Bone tissue is the most important tissue in our body. It performs many functions. Bone tissue in histology is classified as a type of skeletal connective tissue, which also includes cartilage. Cells of skeletal connective tissue, including bone, develop from mesenchyme.
Skeletal connective tissue
Skeletal connective tissue performs many functions:
- Bones are the support of the whole organism. The skeleton allows a person, consisting entirely of soft tissues, to feel confident in space.
- Thanks to the skeleton we can move. Muscles are attached to the bones, which, in turn, form leverage, allowing you to perform any action.
- Depot of many minerals is located in the bone tissue. Bone tissue is involved in the metabolism of phosphates and calcium.
- In the bones, namely in the red bone marrow, hematopoiesis occurs.
The functions of bone tissue in histology are defined as coinciding with the functions of all skeletal connective tissues, however, this tissue has a number of unique properties.
The main feature and difference of bone tissue from other connective tissue is a high content of mineral substances in it, which is 70%. This explains the strength of the bones, because the intercellular substance of the bone connective tissue is in a solid state.
Bone tissue. The chemical composition of bone tissue
Bone tissue should be started by studying its chemical composition. This will allow us to understand its special properties. The content of organic substances in the tissue is from 10 to 20%. Water contains from 6% to 20%, minerals, as mentioned above, most - up to 70%. The main elements of the bone mineral are calcium phosphate and hydroxyapatites. Also high in mineral salts.
The combination of organic and inorganic substances of bone tissue explains the strength, elasticity of bones, their ability to withstand heavy loads. At the same time, too high a content of minerals gives bones significant fragility.
The intercellular substance is formed by 95% type I collagen. Organic matter accumulates on protein fibers. Phosphoproteins contribute to the accumulation of calcium ions in bones. Proteoglycans promote the binding of collagen to mineral compounds, the formation of which, in turn, is helped by alkaline phosphatase and osteonectin, which stimulates the further growth of crystals of inorganic compounds.
Cell components
In histology, bone cells are divided into three types: osteoblasts, osteocytes and osteoclasts. Cellular components interact with each other, forming an integrated system.
Osteoblasts
Osteoblasts are cells of a cubic, oval shape with an eccentrically located nucleus. The size of such cells is approximately 15-20 microns. Organelles are well developed, granular EPS and Golgi complex are expressed, which can explain the active synthesis of exported proteins. In histology, on the preparation of bone tissue, the cytoplasm of cells is stained basophilically.
Osteoblasts are localized on the surface of bone beams in the resulting bone, where they remain in mature bones in the spongy substance. In the formed bones, osteoblasts can be found in the periosteum, in the endosteum covering the medullary canal, in the perivascular space of osteons.
Osteoblasts are involved in osteogenesis. Due to the active synthesis and export of proteins, a bone matrix is formed. Due to alkaline phosphatase, which is active in the cell, there is an accumulation of minerals. Do not forget that osteoblasts are the precursors of osteocytes. Osteoblasts secrete matrix vesicles, the contents of which trigger the formation of crystals from minerals in the bone matrix.
Osteoblasts are divided into active and resting. Active are involved in osteogenesis and produce matrix components. Resting osteoblasts with an endostatic membrane protect bone material from osteoclasts. Resting osteoblasts can be activated during bone remodeling.
Osteocytes
Osteocytes are mature, well-differentiated cells of bone tissue, located one at a time in gaps, also called bone cavities. Cells are oval in shape with numerous processes. The size of osteocytes is approximately 30 microns in length and up to 12 in width. The core is elongated, located in the center. Chromatin is condensed, forming large lumps. Organelles are poorly developed, which may explain the low synthetic activity of osteocytes. The cells are connected to each other by processes through the cellular contacts of the nexus, forming syncytium. According to the processes, there is an exchange of substances between the bone tissue and blood vessels.
Osteoclasts
Osteoclasts, unlike osteoblasts and osteocytes, come from blood cells. Osteocytes are formed during the fusion of several promonocytes, so some authors do not consider them to be cells and are considered symplasts.
By structure, osteoclasts are large, slightly elongated cells. Cell size can vary from 60 to 100 microns. The cytoplasm can be stained both oxyphilic and basophilic, it all depends on the age of the cells.
In the cell, several zones can be distinguished:
- Basal, containing the main organelles and nuclei.
- Corrugated border of microvilli penetrating the bone.
- The vesicular zone, which contains bone-destroying enzymes.
- Bright area of adhesion, contributing to the fixation of the cell.
- Resorption area
Osteoclasts destroy bone tissue, are involved in bone remodeling. The destruction of bone substance, or, in other words, resorption, is an important stage of reconstruction, followed by the formation of a new substance with the help of osteoblasts. The localization of osteoclasts coincides with the location of osteoblasts in depressions on the surfaces of bone beams, in the endosteum and periosteum.
Periosteum
The periosteum consists of osteoblasts, osteoclasts and osteogenic cells, which are involved in bone growth and repair. The periosteum is rich in blood vessels, the branches of which surround the bone, penetrating into its substance.
In histology, the classification of bone tissue is not very extensive. Fabrics are divided into coarse-fiber and lamellar.
Coarse bone
Coarse bone tissue is found mainly in the baby before birth. In an adult, she remains in the sutures of the skull, in the dental alveoli, in the inner ear, at the points of attachment of the tendons to the bones. Coarse fibrous bone in histology is determined by the predecessor lamellar.
The fabric consists of randomly arranged thick bundles of collagen fibers, which are located in a matrix consisting of inorganic substances. In the intercellular substance there are also blood vessels, which are poorly developed. Osteocytes are located in the intercellular substance in the systems of gaps and channels.
Plate bone tissue
All bones of the adult body, with the exception of the tendon attachment sites and areas of cranial sutures, consist of lamellar bone connective tissue.
Unlike coarse fibrous bone, all lamellar components are structured and form bone plates. Collagen fibers within the same plate have one direction.
There are two varieties of lamellar bone tissue in histology - spongy and compact.
Spongy substance
In a spongy substance, the plates are combined into trabeculae, the structural units of the substance. Arcuate plates lie parallel to each other, forming avascular bone beams. The plates are oriented along the direction of the trabecula themselves.
Trabeculae are connected to each other at different angles, forming a three-dimensional structure. Between the bone beams are located bone cells, which makes this substance porous, explaining the name of the tissue. The cells contain red bone marrow and blood vessels that feed the bone.
The spongy substance is located in the inner part of the flat and cancellous bones, in the epiphyses and inner layers of the tubular diaphysis.
Compact bone substance
The histology of lamellar bone tissue should be well studied, because it is this kind of bone tissue that is the most complex and contains many different elements.
Bone plates in a compact substance are located on a circle, they are embedded in each other, forming a dense stack where there are practically no gaps. The structural unit is osteon formed by bone plates. The plates can be divided into several types.
- Outside general records. They are located directly under the periosteum, encircling the entire bone. In spongy and flat bones, a compact substance can only be expressed by such plates.
- Osteon plates. This type of plate forms osteons, concentric plates lying around the vessels. Osteon is the main element of the compact substance of the diaphysis in the tubular bones.
- Insert plates, which are the remnants of collapsing plates.
- Internal general plates surround the bone marrow canal with yellow bone marrow.
The compact substance is localized in the surface layer of the flat and cancellous bones, in the diaphysis and in the surface layers of the pineal gland epiphysis.
The bone is covered with a periosteum containing cambial cells, due to which the bone grows in thickness. The periosteum also contains osteoblasts and osteoclasts.
Under the periosteum is a layer of external general lamina.
In the very center of the tubular bone is a bone marrow cavity covered with endostomy. The endosteum is covered with internal general plates, enclosing it in a ring. Trabeculae of the spongy substance may adjoin the bone marrow cavity, therefore, in some places, the plates may become less pronounced.
Between the outer and inner layers of the general plates is the osteon layer of the bone. In the center of each osteon is the Havers Canal with a blood vessel. Haversian channels communicate with each other through the Volkman transverse channels. The space between the plates and the vessel is called perivascular, the vessel is covered with loose connective tissue, and the perivascular space contains cells similar to the cells of the periosteum. The channel is surrounded by layers of osteon plates. In turn, the osteons are separated from each other by a resorption line, which is often called a cleavage. There are also insertion plates between the osteons, which are the residual material of the osteons.
Between the plates of osteon are bone lacunae with osteocytes enclosed in them. The processes of osteocytes form tubules along which, perpendicular to the plates, the transport of nutrients to the bone occurs.
Collagen fibers allow you to see the bone channels and cavities under the microscope, because the areas lined with collagen are stained with brown.
In histology on a preparation, lamellar bone tissue is stained according to Schmorl.
Osteogenesis
Osteogenesis is direct and indirect. Direct development is carried out from mesenchyme, from connective tissue cells. Indirect - from cartilage cells. In histology, direct osteogenesis of bone tissue is considered before indirect, because it is a simpler and more ancient mechanism.
Direct osteogenesis
From the connective tissue, the bones of the skull, small bones of the hand and other flat bones develop. Four stages can be distinguished in bone formation in this way.
- The formation of a skeletal germ. In the first month, stromal stem cells enter the mesenchyme from somites. There is a multiplication of cells, enrichment of the tissue with blood vessels. Under the influence of growth factors, cells form clusters of up to 50 pieces. Cells secrete proteins, multiply and grow. In the stromal stem cells, the process of differentiation starts, they turn into osteogenic progenitor cells.
- Osteoid stage. In osteogenic cells, protein synthesis and glycogen accumulation occur, organelles become larger, they function more actively. Osteogenic cells synthesize collagen and other proteins, such as bone morphogenetic protein. Over time, cells begin to multiply less often and differentiate into osteoblasts. Osteoblasts are involved in the formation of intercellular substance, which is poor in minerals and rich in organic matter, osteoid. It is at this stage that osteocytes and osteoclasts appear.
- Osteoid mineralization. Osteoblasts are also involved in this process. Alkaline phosphatase begins to work in them, the activity of which contributes to the accumulation of mineral substances. Matrix vesicles filled with protein osteocalcin and calcium phosphate appear in the cytoplasm. Minerals stick to collagen thanks to osteocalcin. The trabeculae increase and, connecting with each other, form a network, where the mesenchyme and blood vessels still remain. The resulting tissue is called primary membranous tissue. Bone tissue is coarse fibrous, forms the primary spongy bone. In this stage, the periosteum is formed from the mesenchyme. Near the blood vessels of the periosteum, cells appear, which will then participate in the growth and regeneration of the bone.
- The formation of bone plates. At this stage, primary membranous bone tissue is replaced by lamellar. Osteons begin to fill the gaps between the trabeculae. Osteoclasts, which form cavities in it, enter the bone from blood vessels. It is osteoclasts that create a cavity for bone marrow, affect the shape of the bone.
Indirect osteogenesis
Indirect osteogenesis occurs with the development of tubular and spongy bones. To understand all the mechanisms of osteogenesis, one needs to be well versed in the histology of cartilage and bone connective tissue.
The whole process can be divided into three stages:
- The formation of a cartilaginous model. In the diaphysis, chondrocytes lack nutrients and become blistering. The prominent matrix vesicles lead to calcification of the cartilage tissue. In histology, cartilage and bone tissue are interconnected. They begin to replace each other. The perichondrium becomes the periosteum. Chondrogenic cells become osteogenic, which, in turn, become osteoblasts.
- Primary cancellous bone formation. In place of the cartilaginous model, coarse fibrous connective tissue appears. Also, a perichondral bone ring is formed, a bone cuff, where osteoblasts form trabeculae directly in the place of the diaphysis. Due to the appearance of a bone cuff, cartilage nutrition becomes impossible, and chondrocytes begin to die. Cartilage and bone tissue in histology are very interconnected. Following the death of chondrocytes, osteoclasts form channels from the periphery of the bone to the depth of the diaphysis, along which the osteoblasts, osteogenic cells and blood vessels move. Enchondral ossification begins, eventually turning into epiphyseal.
- Fabric remodeling. Primary coarse fibrous tissue gradually passes into lamellar.
Bone growth and development
Bone growth in humans goes up to 20 years. The bone grows in width due to the periosteum, in length due to the metaepiphyseal growth plate. In the metaepiphyseal plate, one can distinguish the zone of resting cartilage, the columnar cartilage zone, the cystic cartilage zone and the calcified cartilage zone.
Many factors affect bone growth and development. These may be environmental factors, environmental factors, deficiency or excess of certain substances.
Growth is accompanied by the resorption of old tissue and the replacement of its new young. In childhood, bones grow very actively.
Many hormones influence bone growth. For example, somatotropin stimulates bone growth, but with its excess acromegaly may occur, with a lack of dwarfism. Insulin is necessary for the proper development of osteogenic and stem stromal cells. Sex hormones also affect bone growth. Their increased content at an early age can lead to shortening of bones due to early ossification of the metaepiphyseal plate. Their low content in adulthood can lead to osteoporosis, increase bone fragility. The thyroid hormone calcitonin leads to the activation of osteoblasts, parathyrin increases the number of osteoclasts. Thyroxine affects the centers of ossification, hormones of the adrenal glands - on the processes of regeneration.
Some vitamins also affect bone growth. Vitamin C promotes collagen synthesis. , . A , , . D , . , .
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