Anatomical and morphological structure of bone tissue. Structure and connection of bones

Bone as an organ is part of the system of organs of movement and support, and at the same time it is distinguished by an absolutely unique shape and structure, and a rather characteristic architectonics of nerves and blood vessels. It is built mainly from special bone tissue, which is covered on the outside with periosteum and contains bone marrow on the inside.

Main Features

Each bone as an organ has a certain size, shape and location in the human body. All this is significantly influenced by the various conditions in which they develop, as well as by all kinds of functional loads experienced by the bones throughout the life of the human body.

Any bone is characterized by a certain number of sources of blood supply, the presence of specific locations of their location, as well as a rather characteristic architecture of blood vessels. All these features apply in the same way to the nerves that innervate this bone.

Structure

Bone as an organ includes several tissues that are in certain proportions, but, of course, the most important among them is bone lamellar tissue, the structure of which can be considered using the example of the diaphysis (central section, body) of a long tubular bone.

Its main part is located between the internal and external surrounding plates and is a complex of intercalated plates and osteons. The latter is a structural and functional unit of bone and is examined on specialized histological preparations or thin sections.

Outside, any bone is surrounded by several layers of common or general plates, which are located directly under the periosteum. Specialized perforating channels pass through these layers, which contain blood vessels of the same name. At the border with the bone marrow cavity they also contain an additional layer with internal surrounding plates, penetrated by many different channels expanding into cells.

The bone marrow cavity is entirely lined with the so-called endosteum, which is an extremely thin layer of connective tissue, which includes flattened osteogenic inactive cells.

Osteons

The osteon is represented by concentrically placed bone plates, which look like cylinders of different diameters, nested into each other and surrounding the Haversian canal, through which various nerves pass and. In the vast majority of cases, osteons are placed parallel to the length of the bone, while repeatedly anastomosing with each other.

The total number of osteons is individual for each specific bone. So, for example, as an organ, it includes them in the amount of 1.8 for every 1 mm², and the Haversian canal in this case accounts for 0.2-0.3 mm².

Between the osteons there are intermediate or intercalary plates, running in all directions and representing the remaining parts of old osteons that have already collapsed. The structure of bone as an organ involves the constant occurrence of processes of destruction and new formation of osteons.

The bone plates are cylindrical in shape, and the ossein fibrils fit tightly and parallel to each other. Osteocytes are located between the concentrically lying plates. The processes of bone cells, gradually spreading through numerous tubules, move towards the processes of neighboring osteocytes and participate in intercellular connections. Thus, they form a spatially oriented lacunar-tubular system, which is directly involved in various metabolic processes.

The osteon composition includes more than 20 different concentric bone plates. Human bones pass one or two microvasculature through the osteon canal, as well as various unmyelinated nerve fibers and special lymphatic capillaries, which are accompanied by layers of connective loose tissue, including various osteogenic elements such as osteoblasts, perivascular cells and many others.

The osteon channels have a fairly tight connection with each other, as well as with the medullary cavity and periosteum due to the presence of special penetrating channels, which contributes to the general anastomosis of bone vessels.

Periosteum

The structure of bone as an organ means that it is covered on the outside with a special periosteum, which is formed from connective fibrous tissue and has an outer and inner layer. The latter includes cambial progenitor cells.

The main functions of the periosteum include participation in regeneration, as well as providing protection, which is achieved through the passage of various blood vessels here. Thus, blood and bone interact with each other.

What are the functions of the periosteum?

The periosteum almost completely covers the outer part of the bone, with the only exception being the places where articular cartilage is located and ligaments or tendons of muscles are attached. It is worth noting that with the help of the periosteum, blood and bone are limited from the surrounding tissues.

In itself, it is an extremely thin, but at the same time durable film, which consists of extremely dense connective tissue in which lymphatic and blood vessels and nerves are located. It is worth noting that the latter penetrate into the bone substance precisely from the periosteum. Regardless of whether we are considering the nasal bone or some other bone, the periosteum has a fairly large influence on the processes of its development in thickness and nutrition.

The internal osteogenic layer of this coating is the main place in which bone tissue is formed, and it itself is richly innervated, which affects its high sensitivity. If a bone loses its periosteum, it eventually ceases to be viable and becomes completely dead. When performing any surgical interventions on bones, for example during fractures, the periosteum must be preserved to ensure their normal further growth and healthy condition.

Other design features

Almost all bones (with the exception of the vast majority of cranial bones, which includes the nasal bone) have articular surfaces that ensure their articulation with others. Such surfaces, instead of periosteum, have specialized articular cartilage, which is fibrous or hyaline in structure.

Inside the vast majority of bones there is bone marrow, which is located between the plates of spongy substance or is located directly in the medullary cavity, and it can be yellow or red.

In newborns, as well as in fetuses, the bones contain exclusively red bone marrow, which is hematopoietic and is a homogeneous mass, saturated with formed elements of blood, blood vessels, as well as special Red bone marrow includes a large number of osteocytes, bone cells. The volume of the red bone marrow is approximately 1500 cm³.

In an adult who has already experienced bone growth, the red bone marrow is gradually replaced by yellow, represented mainly by special fat cells, and it is immediately worth noting the fact that only the bone marrow that is located in the bone marrow cavity is replaced.

Osteology

Osteology deals with what the human skeleton is, how bones grow together, and any other processes associated with them. The exact number of described organs in humans cannot be precisely determined because it changes during the aging process. Few people realize that from childhood to old age, people constantly experience bone damage, tissue death and many other processes. In general, more than 800 different bone elements can develop throughout life, 270 of which occur in the prenatal period.

It is worth noting that the vast majority of them grow together while a person is in childhood and adolescence. In an adult, the skeleton contains only 206 bones, and in addition to permanent ones, non-permanent bones may also appear in adulthood, the appearance of which is determined by various individual characteristics and functions of the body.

Skeleton

The bones of the limbs and other parts of the body, together with their joints, form the human skeleton, which is a complex of dense anatomical formations that, in the life of the body, take on mainly exclusively mechanical functions. At the same time, modern science distinguishes a hard skeleton, which appears as bones, and a soft one, which includes all kinds of ligaments, membranes and special cartilaginous compounds.

Individual bones and joints, as well as the human skeleton as a whole, can perform a variety of functions in the body. Thus, the bones of the lower extremities and torso mainly serve as support for soft tissues, while most bones are levers, since muscles that provide locomotor function are attached to them. Both of these functions make it possible to rightly call the skeleton a completely passive element of the human musculoskeletal system.

The human skeleton is an anti-gravity structure that counteracts the force of gravity. While under its influence, the human body must be pressed to the ground, but due to the functions carried by individual bone cells and the skeleton as a whole, no change in the shape of the body occurs.

Functions of bones

The bones of the skull, pelvis and torso provide a protective function against various damage to vital organs, nerve trunks or large vessels:

the skull is a complete container for the organs of balance, vision, hearing and the brain;
the spinal canal includes the spinal cord;
the chest provides protection to the lungs, heart, as well as large nerve trunks and blood vessels;
The pelvic bones protect the bladder, rectum, and various internal genital organs from damage.

The vast majority of bones contain red bone marrow, which is a special organ of hematopoiesis and the immune system of the human body. It is worth noting that bones provide protection from damage, and also create favorable conditions for the maturation of various formed elements of blood and its trophism.

Among other things, special attention should be paid to the fact that bones are directly involved in mineral metabolism, since many chemical elements are deposited in them, among which calcium and phosphorus salts occupy a special place. Thus, if radioactive calcium is introduced into the body, after about 24 hours more than 50% of this substance will accumulate in the bones.

Development

Bone formation is carried out by osteoblasts, and there are several types of ossification:

Endesmal. It is carried out directly in the connective tissue of the primary bones. From various points of ossification on the connective tissue embryo, the ossification procedure begins to spread radially on all sides. The superficial layers of connective tissue remain in the form of periosteum, from which the bone begins to grow in thickness.
Perichondral. Occurs on the outer surface of the cartilaginous rudiments with the direct participation of the perichondrium. Thanks to the activity of osteoblasts located under the perichondrium, bone tissue is gradually deposited, replacing cartilaginous tissue and forming an extremely compact bone substance.
Periosteal. Occurs due to the periosteum, into which the perichondrium is transformed. The previous and this type of osteogenesis follow each other.
Endochondral. It is carried out inside the cartilaginous rudiments with the direct participation of the perichondrium, which ensures the supply of processes containing special vessels into the cartilage. This bone-forming tissue gradually breaks down the worn-out cartilage and forms an ossification point right in the center of the cartilaginous bone model. With further spread of endochondral ossification from the center to the periphery, the formation of spongy bone substance occurs.

How does it happen?

In each person, ossification is functionally determined and begins from the most loaded central areas of the bone. Approximately in the second month of life, primary points begin to appear in the womb, from which the development of the diaphyses, metaphyses and bodies of tubular bones occurs. Subsequently, they ossify through endochondral and perichondral osteogenesis, and right before birth or in the first few years after birth, secondary points begin to appear, from which the development of the epiphyses occurs.

In children, as well as people in adolescence and adulthood, additional islands of ossification may appear, from where the development of apophyses begins. Various bones and their individual parts, consisting of a special spongy substance, ossify endochondral over time, while those elements that include spongy and compact substances ossify peri- and endochondral. The ossification of each individual bone fully reflects its functionally determined phylogenetic processes.

Height

During growth, the bone undergoes restructuring and slight displacement. New osteons begin to form, and in parallel with this, resorption also occurs, which is the resorption of all old osteons, which is produced by osteoclasts. Due to their active work, almost all of the endochondral bone of the diaphysis is eventually resorbed, and instead a full-fledged medullary cavity is formed. It is also worth noting that the layers of perichondral bone also dissolve, and instead of the missing bone tissue, additional layers are deposited on the side of the periosteum. As a result, the bone begins to grow in thickness.

The growth of bones in length is ensured by a special layer between the metaphysis and epiphysis, which persists throughout adolescence and childhood.

Bone tissue consists of cells and intercellular substance (fibers and mineralized amorphous substance).

The following bone tissue cells are distinguished: osteoblasts, osteocytes, osteoclasts. The main function of osteoblasts is to synthesize the intercellular substance of bone. As a result, osteoblasts surround themselves with a matrix and transform into osteocytes. Each osteocyte lies, like a chondrocyte, in a lacunae, but these lacunae, unlike lacunae of cartilage tissue, are connected to each other by tubules in which the processes of osteocytes are enclosed. Osteoclasts, using their enzyme systems, destroy the organic matrix of the bone, after which the inorganic component of the intercellular substance is washed out. Thus, osteoclasts resorb bone in areas where bone remodeling occurs.

The intercellular substance contains collagen fibers consisting of type I collagen. The organic component of the amorphous substance is represented by sulfated glycosaminoglycans in combination with proteins (proteoglycans). The inorganic component consists of calcium phosphate - 95% and calcium carbonate - 10%, as well as small amounts of magnesium, potassium, fluorine and other substances. Calcium phosphate forms hydroxyapatite crystals, which are tightly bound to collagen fibers and lie along their surface. There are two specific glycoproteins: osteonectin (a compound of minerals and collagen) and osteocalcin (calcium-binding protein). This dense, mineralized matrix prevents any diffusion of gases or nutrients. Therefore, bone tissue is richly vascularized.

Students must clearly distinguish bone as an organ from bone tissue. The structure of bone as an organ is studied at the Department of Normal Anatomy. Bones are flat and tubular; bones contain compact and spongy substance; tubular bone has an epiphysis, diaphysis, metaphysis, and apophysis. All these are characteristics of bone as an organ. And bones consist of bone tissue, which comes in two types: lamellar and reticulofibrous. In an adult, the skeleton consists of lamellar bone tissue; reticulofibrous bone tissue forms only the sutures between the bones of the skull and the apophyses of the tubular bones.

Lamellar bone tissue consists of plates formed by bone cells connected to each other by processes, mineralized amorphous substance and collagen fibers oriented in the direction of the applied force.

In the compact substance of bone, plates of bone tissue form osteons - bone plates concentrically located around a blood vessel. Compact bone is very dense and strong. In the cancellous substance, the bony plates form a network in which the plates follow in the direction of the applied force. Between the bone plates in the spongy substance there are blood vessels.

Reticulofibrous bone tissue consists of trabeculae of bone tissue without a specific orientation, which differ from lamellae in the random arrangement of thick collagen fibers. Bone trabeculae form projections and communicate with each other in a wide-loop network. The space between the trabeculae is occupied by loose connective tissue with blood vessels.

Specimen: cross section of tubular bone. Schmorl staining.

At low magnification, examine the outer surface of the bone. The periosteum consists of two layers: the outer fibrous layer (collagen fibers are colored brown) and the inner osteogenic layer (the nuclei of thin pale-colored flattened osteogenic cells can be seen). Osteogenic cells participate in the processes of bone formation and appositional growth. The periosteum contains blood vessels that enter and exit the bone.

Under the periosteum is the outer layer of common lamellae. These are bony plates that run parallel to the periosteum around the entire circumference of the bone.

Further to the center of the slice is a layer of osteons. At low magnification they look like concentric circles around the vessel. Between them are intercalated plates - the remains of old osteons, which look like an osteon sector.

Following the layer of osteons is a layer of inner surrounding lamellae - parallel lamellae of bone on the inside of the bone.

In the center of the cut there is a section of spongy substance - intertwined bone crossbars, and endosteum is a layer that covers the cavities of the spongy bone, the cavities containing the bone marrow, and the Haversian canals of compact bone tissue. On the preparation it is a thin fibrous membrane covering the inner surrounding plates.

Go back to the osteon layer and look at it at high magnification. In the central canal of the osteon there is a blood vessel, around it there are dark brown circles - these are osteon plates. Each plate contains lacunae with bone cells. After the completion of the synthesis of the components of the intercellular substance and their mineralization, osteoblasts remain closed in lacunae with strong mineralized boundaries. The lacunae in which osteocytes are located soon after their formation have a relatively rounded outline; older ones are usually ovoid in shape, as are the osteocytes located in them. This means that bone cells do not have the opportunity to divide (therefore, interstitial bone growth does not occur) and for diffuse nutrition. Osteocytes are nourished by their processes, which are located in small crevices in the mineralized matrix - bone tubules. Bone tubules appear as thin wavy lines radiating from the lacuna. They appear short because they only partially lie in the cut plane, and this is easy to verify when rotating the microscrew. Bone tubules permeate the entire bone plate, and nutrients enter the tubules from blood vessels. The compact bone is pierced by canals in which vessels are located: these are Haversian canals and Volkmann canals. Haversian canals run along the length of the bone and osteon plates are concentrically located along them. Gases and nutrients spread from the Haversian canals along the bone tubules along the processes of osteocytes. Volkmann's canals are easier to detect on longitudinal sections of tubular bone, because they run across the bone, connect the Haversian canals with each other and conduct the vessels of the periosteum to the Haversian canals.

Specimen: development of bone from mesenchyme (cross section of the jaw of an animal embryo). Hematoxylin-eosin staining.

Ossification zones at low magnification look like pink islands of irregular tree-like shape. Examine such an island at high magnification. The bone matrix, which is produced by osteoblasts, turns pink. When osteoblasts complete the synthesis of the organic part of the matrix and it mineralizes, bone cells become embedded in the intercellular substance. They are visible inside the islet - spindle-shaped basophilic osteocytes.

Osteocytes are connected to each other by processes lying in the tubules. They are poorly visible on this preparation. This is due to the fact that the bone is decalcified to prepare the drug. When the mineral component is removed, there is nothing left that would provide sufficient matrix rigidity to maintain the tubule open. The canaliculus collapses. When stained with hematoxylin-eosin, there is insufficient contrast between the osteocyte process and the matrix, so the processes are poorly visible (in the previous preparation, the tubules also collapsed, but the dark brown processes clearly contrasted with the green matrix).

The ossification zone is surrounded by osteoblasts - polygonal cells with eccentrically located nuclei and such basophilic cytoplasm that sometimes the nuclei are poorly distinguishable. Between them, sometimes in the recesses of the island of bone tissue, osteoclasts are found. Osteoclasts are large cells with many nuclei. As a rule, 5-10 nuclei are visible, the rest remain outside the cut plane. Typically, the side of the cell that is closest to the bone surface contains fewer nuclei than the opposite side. The cytoplasm near the bone surface is weakly stained and highly vacuolated. Sometimes bristle-like structures can be seen between the osteoclast and the bone surface, especially if the osteoclast is located in a recess in the niche of the bone island. When they are found, students incorrectly assume that this is an osteoclast brush border. But this structure is actually part of the bone exposed by erosion. These cells destroy formed bone tissue in order to rebuild the trabecula, changing its shape and size.

The spaces between the ossification zones are occupied by pale-colored mesenchyme. Its cells are branched with slightly basophilic cytoplasm. In the mesenchyme, transverse and oblique sections of thin-walled blood vessels are found in large numbers.

Specimen: development of bone in place of cartilage (longitudinal section of the tubular bone of the embryo). Hematoxylin-eosin staining.

Focus on the specimen at low magnification: find the epiphysis, metaphysis, diaphysis. The epiphysis is represented by hyaline cartilage, externally covered with perichondrium. This is a zone of unchanged cartilage.

If you move along the preparation towards the diaphysis, then the zone of columnar cartilage begins, which consists of young, proliferating cartilage cells. Their division ensures the growth of the primordium in length. The cells are small, wedge-shaped, stacked one on top of the other, like stacks of coins, and thus form columns located perpendicular to the plane of the plate. The organization of cartilage cells into columns is apparently maintained due to the fact that bundles of collagen fibrils in the septa of the intercellular substance run in the longitudinal direction. Chondroblasts located near the epiphysis are the youngest and divide more often, and those located closer to the diaphysis are the most mature, which are displaced by dividing cells.

During the maturation process, these cells increase in size, glycogen accumulates in their cytoplasm, and in the preparation they look light - a zone of vesicular cartilaginous cells.

When mature, these cells begin to produce alkaline phosphatase, so the intercellular substance becomes calcified. A basophilic matrix of the calcified cartilage zone is formed. This zone is located on the border with the diaphysis. Move the specimen to the area of the diaphysis and examine the areas of ossification.

When the cartilaginous model of bone increases significantly in size due to peripheral cell division, the chondrocytes in the central part mature and hypertrophy, and the surrounding matrix calcifies. Since it is not able to ensure the diffusion of nutrients to the chondrocytes, they die. The site of cartilage death is reached by blood vessels and osteogenic cells, which gather around the remains of calcified cartilage and differentiate into osteoblasts that produce bone intercellular substance. Thus, the preparation reveals basophilic areas of calcified cartilaginous matrix, which is covered with oxyphilic bone tissue; osteoblasts covering bone trabeculae are also basophilic. These are areas of internal, enchondral ossification. But if you move the specimen and examine the periphery of the diaphysis, you can also find areas of ossification there. Outside, the diaphysis is covered with already formed periosteum, and under it oxyphilic zones of perichondral ossification are found.

Examine the diaphysis at high magnification. Using the same characteristics as in the previous preparation, look for osteoblasts, osteocytes, osteoclasts and mesenchymal cells.

Electron diffraction pattern of an osteoblast. The ultrastructure of an osteoblast is typical of a secretory cell. The main product of its secretory activity is procollagen; in addition, the osteoblast secretes components of the amorphous substance and some enzymes. Therefore, the osteoblast has a well-developed granular EPS, which is distributed randomly throughout the cell. The Golgi apparatus is located on the side of the nucleus that faces the bulk of the cytoplasm and contains spherical and cylindrical sacs. The cell contains numerous mitochondria, several lysosomes and multivesicular bodies. Students will be able to distinguish an osteoblast from other actively secreting cells by a section of calcified electron-dense intercellular substance located in the corner of the microphotograph.

Electron diffraction pattern of an osteocyte. An osteocyte is a small process cell located in a bone lacuna. Bone tissue is an electron-dense substance that forms a narrow chamber - a lacuna.

Since the cell does not actively function, most of it is occupied by the nucleus with a large amount of heterochromatin. Cytoplasmic processes located inside the bone matrix tubules are visible.

The bone consists of a dense compact substance, substantia compacta, located along the periphery, and a spongy substance, substantia spongiosa, located in the center and represented by a mass of bone crossbars located in different directions. The cancellous beams do not run randomly, but correspond to the lines of compression and tension that act on each section of the bone. Each bone has a structure that best suits the conditions in which it is located. In some adjacent bones, the compression (or tension) curves, and therefore the cancellous beams, form a single system.

Figure: The structure of the femur on a cut.
1 - pineal gland; 2 - metaphysis; 3 - apophysis; 4 - spongy substance; 5 - diaphysis; 6 - compact substance; 7 - bone marrow cavity.

The thickness of the compact layer in spongy bones is small. The bulk of bones of this shape is represented by spongy substance. In tubular bones, the compact substance is thicker in the diaphysis, while the spongy substance, on the contrary, is more pronounced in the epiphyses. The medullary canal, located in the thickness of the tubular bones, is lined with a connective tissue membrane - endosteum.
The cells of the spongy substance and the medullary canal of long bones are filled with bone marrow. There are two types of bone marrow: red, medulla ossium rubra, and yellow, medulla ossium flava. In fetuses and newborns, the bone marrow in all bones is red. From 12 to 18 years of age, the red marrow in the diaphysis is replaced by yellow bone marrow. The red brain is built from reticular tissue, in the cells of which there are cells related to hematopoiesis and bone formation. Yellow marrow contains fatty inclusions that give it a yellow color. The outside of the bone is covered with periosteum, and at the junction with the bones - with articular cartilage.
The periosteum, periosteum, is a connective tissue formation consisting of two layers: internal (germinal, or cambial) and external (fibrous). It is rich in blood and lymphatic vessels and nerves, which continue into the thickness of the bone. The periosteum is connected to the bone through connective tissue fibers that penetrate the bone. The periosteum is the source of bone growth in thickness and is involved in the blood supply to the bone. 3a account of the periosteum, the bone is restored after fractures. In old age, the periosteum becomes fibrous, its ability to produce bone substance weakens. Therefore, bone fractures in old age are difficult to heal.
Microscopically, bone consists of bone plates arranged in a certain order. Bone plates consist of collagen fibers impregnated with the ground substance and bone cells. Bone cells are located in bone cavities. From each bone cavity, thin tubules diverge in all directions, connecting with the tubules of neighboring cavities. These tubules contain processes of bone cells that anastomose with each other. Through the tubule system, nutrients are delivered to bone cells and waste products are removed. The system of bone plates surrounding the bone canal is called osteon. Osteon is a structural unit of bone tissue. The direction of the osteon channels corresponds to the direction of the tension and support forces created in the bone during its functioning. In addition to the osteon canals, the bones have perforating nutrient canals that penetrate the outer common plates. They open on the surface of the bone under the periosteum. These channels serve to pass blood vessels from the periosteum into the bone.
Bone plates are divided into osteon plates, concentrically located around the bone canals of the osteon, intercalary plates, located between the osteons, and common plates (external and internal), covering the bone from the outer surface and along the surface of the medullary cavity.
Bone is a tissue whose external and internal structure undergoes change and renewal throughout a person’s life. This is accomplished due to the interconnected processes of destruction and creation that lead to bone restructuring and are characteristic of living bone. Restructuring of bone tissue enables the bone to adapt to changing conditions of function and ensures high plasticity and reactivity of the skeleton.

Figure: Bone structure (diagram).
1 - spongy substance; 2 - osteon channel; 3 - spongy crossbar; 4 - intercalary bone plates; 5 - cells of spongy substance; 6 - compact substance; 7 - perforating nutrient channels; 8 - periosteum; 9 - common external bone plates; 10 - osteons; 11 - osteon bone plates.

Restructuring of bones occurs throughout a person’s life. It occurs most intensely in the first 2 years of the postnatal period, at 8-10 years and during puberty. The child’s living conditions, previous diseases, and the constitutional characteristics of his body affect the development of the skeleton. Physical exercise, labor and associated mechanical factors play a major role in the formation of bones in a growing organism. Sports and physical labor lead to increased bone remodeling and a longer period of its growth. The processes of formation and destruction of bone substance are regulated by the nervous and endocrine systems. If their function is impaired, bone development and growth disorders may occur, including the formation of deformities. Professional and sports stress affects the structural features of bones. Bones that experience heavy loads undergo restructuring, leading to a thickening of the compact layer.
Blood supply and innervation of bones. The blood supply to the bones comes from nearby arteries. In the periosteum, the vessels form a network, the thin arterial branches of which penetrate through the nutrient openings of the bone, pass through the nutrient canals, osteon canals, reaching the capillary network of the bone marrow. The capillaries of the bone marrow continue into the broad sinuses, from which the venous vessels of the bone originate.
The branches of the nearest nerves, forming plexuses in the periosteum, take part in the innervation of the bones. One part of the fibers of this plexus ends in the periosteum, the other, accompanying the blood vessels, passes through the nutrient canals, osteon canals and reaches the bone marrow.

Material taken from the site www.hystology.ru

Bone tissue, like other types of connective tissue, develops from mesenchyme, consists of cells and intercellular substance, performs the function of support, protection and is actively involved in the metabolism of the body. The bones of the skeleton, skull, chest, and spine provide mechanical protection for the organs of the central nervous system and the thoracic cavity. Red bone marrow is localized in the spongy substance of the skeletal bones, where the processes of hematopoiesis and differentiation of the body's immune defense cells take place. Bone deposits salts of calcium, phosphorus, etc. In total, minerals make up 65 - 70% of the dry mass of tissue, mainly in the form of its phosphorus and carbon dioxide compounds (salts). Bone actively participates in the body’s metabolism, which determines its ability to naturally restructure itself in response to changing conditions of its life, metabolic dynamics due to age, diet, activity of the endocrine glands, etc.

Bone cells. Bone tissue contains four different types of cells: osteogenic cells, osteoblasts, osteocytes and osteoclasts.

Osteogenic cells are cells at an early stage of specific differentiation of mesenchyme in the process of osteogenesis. They retain the potency for mitotic division. They are characterized by an oval, chromatin-poor nucleus. Their cytoplasm is weakly stained with basic or acidic dyes. These cells are localized on the surface of bone tissue: in the periosteum, endosteum, Haversian canals and other areas of bone tissue formation. Osteogenic cells multiply and differentiate

Rice. 120. Bone development in mesenchyme (according to Petersen):

A- newly formed intercellular substance of bone tissue; b - osteoblasts.

replenish the supply of osteoblasts, which provide burrowing and reconstruction of the bone skeleton.

Osteoblasts are cells that produce organic elements of the intercellular substance of bone tissue: collagen, glycosaminoglycans, proteins, etc. These are large cubic or prismatic cells located on the surface of the developing bone beams. Their thin processes anastomose with each other. The nuclei of osteoblasts are round with a large nucleolus and located eccentrically. The cytoplasm contains a well-developed granular endoplasmic reticulum and free ribosomes, which determines its basophilia (Fig. 120, 121, 122). The Gol-ji complex is dispersed in the cytoplasm of cells between the nucleus and the developing bone. Numerous oval-shaped mitochondria. A positive reaction to alkaline phosphatase activity is specific to the cytoplasm of osteoblasts.

Osteocytes - bone tissue cells - lie in special cavities of the intercellular substance - lacunae, interconnected by numerous bone tubules. Osteocytes have a flattened oval shape corresponding to the lacuna (22 - 55 µm in length and b - 15 µm in width). Their numerous thin processes, spreading along the bone canaliculi, anastomose with the processes of neighboring cells. The system of lacunae and bone tubules contains tissue fluid and provides the level of metabolism necessary for the life of bone cells (Fig. 123, 124). The morphological organization of the cytoplasm of osteocytes corresponds to the degree of their differentiation. Young developing cells are close to osteoblasts in the composition of organelles and the degree of their development. In more mature bone, the cytoplasm of cells is poorer in organelles, which indicates a decrease in the level of metabolism, in particular protein synthesis.

Osteoclasts are large, multinucleated cells, from 20 to 100 microns in diameter. Osteoclasts are located on the surface of bone tissue in places of its resorption. Cells are polarized. Their surface, facing the resorbable bone, has a larger number of thin, densely located, branching processes, which together form a corrugated border (Fig. 125). Here they are secreted and concentrated

Rice. 121. Scheme of the structure of an osteoblast:

A- on light-optical; B - at the submicroscopic level; 1 - core; 2 - cytoplasm; 3 - development of granular endoplasmic reticulum; 4 - osteoid; 5 - mineralized bone tissue.

Rice. 122. Electron microphotograph of an osteoblast;

1 - core; 2 - nucleolus; 3 - cytoplasmic reticulum; 4 - mitochondria.

Rice. 123. Bone plate from the ethmoid bone of a white mouse: cells and intercellular substance are visible.

Rice. 124. Electron microphotograph of an osteocyte (magnitude 16000):

1 - core; 2 - osteocyte processes; 3 - the main calcified substance surrounding the osteocyte; 4 - alpha cytomembranes of ergastoplasma; 5 - the main non-calcified substance directly adjacent to the osteocyte (according to Dalley and Spiro).

Rice. 125, Scheme of the structure of an osteoclast:

A __ at the light-optical level; B - at the submicroscopic level; I- core; 2 - corrugated edge of the osteoclast; 3 - light zone; 4 - lysosomes; 5 - zone of resorption of intercellular substance; 6 - mineralized intercellular substance.

hydrolytic enzymes involved in bone destruction processes. The area of the corrugated border borders the surrounding area of the cell surface, which is tightly adjacent to the resorbable bone in a light zone containing almost no organelles. The cytoplasm of the central part of the cell and its opposite pole contains numerous nuclei (up to 100 nuclei), several groups of Golgi complex structures, mitochondria, and lysosomes. Lysosome enzymes entering the corrugated border zone are actively involved in bone resorption. Parathyroid hormones (PTH), by enhancing the secretion of lysosome enzymes, stimulate bone resorption. Thyroid calcitonin reduces osteoclast activity. Under these conditions, the processes of the corrugated border are smoothed out, and the cell is separated from the surface of the bone. Bone resorption slows down.

Intercellular substance bone tissue consists of collagen fibers and amorphous substances: glycoproteins, sulfated glycosaminoglycans, proteins and inorganic compounds - calcium phosphate, hydroapatite and various trace elements (copper, zinc, barium, magnesium, etc.). 97% of the body's total calcium is concentrated in bone tissue. In accordance with the structural organization of the intercellular substance, coarse fibrous bone and lamellar bone are distinguished.

Rough fibrous bone characterized by a significant diameter of bundles of collagen fibrils and a variety of their orientation. It is typical for bones of the early stage of animal ontogenesis and some areas of the adult skeleton: dental alveoli, skull bones near bone sutures, the bony labyrinth of the inner ear, the area of attachment of tendons and ligaments. In lamellar bone, collagen fibrils of the intercellular substance do not form bundles. Arranged in parallel, they form layers - bone plates with a thickness of 3 - 7 microns. Adjacent plates always have different fibril orientations. In the plates there are naturally located cellular cavities - lacunae and bone tubules connecting them, in which bone cells - osteocytes and their processes lie (Fig. 126). Tissue fluid circulates through the system of lacunae and bone tubules, ensuring metabolism in the tissue.

Depending on the position of the bone plates, spongy and compact bone tissue is distinguished. In spongy matter, in particular in the epiphyses of tubular bones, groups of bone plates are located at different angles to each other in accordance with the direction of the main mechanical loads of a given section of the skeleton. The cells of spongy bone contain red bone marrow. It is abundantly supplied with blood and actively participates in the mineral metabolism of the body.

In the compact substance, groups of bone plates: 4 - 15 microns thick are closely adjacent to each other. In accordance with the characteristics of vascularization and localization of cambial bone cells - osteoblasts in the compact substance of the diaphysis

Rice. 126. System of osteops of lamellar bone tissue (histological preparation of decalcified tubular bone. Transverse section):

1 - osteon; A- osteon canal with blood vessels; b - bone plates; V- bone lacunae (cavities); d - bone tubules; 2 - system of insert plates; 3 - resorption (commissural) line.

Rice. 127. Scheme of the structure of the tubular bone:

1 - periosteum; 2 - blood vessels; 3 - external general system of bone plates; 4 - Haversian system; 5 - insertion system; 6 - Haversian canal; 7 - Volkman channel; 8 - compact bone; 9 - spongy bone; 10 - internal general system of bone plates.

tubular bones are formed into three layers: the outer common system of plates, the osteonic layer containing osteons and intercalary systems of bone plates, and the internal common (surrounding) system. The plates of the external common system are formed by osteoblasts of the periosteum, while some of the osteoblasts turn into osteocytes and are included in the newly formed bone tissue. The bone plates of the external common system run parallel to the surface of the bone. Through this layer of bone, perforating tubules pass from the periosteum, carrying blood vessels and coarse bundles of collagen fibers into the bone, immured into it during the formation of the outer common plates (Fig. 127).

In the osteon layer of the tubular bone, the osteon channels containing blood vessels, nerves and accompanying connective tissue elements, anastomosing with each other, are mainly oriented longitudinally. The systems of tube-shaped bone plates surrounding these canals - osteons - contain from 4 to 20 plates. On cross sections of the compact substance of tubular bones, they are defined as alternating lighter fibrous (with a circular position of the fibers) and darker granular layers in accordance with the orientation of the collagen fibrils of the intercellular substance. Osteons are delimited from each other by a cement line of the ground substance. Between the osteons are included intercalary, or intermediate, systems of bone plates, which are parts of previously

Rice. 128. Lamellar bone:

A - dense (compact) bone substance; 1 - periosteum; 2 - external common plates; 3 - osteons; a - osteon channel; 4 - system of insert plates; 5 - internal common plates; B - spongy bone; 6 - yellow bone marrow.

Rice. 129. Formation of bone tissue from the mesenchyme of a cat embryo:

O - osteoblast; IN- intercellular substance of bone tissue; F- fibroblast; C - intercellular substance of connective tissue.

formed osteons, preserved during the process of bone restructuring. The latter are very diverse in size, shape and orientation (Fig. 128).

The internal common (surrounding) system of bone plates borders the endosteum of the bone cavity and is represented by plates oriented parallel to the surface of the medullary canal.

Bone histogenesis. Bone, like other types of connective tissue, develops from mesenchyme. There are two types of osteogenesis: directly from mesenchyme and by replacing embryonic cartilage with bone.

Development of bone from mesenchyme- intermembranous ossification. This type of osteogenesis is characteristic of the development of coarse fibrous bone of the skull and mandible. The process begins with the intensive development of connective tissue and blood vessels.

Mesenchymal cells, anastomosing processes with each other, collectively form a network immersed in an amorphous intercellular substance containing individual bundles of collagen fibers. Cells pushed aside by the intercellular substance to the surface of such an osteogenic island become basophilic and differentiate into osteoblasts, which are actively involved in osteogenesis (Fig. 129).

Individual cells, losing the ability to synthesize intercellular substance, with the activity of adjacent osteoblasts, become embedded in it and differentiate into osteocytes. The intercellular substance of young bone is impregnated with calcium phosphate, which accumulates in the bone due to the breakdown of blood glycerophosphate under the action of alkaline phosphatase secreted by fibroblasts. The released phosphoric acid residue reacts with calcium chloride. The resulting calcium phosphate and calcium carbonate impregnate the ground substance of the bone. Surrounding the developing bone, embryonic connective tissue forms the periosteum.

Subsequently, the primary coarse-fiber bone tissue is replaced by lamellar bone. Bone plates form around the blood vessels, forming primary osteons. On the side of the periosteum, external common systems of bone plates develop, oriented parallel to the surface of the bone.

Enchondral ossification. The bones of the torso, limbs, and base of the skull are formed in place of cartilaginous tissue. The beginning of the process is characterized by perichondral ossification, which begins with increased vascularization of the perichondrium, proliferation and differentiation of its cells and intercellular substance, including osteoblasts.

In tubular bones, this process begins in the area of the diaphysis with the formation under the perichondrium of a network of crossbars of coarse-fiber bone - a bone cuff (Fig. 130). As the periosteal bone develops in the middle of its cartilaginous model in the center of ossification, the cartilage tissue naturally changes. Cartilage cells progressively increase in size, become rich in glycogen, and become vascularized. Their kernels shrink. Cell cavities increase. In the area of the diaphysis, a zone of vesicular cartilage is formed (Fig. 131). The connective tissue of the periosteum, penetrating between the crossbars of the bone cuff, introduces into the zone of degenerating cartilage variously differentiated mesenchymal cells of both the hematopoietic series and differentiating cells of bone tissue: osteoclasts and osteoblasts.

Rice. 130. Perichondral and enchondral bone formation of a mammal (according to Bucher):

A- the beginning of the formation of the periosteal cuff; B - beginning of enchondral bone formation; 1 - perichondrium; 2 - perichondral bone; 3 - cartilage with vesicular cells and calcified intercellular substance; 4 - hyaline cartilage of the epiphysis; 5 - column of cartilage cells; 6 - cartilage with vesicular cells; 7 - enchondral bone; 8 - primary bone marrow; 9 - perichondral bone; 10 - osteoblasts.

In adjacent zones of the cartilaginous rudiment of the bone, cells, multiplying, form “cell columns” arranged in parallel rows, longitudinally oriented. The cells in the column are delimited by thin partitions of the ground substance. The intercellular substance between the columns of cells, compacting and calcifying, forms “cartilaginous beams”. Endochondral ossification spreads from the diaphysis of the cartilaginous anlage to its epiphyses; accordingly, in the composition of cell columns it is possible

Rice. 131. Enchondral and perichondral bone development:

1 - osteoblastic layer of the periosteum; 2 - fibrous layer of the periosteum; 3 - perichondrial bone cuff; 4 - cell columns; 5 - osteocytes 6 - osteoblasts; 7 - osteoclast.

identify the zone of cell proliferation that is most distant from the diaphysis (followed by zones of cell maturation, hypertrophy, dystrophy and decay closer to the diaphysis). Blood vessels with osteogenic cells grow into the lacunae that form. As osteoblasts differentiate, they localize to

Rice. 132. Enchondral bone development:

1 - osteoclast; 2 - osteoblast; 3 - remains of calcified cartilage; 4 - newly formed bone; 5 - blood vessel.

the walls of the lacunae and, producing the intercellular substance of bone, form bone tissue on the surface of the preserved cartilaginous plates. The process of replacing cartilage with bone tissue is called enchondral ossification (Fig. 132).

Simultaneously with the development of enchondral bone, an active process of perichordal osteogenesis occurs on the side of the periosteum, forming a dense layer of periosteal bone, extending along its entire length to the epiphyseal growth plate. Periosteal bone is the compact bone substance of the skeleton. Unlike the coarse-fiber bone of the cuff, its structure is

Rice. 133. Section through the epiphysis of the femur of a 4-week-old mouse (according to Shafer):

D- diaphysis; E- pineal gland; E.K.- enchondral bone of the epiphysis; GK - articular cartilage; OZ- zone of ossification of the diaphysis; PK - perichondrial bone of the diaphysis; ZR- columns of cells of the cartilaginous plate.

a typical lamellar bone with characteristic systems of bony plates, expressed to varying degrees depending on the type of animal and the specificity of individual bones of the skeleton.

Later, ossification centers appear in the epiphyses of the bone. The bone tissue developing here replaces the cartilaginous tissue of the entire epiphysis. The latter is preserved only on the articular surface and in the epiphyseal growth plate, which separates the epiphysis from the diaphysis (Fig. 133) during the entire period of growth of the organism until the animal reaches sexual maturity.

Periosteum(periosteum) consists of two layers. Its inner layer contains collagen and elastic fibers, osteoblasts, osteoclasts and blood vessels. The latter penetrate through the nutrient openings of the bone into the bone tissue and into the bone marrow. The outer layer of the periosteum is formed by dense connective tissue. It is directly connected to the muscle tendons and collagen fibers of the ligaments. Individual bundles of collagen fibers of the periosteum are directly included in the bone tissue in the form of “perforating” fibers, providing mechanical strength to the connection between the periosteum and the bone.

Endoost- a layer of connective tissue lining the medullary canal. It contains osteoblasts and thin bundles of collagen fibers that pass into the bone marrow tissue.

LAMILE BONE TISSUE

Mature (secondary), or lamellar bone tissue is formed by bone plates. Lamellar bone tissue forms spongy and compact bone substance. Spongy substance is intertwined bone trabeculae, the cavities between which are filled with bone marrow. The trabecula consists of bony plates and is surrounded on the outside by a single layer of osteoblasts. Trabeculae are located according to the direction of compression and tension forces. Spongy substance fills the epiphyses of long tubular bones and forms the internal contents of short and flat bones of the skeleton. The bulk of the compact substance consists of osteons. The compact substance forms the diaphyses of long tubular bones and covers all other (short and flat) bones of the skeleton with a layer of varying thickness.

Bone plate- a layer of bone matrix with a thickness of 3–7 microns. Osteocytes are located between adjacent plates in the lacunae, and their processes pass through the thickness of the plate in the bone tubules. Collagen fibers within the lamina are oriented in an orderly manner and lie at an angle to the fibers of the adjacent lamina, which provides significant strength to the lamellar bone.

Osteon

Osteon (Fig. 6-56, 6-56A), or Haversian system, is a collection of 4–20 concentric bone plates. In the center of the osteon is the Haversian canal (osteon canal), filled with loose fibrous connective tissue with blood vessels and nerve fibers. Volkmann canals (Fig. 6-58) connect the osteon canals with each other, as well as with the vessels and nerves of the periosteum. Externally, the osteon is limited by a cleavage line (cementation line), separating it from fragments of old osteons. During osteon formation (Fig. 6-57), osteogenic cells located in the immediate vicinity of the Haversian canal vessel differentiate into osteoblasts. On the outside there is a layer of osteoid formed by osteoblasts. Subsequently, the osteoid is mineralized, and osteoblasts, surrounded by a mineralized bone matrix, differentiate into osteocytes. The next concentric layer arises in a similar manner from within. A calcification front runs along the outer surface of the osteoid at the border with the mineralized bone matrix, where the process of deposition of mineral salts begins. The diameter of the osteon (no more than 0.4 mm) determines the distance over which substances effectively diffuse to the peripheral osteocytes of the osteon along the lacunar-tubular system from a centrally located blood vessel.

Rice. 6-56. Osteons in the compact part of the tubular bone. The layer of osteons of the compact substance of the tubular bone is formed by osteons of different generations, between which are located the remains of old osteons in the form of intercalated bone plates.

Rice. 6-56A. Diaphysis of the tubular bone, compact part. Osteons (1) and intercalated bone plates (6) are visible. In the osteon, the osteon canal (2), concentric bone plates (3), bone cavities (4), and commissural line (5) are clearly visible. Schmorl staining.

Rice. 6-57. Osteon formation. In the central part, at the site of the future osteon canal, blood vessels pass through loose connective tissue. This central part is surrounded by a layer of osteoblasts, and on the outside lies a layer of osteoid. The next layer of osteoblasts and the corresponding osteoid layer are formed closer to the center of the osteon and have a smaller diameter. First, the peripheral osteon plates calcify, and then the central ones. As the matrix calcifies, osteoblasts differentiate into osteocytes.