphology similar to that of fibroblasts and can either become a chondroblast or a Pre-osteoblasts develop. The latter then differentiate into osteoblasts, which is promoted by active vitamin A (retinoic acid). Osteoblasts show abundant, partly enlarged (dilated) rough endoplasmic reticulum and are therefore basophilic. In their Golgi apparatus, the glycoproteins and proteoglycans for release into the bone matrix from proteins are generated by sugar coupling. The cell membrane of the osteoblasts shows abundant alkaline phosphatase. Like the osteocytes, they are connected to one another in an epithelial manner via processes that have gap junctions. In addition, they also form tight junctions and locally delimited zonulae adhaerentes to one another. They secrete thatOsteoid (Terminologia histologica: Stratum preosseum, Osteoideum; English: osteoid, preosseus matrix), which is still soft and rich in basic substance and tropocollagen from which the first collagen fibrils then form extracellularly. In addition, osteoblasts secrete substances that stimulate the osteoclasts and also bind to these cells see here. Parathyroid hormone inhibits osteoid synthesis and promotes the formation of osteoclast stimulating factors. Calcitriol, derived from vitamin D, also stimulates osteoblast activity.
Osteocytes (Terminologia histologica: Osteocyti, English: osteocytes), the "walled-in" bone cells with little metabolic activity, stand over up to 200 µm long, 150 nm thin processes (Terminologia histologica: Processus osteocyti, English: osteocyte processes) and there formed gap junctions with one another or the endothelial cells of the Haversian ducts. The processes run in fine tubules with a diameter of up to 400 nm, which interrupt the bone matrix and are called bone canaliculi (Terminologia histologica: Canalculi ossis). Around its lumen there is uncalcified matrix that is "nourished" by diffusion. The cytoplasm of the osteocytes has fewer organelles; Little and only exceptionally dilated rough endoplasmic reticulum, small Golgi apparatus, few ribosomes. The cell bodies of the osteocytes lie in about 30 x 10 µm in elongated oval recesses in the lamellae, which are known as osteocyte lacunae (Terminologia histologica: Lacunae oseocytorum; English: osteocyte lacunae).
Osteoclasts (Terminologia histologica: Osteoclasti, English: osteoclasts) belong to the mononuclear (mononuclear) phagocyte system although they are always multinucleated (up to 25 cell nuclei) giant cells (diameter 30-100 µm) because they are made up of fused monocytes that have migrated out of the blood (real mononuclear cells ) emerge. So that these cells become one Syncythium can fuse, they have to bind as osteoclastic progenito cells with their osteoprotegerin receptor to osteoprogeterin ligands of the cell membrane of osteoblasts. In addition, CSF-1 (Colony stimulating factor 1), Interleukin-1, -6, -11, TNF-alpha and ProstaglandinE2 to be available. In the area of ​​the compact bone tissue, osteoclasts eat their way into the hard substance at the tips of the erosion tunnels, while they form erosion clouds (Terminologia histologica: Lacunae erosionis, English: osteoclastic crypts, erosion lacunae) on the trabeculae of the cancellous bone tissue. In both cases they form one towards the bone substance Absorption seam out. This is a brush-like tuft (Terminologia histologica: Limbus microplicatus, English: ruffled border) with short cell processes and / or small, surface-enlarging folds (microplicae). The calcium salts stored there are destroyed by the release of hydrochloric acid in the direction of the mineralized bone and the bone substance is actively broken down. Protons are pumped towards the bone via a hydrogen ions ATPase stored in the cell membrane in the area of ​​the resorption border, while chloride ions also get there through a channel in the cell membrane. At the points where the fold line merges into the "normal" cell membrane, the cells form a chain of localized small contacts to the bone matrix. Your cytoplasm is poor in organelles in the adhesive zones, but rich in actin filaments and SRC kinase. Integrins bind to the actin filaments via talin, alpha-actinin and vinculin, which then bind with their extracellular domains to osteopontin and bone sialoprotein of the bone matrix. Due to the high content of mitochondria, which provide the energy for the enzyme carbonic anhydrase 2, which is important for hydrogen ion production, the cytoplasm of the osteoclasts acidophilic. The numerous Golgi apparatus produce abundant vesicles, most of which are lysosomes, which release their contents via exocytosis in the area of ​​the resorption margin. In this way, tartrate-resistant acid phosphatase, beta-glucuronidase, aryl sulfatase A and cathepsin K reach the lacuna below the absorption margin. Procollagenase, stromelysin-1 and lysozyme are also delivered here from the cells via a non-lysosomal route. With the hydrochloric acid together they bring about that Destruction of the bone matrix. The calcium ions released in the process are absorbed into the osteoclasts via cell membrane channels and transported back out of the cells in the side facing away from the lacuna. The activity of the osteoclasts is increased indirectly by stimulating the neighboring osteoblasts to secrete osteoclast-stimulating factors, since osteoclasts themselves have no parathyroid hormone receptors. Vitamin D (calcitriol) stimulates the formation and activation of osteoclasts. Calcitonin slows it down by inhibiting the calcium channels, whereby the intracellular calcium increases so that it depolymerizes the actin filaments necessary for matrix adhesion. Osteoclasts are identical to the chondroclasts, which break down the cartilage substance, and the odontoclasts which break down the dentin.
Hem cells (Terminologia histologica: Cellulae vestientes osseorum, English: bone lining cells) are the dormant osteoblasts that occur in the endosteum but also in the periosteum, which are connected to one another via gap junctions and form a flat epithelium, as there is practically no intercellular substance between them. These cells have only a small amount of organelle-poor cytoplasm. They protrude with long, thin processes into fine bone tubules, where they are also connected to the processes of neighboring osteocytes via gap junctions. Only when these cells diverge and the bone matrix is ​​exposed can the monocytic precursor cells of the osteoclasts cadherin-6, which have migrated from the blood as macrophages, combine and only combine to form osteoclasts and attack bones through contact with the border cells.
Types of bone tissue:
1. The Braided bone (Terminologia histologica: Textus osseus reticulofibrosus; English: woven bone) is the first form of formed bone and shows itself in the desmal ossification as well as in the formation of the bony callus in the context of a fracture healing. The basis is a tight, disordered, network-like connective tissue structure on which calcium hydroxylapatite and the other typical bone matrix substances have accumulated. Cavities (lacunae) with osteocytes appear disordered. A lamellar order is not discernible. In adults, braided bone can only be found in the massive interior of the Temporal bones (Pars petrosa ossis temporalis) around the structures of the inner ear (snail = cochlea and organ of equilibrium = labyrinth), in the synostoses (ossified bone-bone connections) e.g. between the cranial bones (= skull sutures = Sutures) and a bit in the compact bone tissue in areas where strong ligaments are anchored to bones.
2. the cancellous bone tissue (Terminologia histologica: Textus osseus spongiosus, Textus osseus trabecularis, English: trabecular bone, spongy bone, cancellous bone), which consists of a spongy, perforated framework made of trabeculae is constantly built up, changed and also reduced in accordance with the longer pressure load. With their plate- to columnar branched trabeculae (Trabeculae; Terminologia histologica: Trabeculae osseae, English: bone trabecula), which are on average approx. 0.2 to a maximum of 0.4 mm thick and up to 0.6 mm long, the structure of the cancellous bone ensures the effective Distribution of compressive and tensile forces e.g .: from the bone surface to the side areas where it merges into the pars compacta. In this context one speaks of a trajectoryn trabeculaearchitecture. The cancellous bone makes approx. 20% of all bone tissue. The layer called Diploë between the inner and outer surfaces of most skull bones also consists of cancellous bone tissue. The trabeculae are roughly crescent-shaped in cross-section Trabecular lamellas built up. These layers, which are elongated parallel to the course of the trabecula, show only a few osteocytes located in lacunae and are organized in lamellar packets that run through collagen-free cement lines are separated and fill out former resorption lumps that arise from the constant bone remodeling that takes place here. Individual special lamellae around fine central canals connected to the endost are rarely found here. The mostlye part of the trabeculae has however no blood vessels of their own and is nourished by diffusion from the adjacent endosteum or the bone marrow located in its cavities.
3. The fascicular bone tissue (Terminologia histologica: Textus osseus fasciculatus, English: bundle bone) forms the transition area from the cancellous bone tissue to the lamellar bone and shows larger bundles of still lamellar bone, which then merge into the trabeculae of the cancellous bone, which are almost no longer equipped with Haversian systems.
4. The compact bone tissue (Terminologia histologica: Textus osseus compactus, English: compact bone) lies as a massive bone mass on and directly below the surface of Long bones and also as Substantia compacta on the bone shaft or Cortical substantia at the bone ends. The inner and outer surface of the cranial bones (tabula interna and externa) is also formed by the stable substantia corticalis. In the temporal bone (os temporale), the compact bone tissue surrounds the structures of the inner ear to be protected. Whereby it is only organized here as a braided bone, otherwise it is in the form of
Lamellar bone tissue (Terminologia histologica: Textus osseus lamellaris, English: lamellar bone). This exists made of 2-4 mm thick boneslats (Terminologia histologica: Lamellae osseae, English: bone lamellas). In this case, collagen fibrils running in the same direction (aniostropic) of one lamella usually alternate with collagen fibrils mostly running in opposite directions of the neighboring lamella, which can be seen in the polarization microscope as a pattern of light and dark bands. The lamellae are mostly oriented in the longitudinal direction of the shaft. Viewed from the inside out, there are the following
Layers of the lamellar bone:
A. Endost (Terminologia histologica: Endosteum, English: endosteum)
The endost, the inner one Periosteum covers the inner surface of the hard bone substance including Haversian canals. In adults, its surface is approx. 15.5 m², of which 11 m² is covered by the cancellous bone, 3.5 m² by the Haversian canals and only 1 m² by the pars compacta. The cells of the endost are important for bone repair in fractures. These are a few mesenchymal stem cells, a few osteoprogenitor cells, resting / active osteoblasts or resting / active osteoclasts. The predominantly dormant, epithelial-like appearing osteoblasts are also referred to as endosteal seam cells. The endostrum continues into the Volkmann's canals, which extend into the interior of the compact bone tissue and often into the periosteum. A narrow area of ​​unmineralized matrix (osteoid) follows beneath the endosteum in the direction of the bone.
B. inner general slats (= inner circumferential lamellae; Terminologia histologica: Lamellae cricumferentiales internae, English: internal circumferential lamellae)
These parallel along the direction of the bone oriented giant lamellae show layers of osteocytes with a surrounding mineralized matrix and are somewhat thinner than the outer general lamellae. The osteocytes located here send their thin processes to the endostrum in order to nourish themselves. The inner circumferential lamellae adjoin the
C. Special slats (= Osteon lamellae; Terminologia histologica: Lamellae osteoni, English: osteon concentric lamellae) are around 5 to 30 rounded around one central channel (Haversian Canal; Terminologia histologica: Canalis osteoni, Canalis centralis; English: osteonic canal, central canal) concentrically formed lamellae of calcified bone matrix with layers in lacunae (Terminologia histologica: Lacunae osteocytorum, English: osteocyte lacunae) of osteocytes in between. These are connected to one another via fine cell processes via gap junctions, but they also send out processes vertically in the direction of the central canal and also in the opposite direction. The basic functional unit of the bone is called this Osteon (= Haversian system; engl: Haversian system; Terminologia histologica: Osteonum primarium, English: primary osteon). This consists of the central canal and the surrounding cells as well as the associated lamellae. It has one on average length of 2.5 mm at a diameter of 0.2 mm.
In the central canals, which are connected to one another and to the inside / outside via Volkmann's canals, there is a central capillary or venule with fenestrated endothelial cells, then loose connective tissue with fine dendritic, only exceptionally myelinated nerve endings and rarely free connective tissue cells, followed by the fringing cells of the endosteum 5 - 30 concentric osteocyte layers. The osteons, which are strongly eosinophilic under the light microscope, end with a collagen-free one 1-2 µm thick Cement line (Terminologia histologica: Linea cementalis, English: cement line, reversal line), which stains basophilically. Very thin (> 0.5 µm) cement lines can often be seen between individual lamellae. Lying in betweenSwitching blades (= interstitial lamellae; Terminologia histologica: Lamellae interstitiales, English: interstitial lamellae). These are old, partially resorbed "gap-filling" special lamellae or Remnants of partially eroded generallamellas without Havers channel. They arise as part of bone remodeling. Follow outwards
D. outer general lamellas (= outer circumferential lamellae; Terminologia histologica: Lamellae cricumferentiales externae, English: external circumferential lamellae) are to the bone surface parallel longitudinally orientede huge (a few hundred µm thick) lamellae made of mineralized bone matrix with osteocytes in lacunae that feed on the periosteum.
E. Periosteum (Terminologia histologica: Periosteum, English: periosteum)
The periosteum, the outer periosteum covers the outer surface of the hard bone substance with the exception of 1. attachment points of muscles, tendons and ligaments, 2. joint surfaces covered with hyaline cartilage, 3. bone parts covered by synovial membrane such as the synovial membrane, e.g.on the neck area of ​​the thigh bone (collum femoris). In the periosteum, a distinction is made between the cell-rich layer located directly on the inside of the bone (Terminologia histologica: Osteogenic stratum, English: osteogenic layer). The cells located here are also important in repairing fractures; they resemble those of the endost and are a few mesenchymal stem cells, a few osteoprogenitor cells, dormant / active osteoblasts or dormant / active osteoclasts. Here, too, the resting cells appearing epithelial-like are summarized under the term seam cells. Below these, in the direction of the bone, follows a narrow area of ​​unmineralized matrix as long as there is still growth in thickness. A firm connection between the periosteum and the bone substance is created by the as originating from the bone matrix Sharpey fibers designated collagen fibers (Terminologia histologica: Fasciculi collageni perforantes, English: perforating collagen fiber bundles). These continue through the stratum osteogenicum into the subsequent outer layer of the periosteum, the stratum fibrosum (Terminologia histologica: Stratum fibrosum, English: fibrous layer). The latter consists of tight, braided collagenous connective tissue. The collagen fibers of the tendons and ligaments attached to the bone also pull as Sharpey fibers directly further into the bone matrix, which interrupts the periosteum at these points. Numerous blood vessels run into the periosteum, some of which continue into the bone. The ones used for this are usually roughly perpendicular to the bone surface trendingn channels are saved asVolkmann channels (Terminologia histologica: Canales osseorum, English: bone canals), one can still distinguish between the perforating canals that extend into the endostrum (Terminologia histologica: Canales perforantes, English: perforating canals) and those that extend only a little way into the corticalis of the bone transverse canals (Terminologia histologica: Canales transversi, English: transverse canals). However, unlike the Haversian canals that are attached to them, these canals have NO surrounding lamellar layers. Functionally, all channels that contribute to the nutrition of the bone are still referred to as nutrient channels (Terminologia histologica: Canales nutricii, Canales nutrientes; English: nutrient canals). All of the named channels contain blood vessels but also fine dendritic nerve endings and are lined by seam cells. The pain sensation in bone trauma is mainly due to the large number of free nerve endings that end in the periosteum.

Bone matrix (Terminologia histologica: Matrix ossea, English: bone matrix)
The uncalcified bone matrix becomes Osteoid called and formed by osteoblasts and osteocytes. The proportion of bone mass in the total mass of the body is approx. 5.7% (approx. 4 kg corresponding to ~ 1,700 cm³ at 70 kg body weight). At around 15%, the water content is significantly lower than in other supporting tissues (hyaline cartilage ~ 70%). If the water is removed from the matrix, approx. 30% organic matter and 70% minerals (inorganic matter) remain. The mostly bundled to form collagen fibers (Fibrae collageni; English collagen fibers) Collagen fibrils from Type 1 make up 90% of the organic component. They take care of elasticity and for a high tensile strenght, attached to it crystalline Calcium hydroxylapatite 3 Approx3(PO4)2 x Ca (OH)2 (Terminologia histologica: Crystallum hydroxylapatiti, English: hydroxylapatite crystal), which makes up over 95% of the inorganic mineral substance, is the basis for the high Compressive strength. However, there are also few other incorporated or bound ions, in particular magnesium ions (Mg+2), Fluoride ions (F-) and carbonate ions CO3-, Phosphate ions (PO4)-3 and citrations and, in small quantities, cations of iron, copper, zinc, strontium and lead. Bone is the most important Calcium storage of the organism (about 1 kg for adults). In addition to the poorly soluble hydroxyapatite, calcium is also found as carbonate (CaCO3) and than calcium hydrogen phosphate (CaHPO), which is easier to mobilize for rapid exchange due to its higher water solubility4) in front. The 10% of the organic matrix components remaining after collagen-1 can be further subdivided into special ones Matrix proteins (Terminologia histologica: Proteinae non collagenosae, English: noncollagen proteins), proteoglycans and glycoproteins. The matrix proteins include type 5 collagen (cross-linked type 1 collagen), osteocalzin (inhibits hydroxyapatite deposition on collagen-1), matrix GIA protein (also inhibits mineralization), metalloproteinase (collagenase for collagen dissolution, stromelysin degrades matrix components). To the Proteoglycansn include decorin and biglycan (regulate collagen fibrillogenesis and inhibit mineralization) and osteoadherin (promotes matrix adhesion of osteoblasts and binds to hydroxyapatite). To the Glycoproteinsn include osteonectin (inhibits cell adhesion to the matrix, inhibits hydroxyapatite crystal formation), osteopontin (promotes integrin-mediated cell adhesion), bone sialoprotein (promotes the formation of hydroxyapatite crystals and is necessary in calcifying matrix), fibronectin (promotes cell adhesion to matrix components) and (stimulates cell proliferation by inhibiting adhesion). In addition, serum albumin and a number of less common components are still present in the bone matrix.
The Mineralization of the bone occurs when calcium hydrogen phosphate crystals (CaHPO4) by exceeding the solubility product of the ions involved (approx+2, PO4-3, HPO4-2 and H2PO4- ) arise through various intermediate stages in Calcium hydroxylapatite convert and finally attach to collagen fibers in the osteoid. However, the product of the ions involved in the extracellular fluid is too low for spontaneous crystal formation, so that the osteoblasts have to help. The alkaline phosphatase in your cell membrane and the cell membrane of the osteocytes is concentrated on fine cell processes, the so-called small cytoplasmic vesicles due to the constriction of small cell membrane-covered vesicles Matrix vesicle (Terminologia histologica: Vesicula matricalis; English: matrix vesicle) pinch off.
In their InnerWith the participation of organic calcium-binding proteins (calbindin-D, phosphatidylserine), the ions are sufficiently enriched, which then lead to Crystal formation leads. The growing crystals lead to Burst the membrane vesicle. The now free crystals continue to grow and attach to the 40 nm gaps in collagen fibrils. This is preferably done on the calcification line (Terminologia histologica: Linea calcificationis; English: calcification front) of the osteoid. Osteocalin and Matrix GLA protein prevent excessive mineralization. With vitamin D.3 Deficiency leads to rickets, which is associated with reduced mineralization and the resulting instability of the bones, with compensatory increased osteoid formation with joint and bone deformation.
Bone formation (Terminologia histologica: Osteogenesis, English: osteogenesis)
A. desmal ossification (woven bone formation; Terminologia histologica: Ossificatio membranacea, Ossificatio desmalis; English: Membranous ossification, intermembranous ossification)
Catch at the desmal or membranous ossification osteogenic mesenchyme densification the 6. Embryonicweek to form in bone tissue, in which mesenchymal cells differentiate into osteoblasts and secrete osteoid around them, whereby they increasingly wall themselves in and become osteocytes. Mineralization takes place later. The osteocytes embedded in the lacunae remain in contact with the mesenchymal tissue surrounding the young bones through their growing processes. Countless small ones arise in the area of ​​a bone to be formed primary trabeculae made of braided bone. On the outside of the bone island, osteoblasts are dense epithelial-like and ensure rapid growth through constant osteoid deposition, with some of them being walled in as new osteocytes over and over again. While this appositional growth (= attachment from outside) closes the gaps between the Bone formation islands gradually become smaller and smaller, the deposition of bone lamellae begins on the primary cancellous atrabeculae formed from the bone islands. The number of osteoclasts increases sharply, as a result of which holes are eaten in the primarily formed woven bone. These are then replaced by newly formed lamellar bones (secondary osteons; Terminologia histologica: Osteona secundaria; English: secondary osteons) until finally no braided bone is left. In addition, new ones are created primary osteons (Terminologia histologica: Osteona primaria; English: primary osteons) around blood vessels of the cavities still remaining between the blood vessels. These grow together with the surrounding bone without the formation of cement lines. Many of these are later broken down by osteoclasts and replaced by newly formed secondary osteons. The growth in size of the resulting bones takes place exclusively appositionally, i.e. through the accumulation of new bone substance on the outer surface of the already existing hard substance.
To the Braided bone, which are also known as covering bones or connective tissue bones (Terminologia histologica: Ossa membranacea; English: membranous bones), include the lower jaw bone (Mandible), the collarbone shafts (corpus claviculae) and following Skull bones: Frontal bone (Os frontale), parietal bone (Os parietale), occipital bone (Os occipitale), upper jawbone (maxilla) and the temporal bone (Squama ossis temporalis). All remaining bones are replacement bones. The process of perichondral ossification is also a desmal bone formation.
B. chondral ossification (replacement bone formation, cartilaginous ossification, indirect ossification; Terminologia histologica: ossificatio chondralis; English: chondral ossification, cartilaginous ossification)
All bones that are not woven bones are created in this way and are called endochondral bones (Terminologia histologica: Ossa endochondralia; English: endochondral bones). First, blastemas of hyaline cartilage develop in the area of ​​compacted mesenchymal cells, whereby the mesenchymal cells become chondroblasts, which are transformed into chondocytes that are no longer able to divide through the excretion of the cartilage matrix. Up to this point, interstitial cartilage growth takes place. After that, the cartilage areas to be ossified only increase in size through appositional growth, i.e. through the accumulation of new cells and new matrix from the stratum chondrogenicum of the perichondrium. The resulting hyaline cartilage then changes within the framework of the enchondral ossification (Terminologia histologica: Ossificatio endochondralis; English: endochondral ossification) in bone tissue. The bone to be formed grows in length.
Begins in the diaphyseal area of ​​long bones from the 7. Embryonicweek one on the thighbone (femur) and humerus (humerus) desmal formation of a bone cuff (Terminologia histologica: Anulus osseus perichondralis; English: perichondral bone annulus) from osteogenic cells of the perichondrium. In the process, bipotent progenitor cells become osteoblasts, which attach bone substance to the outside, whereby some are "walled in" and become osteocytes. This process of deposition of bone substance on the perichondrium, which is mainly essential for the growth of bone thickness, is called perichondral ossification (Terminologia histologica: Ossificatio perichondralis; English: perichondral ossification).
Cartilage cells located under the cuff in the center of the cartilaginous bone structure enlarge (hypertrophy) hypertrophic cartilage arises. The basic substance formed by these cells begins to calcify (central foci of calcification). The hypertrophic cartilage cells (Terminologia histologica: Chondrocyti hypertrophici; English: hypertrophic chondrocytes) secrete vascular endothelial growth factor (English: vasular endothelial growth factor, VEGF), causing it to Capillary ingrowth comes through the still thin bone cuff to the calcification center. Osteoclasts, which are identical to chondroclasts, eat their way through bone and cartilage. Mesenchymal cells following the capillaries, which differentiate into osteoblasts and later also into fibroblastic reticulum cells (important for bone marrow formation). In addition, monocytes emerging from the blood fuse here to form further osteoclasts. The osteoblasts form spongy bone matrix in the cavities created by the osteoclasts, whereby a primary ossification center (Terminologia histologica: Centrum ossificationis primarium; Centrum ossificationis diaphysiale; English: primary ossification center, diaphysial ossification center) becomes visible. As long as it is still a small spherical area, one speaks of primary bone nucleus (Terminologia histologica: Gemma osteogenica primaria; English: primary osteogenic nucleus). In just a few days, the woven bone of the primary cancellous bone has filled the entire diaphyseal system and in its interstices, the primary bone marrow cavity (Terminologia histologica: Cavitas medullaris primaria; English: primary medullary cavity) settles blood-forming bone marrow at. Further bone growth from the outside (appositional growth) with resorption inside, the bone thickness gradually increases.
In the area of ​​the epiphyses and metaphyses there is still hyaline cartilage. The growth in length takes place in the metaphysis between the epiphysis and the diaphysis through interstitial growth, i.e. cell division and thus the multiplication of the chondroblasts. This growth area is referred to as the epiphyseal plate, the growing cartilage tissue located here as the epiphyseal cartilage (Terminologia histologica: Cartilago epiphysialis; English: epiphysial cartilage). Only after the birth does the actual bone formation take place in this area enchondral ossification instead of and only with the final completion of body growth in adulthood, complete ossification will also be achieved in this area.
A distinction is made between the following 6 Zones in the area the epiphyseal plate; Terminologia histologica: Centrum ossificationis secundarium, Centrum ossificationis epiphysiale; English: secondary ossification center, epiphysial ossification center) viewed from the epiphysis in the direction of the diaphysis:
1. The Reserve zone (Terminologia histologica: Zona quiescens; English: resting zone, quiescent zone) is located between the epiphysis and the epiphyseal plate and mostly shows individually lying chondocytes / chondroblasts in hyaline cartilage.
2. The Proliferation zone (Terminologia histologica: Zona proliferationis; English: proliferation zone) shows a very high division activity of the chondroblasts and is the essential area for the predominantly longitudinal growth of the bone. The epiphysis and diaphysis are gradually pushed further apart. The cells in the proliferation zone usually appear stacked flat on top of each other, which is why one also speaks of columnar cartilage. One recognizes many parallel cell columns (Terminologia histologica: Columnellae chondrocytorum; English: chondrocyte columns), which are separated by only a little inter-territorial matrix. As Maturation zone (Maturation zone) is sometimes also referred to as the transition area between the proliferation zone and the hypertrophy zone. This is where the first increased glycogen storage in the cytoplasm of the chondocytes can be seen.
3. The Zone of hypertrophy (Terminologia histologica: Zona hypertrophica; English: hypertrophic zone) with very metabolically active hypertrophic cartilage cells (Terminologia histologica: Chondrocyti hypertrophici; English: hypertrophic chondrocytes), which increase in thickness and increasingly store glycogen. The cells release PDGF (platelet derived growth factor), which promotes the immigration and growth of osteoblasts. They produce collagen 10, which forms a fine network around the cells, and matrix GIA protein which counteracts excessive calcification. Due to the fixation, the hypertrophic cartilage cells often appear blistered in light microscopic specimens, which is why they are also called Bladder cartilage cells (Terminologia histologica: Chondrocyti hypertrophici; English: hypertrophic chondrocytes).
4. The Calcification zone (Terminologia histologica: Zona calcificationis; English: calcification zone) shows one Calcium deposits in the interterritorialen matrix between the cell columns, so that here calcified cartilage (Terminologia histologica: Cartilago calcificata; English: calcified cartilage) arises. There is no calcium deposit between the lined up cells themselves, but the dissolution of the matrix.
5. The Resorption zone (Opening zone; Terminologia histologica: Zona erosionis; English: erosion zone) shows the result of this, the chondocytes now lie directly next to each other in parallel tunnel-shaped cavities (Terminologia histologica: Cavitates cartilaginis; English: cartilaginous lacunae), the walls of which are calcified cartilage septa (Terminologia histologica: Trabeculae cartilagineae; English: cartilaginous trabeculae) are. The chondocytes show the first signs of programmed cell death (apoptosis).
6. The Ossification zone (Terminologia histologica: Zona ossificationis; English: ossification zone) begins where the last Chondocytes by Apoptosis have died. The remaining elongated cavities are called erosion lacunae (Terminologia histologica: Lacunae erosionis; English: erosion lacunae). In part, they are expanded somewhat by chondroclasts that come here from below from the primary bone marrow cavity. The remaining calcified cartilage septa are soon covered with a thin layer of braided bone by osteoblasts that migrate from the bone marrow cavity with capillaries. This is now referred to as primary trabeculae (Terminologia histologica: Trabeculae osseae primariae; English: primary bone trabeculae). With a light microscope, in addition to the basophilic calcified cartilage matrix, thin acidophilic areas of young bone can be seen in the ossification zone, which are called endochondral bone (Terminologia histologica: Textus osseus endochondralis; English: endochondral bone). A little further down towards the bone marrow cavity there are many osteoclasts that dissolve the ends of the trabeculae just mentioned and thus create space for the steadily growing marrow cavity. Only on pageIn the area below the epiphyseal plate, primary osteons are formed along capillaries from the medullary cavity, since here the corticalis of the diaphysis has to be continued in the sense of the increase in bone length. These new osteons can later become too secondaryn Bone trabeculan (Terminologia histologica: Trabeculae osseae secundariae; English: secondary bone trabeculae).
secondary ossification: Before they finally ossify, they show up in the epiphyseal plate secondary bone nuclei (Terminologia histologica: Gemmae osteogenicae secundariae; English: secondary osteogenic nuclei), the remaining chondocytes hypertrophy into bladder cartilage cells, form calcified cartilage (Terminologia histologica: Cartilago calcificata, English: calcified cartilage) and then finally die off. Immigrating chondroclasts clear away the remaining cartilage tissue and accompanying osteoblasts ultimately form bones in the area now known as the epiphyseal line (Linea epiphysialis). Only in the region near the joint does the epiphyseal cartilage remain as hyaline articular cartilage throughout life.
After birth they already show up in some bones in the central area Epiphysessecondary ossification centers (Terminologia histologica: Centrae ossificationes secundariae, Centrae ossificationes epiphysialiae; English: secondary ossification centers, epiphysial ossification centers). They are first found in the distal femoral and proximal tibial epiphysis (lower femur and upper tibial piphysis). Based on the occurrence of the detectable in x-rays Bone nuclei you can do that relatively exactly Age of a child determine, since this takes place according to a genetically generally prescribed rigid schedule.
Flat, short and irregular bones also ossify in principle according to the scheme described here, sometimes there are cartilage plates analogous to the epiphyseal plates for longitudinal growth while latitudinal growth always occurs appositionally.
Bone remodeling
Since bones are alive, the mechanical requirements are followed here constant renovation processes instead of. In the cancellous bone, around 28% of the matrix is ​​renewed in the compacta around 4% of the matrix per year. Usually there is a balance between assembly and dismantling. If degradation processes predominate, osteoporosis occurs, whereas osteopetrosis predominates. When osteoclasts undergo bone remodeling at a rate of up to 40 µm per day If they grow into the hard substance of the bone, they eat cavities either as resorption lacunae (Howship lacunae; Terminologia histologica: Cavitates resorptionis; English: resorption cavities) in cancellous bone or erosion canals (Terminologia histologica: Canales erosiones, English: erosion canals) in compact bone tissue. The round shape of the bowl-shaped, 60 µm deep and 100 µm wide Absorption lacunae is the reason for the curved lamellar structure of the trabeculae in the cancellous bone. In the compact bone tissue they have Erosion channels 200 µm in diameter and several millimeters long. A cone-shaped one forms at its tip Erosion line (Terminologia histologica: Linea erosionis, Linea resorptionis; English: erosion front) from adjacent osteoclasts, followed by a capillary on the inside, which is accompanied by sparse connective tissue, and one on the outside monocytic reversal zone with macrophages created from monocytes, which remove the decalcified matrix. The subsequent osteoblastic occlusion zone shows a large number of osteoblasts, which close the defect again by forming new bone lamellae. If after several weeks the osteoclasts at the tip of the tunnel stop their activity (probably when you come across connective tissue located in Haversian or Volkmann canals) the defect closes, the osteoblasts, which were not walled in during the course of the renewal, become seam cells and the new special lamella is finished.
Fracture healing
In the healing of bone fractures in the context of injuries, the dormant seam cells in the osteogenic stratum are stimulated by the terminal and periosteum and proliferate. A primary fracture healing is possible if the periosteum does not tear and the fracture gap is less than 1 mm wide. Contact healing then takes place during which special lamellae grow over the gap from both sides and the bone is restored to function after approx. 3 weeks. Fracture gaps around 1 mm lead to the healing of the gap, with very little capillary-rich connective tissue growing into the gap, nearby osteoprogenitor cells and seam cells are activated and an ossification coat soon forms around the capillaries. Mostly, however, it comes to secondary fracture healing in which larger fracture cracks occur with dislocation of the fracture ends. Bleeding and hematoma formation occur. Osteoclasts settle at the bone ends, which die due to a lack of blood supply, which clear the matrix until they encounter "surviving cells. Fibroblasts, capillaries and osteogenic cells migrate into the hematoma, which is cleared away by macrophages. In approx. 1 week Connective tissue callus formed from fibrous scar tissue. Since the progenitor cells also quickly differentiate, chondroblasts are formed in addition to osteoblasts and initially an osseous-hyaline cartilage replacement tissue is formed provisional callus. Over the course of weeks to months, this then completely changes into lamellar bones (Lamellar callus), if no shifting movements occur at the breakage due to immobilization. If there is insufficient immobilization, the cartilage tissue remains intact and stability is not achieved in the long term.

-> hyaline, elastic cartilage, connective tissue, local connective tissue cells, bone marrow
-> Electron microscopic atlas general overview
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Most of the pictures were made available by Prof. H. Wartenberg; Other images, page & copyright H. Jastrow.