What functions does bone tissue perform? The structure of bone tissue

Bone tissue is a type of connective tissue and consists of cells and intercellular substance, which contains a large amount of mineral salts, mainly calcium phosphate. Minerals make up 70% of bone tissue, organic substances - 30%.

Functions of bone tissue:

Support;

Mechanical;

Protective;

Participation in the mineral metabolism of the body - a depot of calcium and phosphorus.

Classification of bone tissue

There are two types of bone tissue:

Reticulofibrous (coarse fibrous);

Lamellar (parallel fibrous).

In reticulofibrous bone tissue, the bundles of collagen fibers are thick, tortuous, and arranged in a disorderly manner. In the mineralized intercellular substance, osteocytes are randomly located in the lacunae. Lamellar bone tissue consists of bone plates in which collagen fibers or their bundles are located parallel in each plate, but at right angles to the course of the fibers in adjacent plates. Osteocytes are located between the plates in the lacunae, while their processes pass through the plates in the tubules.

In the human body, bone tissue is presented almost exclusively in the lamellar form. Reticulofibrous bone tissue occurs only as a stage in the development of some bones (parietal, frontal). In adults, they are located in the area of attachment of tendons to bones, as well as at the site of ossified sutures of the skull (sagittal suture of the squama of the frontal bone).

Bone cells: osteoblasts, osteocytes, osteoclasts. The main cells in formed bone tissue are osteocytes. These are process-shaped cells with a large nucleus and weak cytoplasm (nuclear type cells). Cell bodies are localized in bone cavities - lacunae, and processes - in bone tubules. Numerous bone tubules, anastomosing with each other, penetrate the entire bone tissue, communicating with the perivascular spaces, and form the bone tissue drainage system. This drainage system contains tissue fluid, through which the exchange of substances is ensured not only between cells and tissue fluid, but also with intercellular substance. The ultrastructural organization of osteocytes is characterized by the presence in the cytoplasm of a weakly defined granular endoplasmic reticulum, a small number of mitochondria and lysosomes, and no centrioles. Heterochromatin predominates in the nucleus. All these data indicate that osteocytes have insignificant functional activity, which consists in maintaining metabolism between cells and the intercellular substance. Osteocytes are the definitive cell form and do not divide. They are formed from osteoblasts.

Osteoblasts are found only in developing bone tissue. They are absent in formed bone tissue, but are usually contained in an inactive form in the periosteum. In developing bone tissue, they cover the periphery of each bone plate, tightly adjacent to each other, forming a kind of epithelial layer. The shape of such actively functioning cells can be cubic, prismatic, or angular. The cytoplasm of osteoblasts contains a well-developed granular endoplasmic reticulum and a lamellar Golgi complex, and many mitochondria. This ultrastructural organization indicates that these cells are synthesizing and secreting. Indeed, osteoblasts synthesize collagen protein and glycosaminoglycans, which are then released into the intercellular space. Due to these components, the organic matrix of bone tissue is formed. Then these same cells provide mineralization of the intercellular substance by secreting calcium salts. Gradually, releasing intercellular substance, they become walled up and turn into osteocytes. In this case, intracellular organelles are significantly reduced, synthetic and secretory activity is reduced, and the functional activity characteristic of osteocytes is preserved. Osteoblasts, localized in the cambial layer of the periosteum, are in an inactive state, synthetic and transport organelles are poorly developed. When these cells are irritated (in the case of injuries, bone fractures, and so on), a granular endoplasmic reticulum and lamellar complex quickly develop in the cytoplasm, active synthesis and release of collagen and glycosaminoglycans occurs, the formation of an organic matrix (bone callus), and then the formation of definitive bone tissue . In this way, due to the activity of osteoblasts of the periosteum, bone regeneration occurs when they are damaged.

Oteoclasts are bone-destructive cells and are absent in formed bone tissue. But they are contained in the periosteum and in places of destruction and restructuring of bone tissue. Since local processes of bone tissue restructuring are continuously carried out during ontogenesis, osteoclasts are necessarily present in these places. During the process of embryonic osteohistogenesis, these cells play an important role and are found in large numbers. Osteoclasts have a characteristic morphology: firstly, these cells are multinucleated (3-5 or more nuclei), secondly, they are quite large cells (about 90 microns in diameter), thirdly, they have a characteristic shape - the cell is oval in shape , but the part of it adjacent to the bone tissue is flat. In this case, two zones are distinguished in the flat part:

The central part is corrugated and contains numerous folds and islands;

The peripheral (transparent) part is in close contact with the bone tissue.

In the cytoplasm of the cell, under the nuclei, there are numerous lysosomes and vacuoles of different sizes. The functional activity of the osteoclast is manifested as follows: in the central (corrugated) zone of the cell base, carbonic acid and proteolytic enzymes are released from the cytoplasm. The released carbonic acid causes demineralization of bone tissue, and proteolytic enzymes destroy the organic matrix of the intercellular substance. Fragments of collagen fibers are phagocytosed by osteoclasts and destroyed intracellularly. Through these mechanisms, resorption (destruction) of bone tissue occurs and therefore osteoclasts are usually localized in the recesses of bone tissue. After the destruction of bone tissue, due to the activity of osteoblasts moving out of the connective tissue of blood vessels, new bone tissue is built.

The intercellular substance of bone tissue consists of the main substance and fibers, which contain calcium salts. The fibers consist of type I collagen and are folded into bundles, which can be arranged in parallel (ordered) or disordered, on the basis of which the histological classification of bone tissue is based. The main substance of bone tissue, like other types of connective tissues, consists of glycosaminoglycans and proteoglycans, but the chemical composition of these substances differs. In particular, bone tissue contains less chondroitinsulfuric acids, but more citric and other acids that form complexes with calcium salts. In the process of bone tissue development, an organic matrix substance and collagen (ossein, type II collagen) fibers are first formed, and then calcium salts (mainly phosphates) are deposited in them. Calcium salts form crystals of hydroxyapatite, deposited both in the amorphous substance and in the fibers, but a small part of the salts is deposited amorphously. Providing bone strength, calcium phosphate salts are also a depot of calcium and phosphorus in the body. Therefore, bone tissue takes part in mineral metabolism.

When studying bone tissue, the concepts of bone tissue and bone should be differentiated.

3. Bone is an anatomical organ, the main structural component of which is bone tissue. Bone as an organ consists of the following elements:

Bone;

Periosteum;

Bone marrow (red, yellow);

Vessels and nerves.

The periosteum (periosteum) surrounds the bone tissue along the periphery (with the exception of the articular surfaces) and has a structure similar to the perichondrium. The periosteum is divided into outer fibrous and inner cellular or cambial layers. The inner layer contains osteoblasts and osteoclasts. A pronounced vascular network is localized in the periosteum, from which small vessels penetrate into the bone tissue through perforating channels. Red bone marrow is considered as an independent organ and belongs to the organs of hematopoiesis and immunogenesis.

Bone tissue in formed bones is represented only in a lamellar form, but in different bones, in different parts of the same bone, it has a different structure. In flat bones and epiphyses of tubular bones, bone plates form crossbars (trabeculae) that make up the spongy bone. In the diaphyses of tubular bones, the plates are adjacent to each other and form a compact substance. However, even in compact matter, some plates form osteons, while other plates are common.

The structure of the diaphysis of the tubular bone

On a cross section of the diaphysis of the tubular bone, the following layers are distinguished:

Periosteum (periosteum);

Outer layer of common or general plates;

Osteon layer;

Inner layer of common or general plates;

Internal fibrous lamina endosteum.

The outer common plates are located under the periosteum in several layers, but do not form complete rings. Osteocytes are located between the plates in the lacunae. Perforating channels pass through the outer plates, through which perforating fibers and vessels penetrate from the periosteum into the bone tissue. With the help of perforating vessels, trophism is ensured in bone tissue, and perforating fibers connect the periosteum with bone tissue.

The osteon layer consists of two components: osteons and intercalary plates between them. Osteon is a structural unit of the compact substance of tubular bone. Each osteon consists of:

5-20 concentrically layered plates;

The osteon channel, in which vessels pass (arterioles, capillaries, venules).

There are anastomoses between the canals of adjacent osteons. Osteons make up the bulk of the bone tissue of the diaphysis of the tubular bone. They are located longitudinally along the tubular bone, according to the lines of force and gravity, and provide a supporting function. When the direction of the force lines changes as a result of a fracture or curvature of bones, non-load-bearing osteons are destroyed by osteoclasts. However, such osteons are not completely destroyed, but part of the bone plates of the osteon along its length are preserved, and such remaining parts of the osteons are called intercalated plates. During postnatal ontogenesis, bone tissue is constantly restructured - some osteons are destroyed (resorbed), others are formed, and therefore there are always intercalary plates between the osteons, as remnants of previous osteons.

The inner layer of the common plates has a structure similar to the outer one, but it is less pronounced, and in the area of the transition of the diaphysis into the epiphyses, the common plates continue into trabeculae.

Endosteum is a thin connective tissue plate lining the cavity of the diaphysis canal. The layers in the endosteum are not clearly defined, but among the cellular elements there are osteoblasts and osteoclasts.

Bone- a type of connective tissue from which bones are built - the organs that make up the bony skeleton of the human body. Bone tissue consists of interacting structures: bone cells, the intercellular organic matrix of bone (the organic skeleton of bone) and the main mineralized intercellular substance. Cells occupy only ≈1-5% of the total volume of bone tissue of the adult skeleton. There are four types of bone tissue cells.

Osteoblasts- germ cells that perform the function of creating bone. They are located in areas of bone formation on the external and internal surfaces of the bone.

Osteoclasts- cells that perform the function of resorption and destruction of bone. The joint function of osteoblasts and osteoclasts underlies the continuous, controlled process of bone destruction and reconstruction. This process of bone tissue remodeling underlies the body’s adaptation to various physical loads by selecting the best combinations of stiffness, firmness and elasticity of the bones and skeleton.

Osteocytes- cells derived from osteoblasts. They are completely immured in the intercellular substance and are in contact with each other with their processes. Osteocytes provide metabolism (proteins, carbohydrates, fats, water, minerals) of bone tissue. Undifferentiated mesenchymal bone cells (osteogenic cells, contour cells). They are found mainly on the outer surface of the bone (at the periosteum) and on the surfaces of the internal spaces of the bone. From them new osteoblasts and osteoclasts are formed.

Intercellular substance represented by an organic intercellular matrix built from collagen (ossein) fibers (≈90-95%) and basic mineralized substance (≈5-10%).

Collagen The extracellular matrix of bone tissue differs from collagen in other tissues in its high content of specific polypeptides. Collagen fibers are mainly located parallel to the direction of the level of the most likely mechanical loads on the bone and provide elasticity and elasticity to the bone.

Main substance(the ground substance) consists mainly of extracellular fluid, glycoproteins and proteoglycans (chondroitin sulfates, hyaluronic acid). The function of these substances is not yet entirely clear, but it is certain that they are involved in controlling the mineralization of the main substance - the movement of the mineral components of bone.

Minerals, located as part of the main substance in the organic matrix of bone, are represented by crystals built mainly from calcium and phosphorus (hydroxyapatite Ca10(PO4)6(OH)2). The normal calcium/phosphorus ratio is ≈1.3-2.0. In addition, magnesium, sodium, potassium, sulfate, carbonate, hydroxyl and other ions were found in the bone, which may take part in the formation of crystals. Each collagen fiber of compact bone is constructed from periodically repeating segments. The length of the fiber segment is ≈64 nm (64.10-10 m). Each fiber segment is adjacent to hydroxyapatite crystals, tightly encircling it.

In addition, segments of adjacent collagen fibers overlap each other. Accordingly, like bricks when laying a wall, hydroxyapatite crystals overlap each other. This close fit of collagen fibers and hydroxyapatite crystals, as well as their overlap, prevents “shear failure” of the bone under mechanical loads. Collagen fibers provide the bone's elasticity, its resilience, its resistance to stretching, while crystals provide its strength, its rigidity, its resistance to compression. Bone mineralization is associated with the characteristics of bone tissue glycoproteins and the activity of osteoblasts. There are coarse-fiber and lamellar bone tissue. In coarse-fibered bone tissue (predominant in embryos; in adult organisms observed only in the area of cranial sutures and tendon attachments), the fibers run in a disordered manner. In lamellar bone tissue (the bone of adult organisms), the fibers, grouped into individual plates, are strictly oriented and form structural units called osteons.

Note in the body:

From 208 to 214 individual bones.
Native bone is composed of 50% inorganic material, 25% organic matter, and 25% water associated with collagen and proteoglycans.
90% of organic matter is collagen type 1 and only 10% is other organic molecules (glycoprotein osteocalcin, osteonectin, osteopontin, bone sialoprotein and other proteoglycans).
Bone components are represented by: organic matrix - 20-40%, inorganic minerals - 50-70%, cellular elements 5-10% and fats - 3%.
Macroscopically, the skeleton consists of two components - compact or cortical bone; and reticular or spongy bone.
The average weight of the skeleton is 5 kg (weight greatly depends on age, gender, body structure and height).
In an adult body, the cortical bone accounts for 4 kg, i.e. 80% (in the skeletal system), while cancellous bone makes up 20% and weighs on average 1 kg.
The entire volume of skeletal mass in an adult is approximately 0.0014 m³ (1,400,000 mm³) or 1,400 cm³ (1.4 liters).
The surface of the bone is represented by periosteal and endosteal surfaces - a total of about 11.5 m² (11,500,000 mm²).
The periosteal surface covers the entire outer perimeter of the bone and constitutes 4.4% of roughly 0.5 m² (500,000 mm²) of the entire bone surface.
The inner (endosteal) surface consists of three components
1. intracortical surface (surface of the Haversian canals), which is 30.4% or roughly 3.5 m² (3,500,000 mm²);
2. the surface of the inner side of the cortical bone is about 4.4% or roughly 0.5 m² (500,000 mm²) and
3. surface of the trabecular component of cancellous bone 60.8% or roughly 7 m² (7,000,000 mm²).
Spongy bone 1 gr. on average has a surface area of 70 cm² (70,000 cm²: 1000 g), while cortical bone is 1 g. has about 11.25 cm² [(0.5+3.5+0.5) x 10000 cm²: 4000 g], i.e. 6 times less. According to other authors, this ratio may be 10 to 1.
Typically, during normal metabolism, 0.6% of the cortical and 1.2% of the cancellous bone surface undergoes destruction (resorption) and, accordingly, 3% of the cortical and 6% of the cancellous bone surface are involved in the formation of new bone tissue. The remaining bone tissue (more than 93% of its surface) is in a state of rest or rest.

They form the bony skeleton of the human body. Bone tissue is characterized by a very high degree of mineralization (70%), mainly due to calcium phosphate. The intercellular substance is represented mainly by collagen fibers; the main adhesive substance is very small. Collagen proteins predominate among organic substances.

The following types of bone tissue are distinguished:

- coarse fibrous or reticular fibrous textile. This tissue is present during embryogenesis. In adults, the sutures of the flat bones of the skull are built from it;

- lamellar bone tissue.

The cellular composition of these two types of tissues is the same. Eat osteoblasts- cells that form bone tissue. They are large, round or cubic in shape, with a well-developed protein synthesizing apparatus that produces collagen fibers. There are many of these cells in the growing body and during bone regeneration. Osteoblasts turn into osteocytes. They have a small oval body and long thin processes, which are located in the bone tubules and anastomose with each other. These cells do not divide and do not produce intercellular substance.

Osteoclasts- very large cells. They come from blood monocytes, are macrophages of bone tissue, multinucleated, they have a well-developed lysosomal apparatus and have microvilli on one of the surfaces. Hydrolytic enzymes are released from the cell into the microvilli zone, which break down the protein matrix of the bone, as a result of which calcium is released and washed out of the bones.

The intercellular substance contains collagen (ossein) fibers. These fibers are wide, ribbon-shaped and in lamellar bone tissue are located parallel and firmly glued together by the ground substance. It is these fibers that form bone plates.

In adjacent bone plates, collagen fibers run at different angles, due to this, high strength of bone tissue is achieved. Between the bone plates there are bodies of osteocytes, the processes of which penetrate the bone plates. In coarse-fibered bone tissue, bone fibers run randomly, intertwine with each other and form bundles. Osteocytes lie between the fibers.

The bones of an adult are built from lamellar bone tissue, and it forms a compact bone substance containing osteons and cancellous bone (it lacks osteons).

The epiphyses of tubular bones are built from spongy bone tissue, and the diaphyses are made from compact bone substance.

Bone tissue has several functions. First of all, it is a supporting function, providing fixation of internal organs, as well as ligaments and muscles (musculoskeletal system). Due to the supporting function of the bone, the skeleton can withstand not only the weight of the body, but also heavy loads. Representing levers of varying lengths movably connected at joints, bones ensure movement of the body in space due to the contractile activity of muscles. The protective function of bone tissue is most clearly manifested in relation to the central nervous system (brain and spinal cord) and bone marrow, covered with solid bone covers. Both of these functions of bone tissue can be called mechanical and their implementation is associated with the structural features of the main types of tissue - spongy or trabecular and dense or lamellar. Thus, trabecular bone tissue, due to its spongy structure at the ends of long bones, absorbs shock transmitted through the joints and is capable of bending and returning to its original shape. Lamellar dense bone is much more resistant to bending and twisting. In addition to supporting and protective, bone tissue also performs other functions in the body: storage reservoir and metabolic, participates in protecting the internal environment from chronic acidosis, is a trap for heavy metals and radioactive isotopes entering the body, and participates in the histogenesis of hematopoietic tissue.

Bone is a constantly renewed tissue, in which individual areas are constantly destroyed, and new ones are formed in their place. Within 10 years, almost all bone tissue in an adult is renewed (physiological regeneration). Consequently, two main processes continuously coexist in bone tissue: resorption and tissue formation. These processes are associated with the activity of bone tissue cells: osteoblasts, osteocytes and osteoclasts.

Bone tissue consists of an organic matrix or osteoid, which accounts for about 35% of the mass, and a mineral component (65%). The functional and structural unit of compact bone is the osteon, which is a concentrically located plate around the Haversian canal, in which blood vessels pass. In cancellous bone, trabeculae are found in cavities that include the bone marrow and are surrounded by numerous blood capillaries. The organic matrix is synthesized by osteoblasts, which have high synthetic activity and secrete collagen and proteoglycans, phospholipids and alkaline phosphatase necessary for bone mineralization. During bone formation, osteoblasts are surrounded on the periphery by mineralized areas of tissue and turn into osteocytes, the main function of which is to maintain the metabolism of already mineralized bone areas. The third type of cells - osteoclasts - is located on the surface of the bone in special recesses or resorption niches formed due to the activity of these cells. Osteoclasts, through exocytosis, release H-ions that dissolve bone minerals and secrete lysosomal enzymes (hydrolases and collagenases) that destroy the bone matrix.

Bone tissue is a specialized type of connective tissue with highly mineralized intercellular substance. The bones of the skeleton are built from these tissues.

Characteristics of cells and intercellular substance.

Bone tissue consists of:

A. Cell:

1) Osteocytes – the predominant number of bone tissue cells that have lost the ability to divide. They have a process form and are poor in organelles. Located in bone cavities, or gaps, which follow the contours of the osteocyte. Osteocyte processes penetrate the bone tubules and play a role in its trophism.

2) Osteoblasts – young cells that create bone tissue. In bone, they are found in the deep layers of the periosteum, in places of formation and regeneration of bone tissue. These cells come in various shapes (cubic, pyramidal or angular), contain one nucleus, and in the cytoplasm a well-developed granular endoplasmic reticulum, mitochondria and Golgi complex.

3) Osteoclasts – cells that can destroy calcified cartilage and bone. They are large in size (their diameter reaches 90 microns), contain from 3 to several dozen nuclei . The cytoplasm is slightly basophilic, rich in mitochondria and lysosomes. The granular endoplasmic reticulum is relatively poorly developed.

B. Intercellular substance, consisting of:

main substance, which contains a relatively small amount of chondroitinsulfuric acid and a lot of citric and other acids that form complexes with calcium (amorphous calcium phosphate, hydroxyapatite crystals).

collagen fibers, forming small bunches.

Depending on the location of collagen fibers in the intercellular substance, bone tissue classified on the:

1. Reticulofibrous bone tissue.

2. Lamellar bone tissue. bone plates

Reticulofibrous bone tissue.

In it, collagen fibers have a random arrangement. This tissue is found mainly in embryos. In adults, it can be found at the site of cranial sutures and at the sites of attachment of tendons to bones.

The structure of lamellar bone tissue using the example of the diaphysis of a tubular bone.

This is the most common type of bone tissue in the adult body. It consists of bone plates, formed by bone cells and mineralized amorphous substance with collagen fibers oriented in a certain direction. In adjacent laminae, the fibers usually have different directions, which results in greater strength of the lamellar bone tissue. The compact and spongy substance of most flat and tubular bones of the skeleton is built from this tissue.

Bone as an organ.

Bone is an independent organ, consisting of tissues, the main one being bone.

Histological structure of tubular bone

It consists of the epiphyses and diaphysis. From the outside, the diaphysis is covered with periosteum, or periostomy(Figure 6-3). The periosteum has two layers: outer(fibrous) – formed mainly by fibrous connective tissue and internal(cellular) – contains cells osteoblasts. The vessels and nerves that feed the bone pass through the periosteum, and collagen fibers, which are called perforating fibers. Most often, these fibers branch only in the outer layer of the common plates. The periosteum connects the bone with surrounding tissues and takes part in its trophism, development, growth and regeneration.

The compact substance that forms the bone diaphysis consists of bone plates arranged in a certain order, forming three layers:

outer layer of common lamellae. In him the plates do not form complete rings around the diaphysis of the bone. This layer contains perforating channels, through which vessels enter from the periosteum into the bone.

average,osteon layer - formed by concentrically layered bone plates around the vessels . Such structures are called osteons, and the plates that form them are osteon plates. Osteons are a structural unit of the compact substance of tubular bone. Each osteon is delimited from neighboring osteons by the so-called cleavage line. The central canal of the osteon contains blood vessels with accompanying connective tissue. . All osteons are generally located parallel to the long axis of the bone. The osteon canals anastomose with each other. The vessels located in the osteon canals communicate with each other, with the vessels of the bone marrow and periosteum. In addition to osteon plates, this layer also contains insert plates(remains of old destroyed osteons) , which lie between the osteons.

inner layer of common laminae well developed only where the compact bone substance directly borders the medullary cavity.

The inside of the compact substance of the diaphysis is covered with endosteum, which has the same structure as the periosteum.

Rice. 6-3. The structure of the tubular bone. A. Periosteum. B. Compact bone substance. V. Endost. D. Bone marrow cavity. 1. Outer layer of common plates. 2. Osteonic layer. 3. Osteon. 4. Osteon channel. 5. Insert plates. 6. Inner layer of common plates. 7. Bone trabecula of spongy tissue. 8. Fibrous layer of the periosteum. 9. Blood vessels of the periosteum. 10. Perforating channel. 11. Osteocytes. (Scheme according to V. G. Eliseev, Yu. I. Afanasyev).

The skeleton of any adult human includes 206 different bones, all of them different in structure and role. At first glance, they appear hard, inflexible and lifeless. But this is a mistaken impression; various metabolic processes, destruction and regeneration continuously occur in them. They, together with muscles and ligaments, form a special system called “musculoskeletal tissue,” the main function of which is musculoskeletal. It is formed from several types of special cells that differ in structure, functional features and significance. Bone cells, their structure and functions will be discussed further.

The structure of bone tissue

Features of lamellar bone tissue

It is formed by bone plates having a thickness of 4-15 microns. They, in turn, consist of three components: osteocytes, ground substance and collagen thin fibers. All bones of an adult are formed from this tissue. The collagen fibers of the first type lie parallel to each other and are oriented in a certain direction, while in neighboring bone plates they are directed in the opposite direction and intersect almost at a right angle. Between them are the bodies of osteocytes in the lacunae. This structure of bone tissue provides it with the greatest strength.

Cancellous bone

The name "trabecular substance" is also found. If we draw an analogy, the structure is comparable to an ordinary sponge, built from bone plates with cells between them. They are arranged in an orderly manner, in accordance with the distributed functional load. The epiphyses of long bones are mainly built from spongy substance, some are mixed and flat, and all are short. It can be seen that these are mainly light and at the same time strong parts of the human skeleton, which experience loads in different directions. The functions of bone tissue are in direct relationship with its structure, which in this case provides a large area for the metabolic processes carried out on it, gives high strength combined with low mass.

Dense (compact) bone substance: what is it?

The diaphyses of the tubular bones consist of a compact substance; in addition, it covers their epiphyses from the outside with a thin plate. It is pierced by narrow channels, through which nerve fibers and blood vessels pass. Some of them are located parallel to the bone surface (central or Haversian). Others emerge on the surface of the bone (nutrient openings), through which arteries and nerves penetrate inward, and veins penetrate outward. The central canal, together with the bone plates surrounding it, forms the so-called Haversian system (osteon). This is the main content of the compact substance and they are considered as its morphofunctional unit.

Osteon is a structural unit of bone tissue

Its second name is the Haversian system. This is a collection of bone plates that look like cylinders inserted into each other, the space between them is filled by osteocytes. In the center is the Haversian canal, through which the blood vessels that ensure metabolism in bone cells pass. Between adjacent structural units there are intercalary (interstitial) plates. In fact, they are the remnants of osteons that existed previously and were destroyed at the moment when the bone tissue underwent restructuring. There are also general and surrounding plates; they form the innermost and outer layers of the compact bone substance, respectively.

Periosteum: structure and significance

Based on the name, we can determine that it covers the outside of the bones. It is attached to them with the help of collagen fibers, collected in thick bundles, which penetrate and intertwine with the outer layer of bone plates. It has two distinct layers:

external (it is formed by dense fibrous, unformed connective tissue, it is dominated by fibers located parallel to the surface of the bone);
the inner layer is well defined in children and less noticeable in adults (formed by loose fibrous connective tissue, which contains spindle-shaped flat cells - inactive osteoblasts and their precursors).

The periosteum performs several important functions. Firstly, trophic, that is, it provides the bone with nutrition, since it contains vessels on the surface that penetrate inside along with the nerves through special nutrient openings. These channels feed the bone marrow. Secondly, regenerative. It is explained by the presence of osteogenic cells, which, when stimulated, transform into active osteoblasts that produce matrix and cause the growth of bone tissue, ensuring its regeneration. Thirdly, the mechanical or support function. That is, ensuring the mechanical connection of the bone with other structures attached to it (tendons, muscles and ligaments).

Functions of bone tissue

Among the main functions are the following:

Motor, support (biomechanical).
Protective. Bones protect the brain, blood vessels and nerves, internal organs, etc. from damage.
Hematopoietic: hemo- and lymphopoiesis occurs in the bone marrow.
Metabolic function (participation in metabolism).
Reparative and regenerative, consisting in the restoration and regeneration of bone tissue.
Morph-forming role.
Bone tissue is a kind of depot of minerals and growth factors.